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... Rock-Eval® thermal analysis served for almost five decades as a standard technique in the petroleum industry (Espitalie et al., 1985b(Espitalie et al., , a, 1986Lafargue et al., 1998;Behar et al., 2001). Since the 1990's after several upgrades on its hardware, the technique saw new alternative applications, such as characterization of recent sediments, soil organic matter analysis and even analysis of pure biochemical compounds (Lafargue et al., 1998;Di-Giovanni et al., 2000;Disnar et al., 2003;Hetényi et al., 2005;Carrie et al., 2012a;Baudin et al., 2015;Gregorich et al., 2015;Barré et al., 2016;Sebag et al., 2016;Soucémarianadin et al., 2018). ...
... The first large-scale soil monitoring projects with focus on soil organic carbon (SOC) were initiated (e.g., RMQS in France, Arrouays et al., 2003 One promising analytical technique in SOM research is Rock-Eval® thermal analysis. A timeefficient and inexpensive method, it can be used on large sample sets to quantify soil organic and inorganic carbon and characterize SOC bulk chemistry and thermal stability (Saenger et al., 2013;Gregorich et al., 2015;Sebag et al., 2016;Soucémarianadin et al., 2018). Moreover, a strong empirical link exists between parameters obtained with this method and in situ observed SOM biogeochemical stability (Barré et al., 2016;Poeplau et al., 2019). ...
... Simultaneous and continuous detection of effluents generates five thermograms in total that describe the evolution of carbon containing gases (HC, CO and CO2) during the analysis. A large number of Rock-Eval® parameters can be calculated from the five thermograms (Behar et al., 2001;Saenger et al., 2013;Sebag et al., 2016;Cécillon et al., 2018Cécillon et al., , 2021Khedim et al., 2021). Parameters obtained with this method are characteristic of the SOM and its interaction with the soil mineral matrix, since soil samples are analysed with no previous isolation of SOM or removal of carbonates. ...
Thesis
Soils store twice the amount of carbon that is found in atmosphere and vegetation combined. They act as a buffer between solid earth and atmosphere and exercise a major control on the atmospheric concentration of CO2 through the release or sink of greenhouse gases. Organic carbon in soils in the form of organic matter is essential to soil health and fertility, to nutrient availability and water quality. The performance of the most valuable tool at our disposal for understanding and predicting the evolution of this reservoir, soil organic carbon (SOC) dynamics models, is currently limited by a missing key: the ability to estimate the proportion of SOC that will remain unchanged over projection-relevant timescales. This important amount of carbon present in soils for centuries or millennia, and therefore considered “stable”, can vary greatly from one location to another. The goal of my thesis was to explore a new approach based on thermal analysis and machine learning, to characterise SOC, estimate the proportion of “stable” carbon in soil samples, and use this information to improve the accuracy of SOC dynamics models. In a second step, I focused on the thermal analysis technique in the heart of this approach to understand better the important information it offers, based on model laboratory experiments. Finally, the main results of my thesis consist of a complete and validated operational approach improving the accuracy of SOC models with a clear and significant value for “climate-smart” soil management, while the experimental part offers new insights into the working principle, limitations and possibilities of the thermal analysis technique at the heart of this approach.
... Several additional RE6 parameters indicative of OM stability were also calculated. The I-and the R-index were determined from the S2 signal (Sebag et al., 2016). The R-index corresponds to the relative contribution of thermally stable OM with refractory compounds released at higher temperatures. ...
... The R-index corresponds to the relative contribution of thermally stable OM with refractory compounds released at higher temperatures. The I-index gives information on the pool of thermally labile immature OM with compounds released at lower temperatures (Sebag et al., 2016). They are both related to compost maturity (Albrecht et al., 2015) and the proportion of OC in particulate OM fractions isolated from soil ). ...
... The R-index is related to the contribution of OM compounds with higher thermal stability. In contrast, the I-index refers to contribution of OM pools presenting lower thermal stability (Sebag et al., 2016). The I-indices recorded for bulk samples and fractions of compost and vermicompost material, with and without minerals, are in the range of values recorded by Schomburg et al. (2018) for decomposed OM in sediments. ...
Article
Stabilization of organic matter (OM) against decomposition and its de-stabilization leading to mineralization are important processes controlling greenhouse gas emissions and carbon sequestration in soils. Soil organisms, particularly earthworms, may greatly influence these processes through their potential impact on the interaction of OM with clay minerals, a mechanism which may protect OM at medium (years) to long (decades, centuries) time scales. The aim of this study was to use earthworms and pure minerals in order to enhance carbon sequestration through the formation of aggregates containing particulate OM and organo-mineral associations during a composting experiment. To this end we compared OM transformations in composting treatments with and without (i) clay minerals (montmorillonite) and (ii) epigeic earthworms (Eisenia andrei and Eisenia foetida). We used density fractionation of the end-products to quantify the amount of aggregates including organo-mineral associations formed during 196 days. Their chemical composition and stability were analyzed by elemental analyses, Rock-Eval 6 thermal analysis and solid-state ¹³C nuclear magnetic resonance spectrometry. Our results indicated that the addition of minerals led to the formation of heavy fractions most probably containing aggregates and organo-mineral associations in treatments with and without earthworms. While OM showed higher oxidative transformation in all treatments, addition of earthworms changed organic carbon (OC) and nitrogen (N) concentrations in specific density fractions and increased the thermal stability of OM in heavy density fractions. These fractions contained higher proportions of aromatic and proteinaceous material. We conclude that earthworms under composting conditions may be able to effectively stabilize OM, though the formation of aggregates and/or organo-mineral associations containing greater proportions of microbial-derived material.
... Three parameters are measured during Rock Eval pyrolysis: (i) the Tmax (in°C), i.e. the temperature corresponding to the optimum hydrocarbon release during pyrolysis; (ii) the amount of pyrolyzed carbon in a N 2 atmosphere, or PC (Behar et al., 2001); (iii) the residual carbon, or RC, as the carbon content measured during the oxidation phase (Behar et al., 2001). In addition, the two indices proposed by Sebag et al. (2016) have been used, i.e. the I-index, emphasizing the degree of transformation of the immature organic fraction (related to SOM stabilization), and the R-index highlighting the contribution of the most refractory fraction or persistent SOM. ...
... Thermal stability of the OM was assessed using the I and R indices calculated from the pyrogram curves resulting from the Rock Eval pyrolysis (Sebag et al., 2016). The relationships between soil physicochemical parameters and soil diversity of the area were assessed using a principal component analysis (PCA) with eight physicochemical variables, i.e. ...
... I and R indices (Fig. 4) cluster different sets of samples, and trends are organized by soil types. As expected (Sebag et al., 2016), there is no trend observable neither in the distribution of Arenosol samples nor in Calcisols (but the number of samples is limited); they have relatively homogeneous values of R-index ( ...
Article
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The study area of the Chobe Enclave (northern Botswana) is defined as mostly covered by Arenosols in available maps. However, recent explorations of the area showed that soils are more diverse than expected. This is because of complex interactions between current alluvial deposition processes, paleo-environmental effects (ancient alluvial deposition, ancient wind-blown sand deposits) and ongoing hydrological effects and colluvial effects on topographic gradients. An in-depth exploration of both soils and vegetation in the area was conducted with the aim (i) to survey the soil diversity at the Chobe Enclave, (ii) to study soil dynamics and identify the key factors of this diversity, and (iii) to create a soil map based on the analysis of the soil-vegetation relationship. For this purpose, thirty-six soil profiles were extensively described according to the World Reference Base for soil resources. In order to better classify these soils, physicochemical parameters, such as pHH2O, exchangeable cations, and particle size distributions, were measured for a specific set of soils (n = 16), representative of their diversity. To assess Soil Organic Matter (SOM) dynamics, samples were studied using Rock Eval pyrolysis. Results show a high soil diversity and heterogeneity with the presence of (i) Arenosols, as expected, but also of (ii) organic-rich soils, such as Chernozems, Phaeozems, and Kastanozems, (iii) salty/sodic soils, such as Solonchaks and Solonetz, and finally (iv) calcium-rich soils, such as Calcisols. Analyses of the different actors driving the soil diversity emphasized the importance of the surficial geology, composed of different sand deposits (red sands/white sands), carbonate and diatomite beds, as well as ancient salt deposits, in which high proportions of exchangeable Na⁺ were found, associated with high pHH2O (up to 11.3). In addition, as a parameter, the topography creates a complex hydrological system in the Chobe Enclave and therefore, induces a notable soil moisture gradient. Moreover, this study stressed the key role of termites: not only do they modify physicochemical patterns of soils, but they also decay and incorporate large quantities of fresh plant materials into soils. Finally, the analysis of Organic Matter (OM) showed that the Soil Organic Carbon (SOC) is composed essentially by recalcitrant Organic Carbon (OC) substances, such as charcoal, a common carbon type of tropical soils.
... One promising analytical technique in SOM research is Rock-Eval® thermal analysis. A time-efficient and inexpensive method, it can be used on large sample sets to quantify soil organic and inorganic carbon and characterize SOC bulk chemistry and thermal stability (Gregorich et al., 2015;Saenger et al., 2013;Sebag et al., 2016;Soucémarianadin et al., 2018a). Moreover, a strong empirical link exists between parameters obtained with this method and in situ observed SOM biogeochemical stability (Barré et al., 2016;Poeplau et al., 2019). ...
... Simultaneous and continuous detection of effluents generates five thermograms in total that describe the evolution of carbon containing gases (HC, CO and CO 2 ) during the analysis. A large number of Rock-Eval® parameters can be calculated from the five thermograms (Behar et al., 2001;Saenger et al., 2013;Sebag et al., 2016;Cécillon et al., 2018Cécillon et al., , 2021Khedim et al., 2021). Parameters obtained with this method are characteristic of the SOM and its interaction with the soil mineral matrix, since soil samples are analysed with no previous isolation of SOM or removal of carbonates. ...
... Finally, we calculated three additional parameters proposed in previous soil studies using the Rock-Eval® technique. These were: The I-index (assessing the preservation of thermally labile "immature" hydrocarbons; Sebag et al., 2016), the R-index (describing the proportion of refractory SOM released as hydrocarbons after 400 • C; Sebag et al., 2016), and the thermolabile hydrocarbon index (TLHC-index corresponding to the proportion of hydrocarbons evolved between 200 • C and 450 • C; Saenger et al., 2015). A detailed description of the definition, units and equations used to calculate all parameters can be found in Supplementary Table 1. ...
Article
Soil sampling depths strongly vary across soil studies. Stocks of elements (such as C, N) or organic matter in a soil layer can be simply calculated from stocks measured in its sublayers. This calculation is less obvious for other soil characteristics, such as soil organic carbon (SOC) persistence, complicating the comparison of results from different studies. Here, we tested whether Rock-Eval® parameters of a soil layer, characterizing soil organic matter and its biogeochemical stability, can be determined using Rock-Eval® data measured on its sublayers. Soil samples collected in 10 plots located in eight French forest sites, taken up at two different depths (0–30 cm, 30–50 cm), and their mixtures were analysed with Rock-Eval®. Expected values for the Rock-Eval® parameters of the soil mixtures were calculated either: (1) as the weighted mean of Rock-Eval® parameters measured on the two sublayers, or (2) based on a signal reconstructed as the weighted mean of Rock-Eval® thermograms recorded on the two sublayers. Our results showed a good agreement between measured and expected Rock-Eval® parameter values. However, when the clay content strongly differed between the two soil sublayers, the amount of pyrolyzed hydrocarbons measured on the soil mixtures was slightly lower than expected. We conclude that it is reasonable to calculate Rock-Eval® parameters of a soil layer, from the Rock-Eval® signature of its sublayers. Our findings facilitate the harmonization of Rock-Eval® data from large scale soil studies using different sampling depths.
... In addition to the previously mentioned indices typifying OM sources, two S2-derived indices; the I-and R-indices (Disnar et al., 2003;Sebag et al., 2006Sebag et al., , 2016Marchand et al., 2008;Albrecht et al., 2014), have been used to qualitatively characterize OM degradation. The S2 thermograms, which measure the amount of pyrolysed HC, are normalized to 100% and divided into 5 temperature ranges, from the most thermally labile to the most thermally refractory organic fraction (A1: 280-340°C, A2: 341-400°C, A3: 401-460°C, A4: 461-550°C, A5: 551-650°C). ...
... In the I-R diagram (Fig. 7B), the whole dataset indicates a strong correlation between these two indices. Following Sebag et al. (2016), this correlation indicates that the thermal stability of sedimentary OM is controlled by degradation processes. Indeed, an OM mixture from different sources would generate poorly related I-R indices (Sebag et al., 2016). ...
... Following Sebag et al. (2016), this correlation indicates that the thermal stability of sedimentary OM is controlled by degradation processes. Indeed, an OM mixture from different sources would generate poorly related I-R indices (Sebag et al., 2016). Towuti water temperatures ranging between 28 and 30°C (Costa et al., 2015) likely enhance degradation, which is further facilitated by methanogenesis, the most important OM mineralization pathway (444 ± 172 mmol m À2 yr À1 of organic carbon) after burial in Towuti sediments Vuillemin et al., 2018). ...
Article
Full-text available
Qualitative and quantitative changes of organic and carbonate carbon in sedimentary records are frequently used to reconstruct past environments, paleoproductivity and sediment provenance. Amongst the most commonly used proxies are Total organic carbon (TOC), Mineral carbon (MinC), as well as Hydrogen (HI) and Oxygen Indices (OI) of organic matter (OM). Rock Eval pyrolysis enables the assessment of these quantitative and qualitative parameters with a single analysis. This is achieved through transient pyrolysis of the samples up to 650°C followed by combustion up to 850°C, with hydrocarbons, CO and CO2 measured during the thermal decomposition of both OM and carbonate minerals. Carbonate minerals with low thermal cracking temperatures such as siderite (<400°C) can induce significant matrix effects, which bias the TOC, MinC and OI Rock-Eval parameters. Here we assess the applicability of End-Member Analysis (EMA) as a means to correct Rock-Eval thermograms for siderite matrix effects. For this, we performed Rock-Eval pyrolysis on sideritic sediments of Lake Towuti (Indonesia). New thermal boundaries were constrained in Rock-Eval thermograms using EMA to limit siderite matrix effects and improve TOC, MinC, and OI calculations. Our approach allowed us to: 1) evaluate the influence of siderite matrix effects on Rock-Eval thermograms; 2) properly exploit a Rock-Eval dataset to characterize the type and sources of OM in siderite-rich sediments; and 3) identify the OM behind degradation and mineralization processes. The Rock-Eval dataset revealed sediments with a substantial amount of refractory OM, especially in those where TOC is high and HI characteristic of autochthonous biomass. These results, associated to alternative indices used to assess OM preservation, suggest that refractory OM is residually enriched following strong degradation of labile compounds. Finally, relatively labile and refractory organic fractions may be consumed in the formation of siderite during this sequential process of OM mineralization.
... Among thermal analyses used to characterize SOM, Rock-Eval (RE) analysis has provided promising results showing that SOC thermal stability observed in RE thermograms results can be related to SOC biogeochemical stability (Gregorich et al., 2015;Saenger et al., 2015;Barré et al., 2016;Soucémarianadin et al., 2018a;Poeplau et al., 2019). RE was also shown to provide information on organic matter evolutions in soils at various depth or during composting (Hetényi et al., 2005;Hetényi et al., 2006;Sebag et al., 2006;Albrecht et al., 2015), and it has been specifically applied to characterize litters, bulk soil, organic layers and POM fractions in mineral soils (Disnar et al., 2003;Saenger et al., 2015;Sebag et al., 2016). ...
... The ether + alcohol and carbonyl + carboxyl ratios are considered as representative of labile C compounds (Baldock et al., 1992;Sarkhot et al., 2007;Ng et al., 2014) and their observed decrease with depth could correspond to the decomposition of POM from the surface to the deep layer (Baldock et al., 1997). Similarly, with depth, HI decreased while T 50_CO2_PYR increased: these opposite trends match what is generally observed for bulk soil OC and organic material (Disnar et al., 2003;Sebag et al., 2016;Soucémarianadin et al., 2018a). These evolutions have been linked to an increased decomposition and a decrease in labile C compounds, resulting in a more thermally and biogeochemically stable OC (Sebag et al., 2006;Albrecht et al., 2015;Barré et al., 2016;Cécillon et al., 2018). ...
... Free POM (light fraction) turnover has been shown to increase by an order of magnitude from the 0-5 cm layer (turnover < 10 years) to the 10-20 cm layer (turnover ≈ 100 years) (Schrumpf and Kaiser, 2015). In the mineral soil, an increased SOC decomposition and turnover (Balesdent et al., 2018;Cécillon et al., 2018) with depth, time since bare-fallow or increase in soil temperature has been linked to a decrease in HI (Barré et al., 2016;Sebag et al., 2016;Soucémarianadin et al., 2018b;Poeplau et al., 2019). Similarly to SOC, it would be expected that POM with lower HI is less energy-rich and would require more energy to further break down (Barré et al., 2016), resulting in a greater thermal and biogeochemical stability. ...
Article
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In temperate forests, soils contain a significant part of the ecosystem carbon (C) stock that can be subjected to C losses upon global changes. In forest soils, particulate organic matter (POM) is a major contributor to the labile C pool and its dynamics can significantly influence the overall total soil organic carbon stock. However, POM has been overlooked in forest soils, specifically in deep horizons. We isolated the POM fraction of mineral soil samples collected in 52 French forest sites, using a size-(> 50 μm) and density-(< 1.6 g·cm −3) fractionation scheme. These soil samples presented variability in terms of depth (0-10 cm; 40-80 cm), soil class (dystric Cambisol, eutric Cambisol, entic Podzol) and vegetation type (deciduous, coniferous). First, we determined the POM chemical composition and thermal stability using elemental analysis, mid infrared-attenuated total reflectance spectroscopy and Rock-Eval thermal analysis. Then, we assessed how depth, soil class and vegetation type influenced POM chemistry and thermal stability in these temperate forest soils. Depth, soil class and vegetation type were all important factors influencing POM chemistry and thermal stability. Variations in POM chemistry (higher C/N ratio, lower ether + alcohol and carbonyl + carboxyl ratios and decrease in hydrogen-rich compounds) and increase in thermal stability with depth suggested different POM input sources for the surface and deep soil layers and an increased biogeochemical stability of POM in deep soil layers. Whatever the vegetation, POM in eutric Cambisols had lower aliphatic and higher aromatic ratios than POM in dystric Cambisols. POM in soils under deciduous trees had higher aliphatic and carbonyl + carboxyl ratios and lower aromatic ratio, more hydrogen-rich and less oxygen-rich compounds than POM in soils under coniferous trees, reflecting the difference in litter chemistry between the two vegetation types. POM from deciduous plots was also significantly more thermally stable than from coniferous plots, suggesting a higher bio-geochemical stability for POM in deciduous forest soils. This study highlights the variations in POM chemistry and thermal stability existing within and among soil profiles and the role of depth, soil class and vegetation type in these variations. It appears that if POM can be regarded as a labile carbon fraction in soils, its lability varies depending on the ecosystem (soil, vegetation) and depth considered.
... Thus, OM that resisted enzymatic decay over decades was characterized by a higher RE thermal stability. Several RE indices have been developed, which can be linked to chemical SOM properties (Disnar et al., 2003) or SOC dynamics (Sebag et al., 2016). It has been concluded that the RE method is sensitive to changes associated with SOC biogeochemical processes and could have the potential for landscape-scale applications due to its time-and cost-effectiveness (Saenger et al., 2013). ...
... In addition to these 25 temperature parameters, we derived two standard RE indices (hydrogen index (HI) and oxygen index (OI RE6 )) that have been shown to reflect bulk SOM chemistry (Espitalié et al., 1977;Lafargue et al., 1998). We also derived three further indices that reflect the thermostability of hydrocarbons (thermolabile hydrocarbon index (TLHC), I-Index, emphasizing the transformation of the immature organic matter fraction, and R-Index, highlighting the contribution of more refractory organic matter) (Sebag et al., 2016) ...
... The observations thus indicate that stabilized and transformed SOC is depleted in hydrogen and relatively enriched in oxygen, i.e., in a high oxidation state. This is also in line with findings that HI decreases and OI RE6 increases with soil depth (Sebag et al., 2016) and that there are significant positive (HI) and negative (OI RE6 ) correlations with the proportion of particulate organic matter in forest samples (Soucémarianadin et al., 2018). Furthermore, it has been shown that the energy density of organic matter declines significantly with bare fallow duration (Barré et al., 2016), which can be explained by the high energy content of CeH bonds compared with compounds of higher nominal carbon oxidation states (LaRowe and Van Cappellen, 2011). ...
Article
Soil warming can increase soil organic carbon (SOC) mineralization, triggering a positive climate‑carbon cycle feedback loop. Globally, many soil warming experiments have examined losses of bulk SOC, but few have assessed changes in quality. Accurate knowledge of the latter is required for an in-depth understanding and improved prediction of SOC feedback to climate change. In this study, we used Rock-Eval thermal analysis (RE6) to characterize shifts in SOC thermal stability and bulk chemistry after six years of geothermal warming by 0.6 °C, 1.8 °C, 3.9 °C, 9.9 °C, 16.3 °C, 40 °C, and 80 °C in an Icelandic grassland topsoil (0–10 cm). We also used the strong warming-induced depletion of SOC (up to 92% in the 80 °C soil) in comparisons of chemical oxidation-resistant and biogeochemically resistant SOC, which are generally assumed to be similar in nature. Sodium hypochlorite (NaOCl) and hydrogen peroxide (H2O2) were used for oxidation. Warming-resistant SOC was strongly depleted in hydrocarbons and enriched in oxygen, confirming that SOC oxidation state, and thus energy content, is an important driver for biogeochemical stability. This was supported by findings that thermal stability, i.e., the amount of energy (temperature) necessary to pyrolyze or oxidize SOC, strongly increased with warming intensity. Of the 31 RE6 parameters tested, the most warming-sensitive were hydrogen index (HI, ρ = −0.84), oxygen index (OIRE6, ρ = 0.83), proportion of total pyrolyzed carbon released as hydrocarbons at 200–650 °C (S2/PC, ρ = −0.86), and the temperature at which a certain proportion of CO2 evolved during pyrolysis (ρ > 0.8). Chemical oxidation of unwarmed soil caused average relative SOC losses of 61% (NaOCl) and 91% (H2O2) and shifts in RE6 properties that differed strongly from warming-induced shifts at comparable SOC losses. Chemical oxidation-resistant SOC was more enriched in oxygen, but slightly enriched in hydrocarbons, and less thermostable than comparable naturally depleted SOC at the same time. A certain overlap, especially for NaOCl-treated soils, is likely, while H2O2-oxidized soils showed very distinct RE6 properties. We concluded that i) soil warming leads to strong shifts in SOC bulk chemistry and thermal stability and ii) H2O2 should be avoided in isolation of a slow SOC kinetic pool.
... bacterial community structure, bacterial diversity, fungi presence, and enzymatic activity influenced microbial community carbon use efficiency [6]. Here, we analyzed the formed SOM after four months of growth on cellobiose, using a method commonly used to quantify thermal stability and gradual stabilization of SOM [10]. The hydrocarbon compounds released at each temperature for each sample during the pyrolytic phase of Rock-Eval ® was used to calculate the Bray-Curtis-based chemical dissimilarity of the soil samples as a proxy for soil C composition, and the and the Rock-Eval ® thermal stability index (R-index) was calculated as a proxy for C persistence, as previously [10]. ...
... Here, we analyzed the formed SOM after four months of growth on cellobiose, using a method commonly used to quantify thermal stability and gradual stabilization of SOM [10]. The hydrocarbon compounds released at each temperature for each sample during the pyrolytic phase of Rock-Eval ® was used to calculate the Bray-Curtis-based chemical dissimilarity of the soil samples as a proxy for soil C composition, and the and the Rock-Eval ® thermal stability index (R-index) was calculated as a proxy for C persistence, as previously [10]. Bacterial or fungal diversity did not drive SOM composition. ...
... Moreover, the ordination first axis was strongly correlated with the Rock-Eval ® R-index (ρ = −0.95, P < 0.0001) which quantifies the relative contribution of thermally stable compounds [10] (i.e., compounds that require higher activation energy for thermal-decomposition). Thus, this suggests that distinct microbial communities produced SOM with different degrees of thermal stability. ...
Article
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The largest terrestrial carbon sink on earth is soil carbon stocks. As the climate changes, the rate at which the Earth’s climate warms depends in part on the persistence of soil organic carbon. Microbial turnover forms the backbone of soil organic matter (SOM) formation and it has been recently proposed that SOM molecular complexity is a key driver of stability. Despite this, the links between microbial diversity, chemical complexity and biogeochemical nature of SOM remain missing. Here we tested the hypotheses that distinct microbial communities shape the composition of SOM, and microbial-derived SOM has distinct decomposition potential depending on its community of origin. We inoculated microbial communities of varying diversities into a model soil matrix amended with simple carbon (cellobiose) and measured the thermal stability of the resultant SOM. Using a Rock-Eval ® ramped thermal analysis, we found that microbial community composition drives the chemical fingerprint of soil carbon. While diversity was not a driver of SOM composition, bacteria-only communities lead to more thermally labile soil C pools than communities with bacteria and fungi. Our results provide direct evidence for a link between microbial community structure, SOM composition, and thermal stability. This evidence demonstrates the relevance of soil microorganisms in building persistent SOM stocks.
... Total Organic Carbon (TOC), Mineral Carbon (MINC), Hydrogen (HI) and Oxygen (OI) Indices were calculated by integrating the amounts of Hydrocarbon Compounds (HC), CO, and CO 2 produced during thermal cracking of OM and oxidative decomposition of carbonate, between defined temperature limits (Lafargue et al., 1998;Behar et al., 2001). The I-index and R-index were computed according to Sebag et al. (2016). By construction, the R-index relates to the most thermally resistant and refractory pools of organic matter, while the I-index is related to the ratio between thermally labile and thermally resistant pools (see details in Sebag et al., 2016). ...
... The I-index and R-index were computed according to Sebag et al. (2016). By construction, the R-index relates to the most thermally resistant and refractory pools of organic matter, while the I-index is related to the ratio between thermally labile and thermally resistant pools (see details in Sebag et al., 2016). As they arise from a mathematical composition, these two indexes may be inversely correlated to each other when OM stabilization results from progressive decomposition of organic components according to their biogeochemical stability. ...
... As they arise from a mathematical composition, these two indexes may be inversely correlated to each other when OM stabilization results from progressive decomposition of organic components according to their biogeochemical stability. Previous studies show that this property is verified with both indices highly correlated along a constant line ("humic trend") in composts and undisturbed soil profiles (Albrecht et al., 2015;Matteodo et al., 2018;Schomburg et al., 2018Schomburg et al., , 2019Sebag et al., 2016). For comparison, we used the Matteodo's dataset composed of 46 soil profiles selected across various ecounits in Swiss Alps (Matteodo et al., 2018). ...
Article
Complex sequences of paleosols are often formed by the interaction between pedogenesis and geomorphological evolution. Their study, particularly in mountain areas, is useful to reconstruct past environmental conditions as well as climate shifts, and to gather information on the morphodynamical processes affecting the landscape through time. Since the combined role that all different factors can play in the soil formation and evolution through time and space influences the formation and evolution of those complex paleosol sequences, a multidisciplinary study was conducted at the NW slope of Mt. Cusna (Northern Apennines, Italy). This work aims to reconstruct and to evaluate how the interactions between the geomorphological context, the Holocene climate variations, and the modification of the vegetation cover and composition influence the soil development of this area. A combination of routine soil analyses (i.e., grain-size distributions, total organic carbon, total nitrogen, pH, and Fe/Al extractions), soil micromorphology and the Rock-Eval® pyrolysis allowed to characterize and to correlate the different soil units constituting a toposequence of six soil profiles. The presence of different pedological units that can be correlated along the slope underlines the occurrence of separate events of pedogenesis, spatio-temporally linked to recognizable stability phases at slope scale. These phases of biostasy, characterized by vegetation cover and soil development, alternate to phases of rhexistasy, characterized mainly by slope instability (i.e., aggradation/degradation). In detail, in the Mt. Cusna toposequence three different soil units, linked to three different stability phases, have been identified: the earliest stability phase, characterized by the presence of well-developed Luvisols, the subsequent stability phase typified by less expressed Luvisols, and the ongoing stability phase with Leptosols. This latter pedogenetic phase, in some cases, is superimposed to the previous one, so affecting the exhumed paleosols. In this light, the Mt. Cusna toposequence characterization allowed to enlighten the complexity of soil polygenesis in higher detail than the previous studies, not only reconstructing the past environmental conditions but also inferring the succession of phases of slope stability and phases characterized by erosion and deposition processes.
... Pyrolysis techniques appear better suited to assess biological stability, as persistent OM tends to disintegrate at higher temperatures than labile OM (Barré et al., 2016). The Rock-Eval pyrolysis technique is now widely employed for routine analysis of OM in soil samples (see Sebag et al., 2016 for a review). This method quantifies total organic and inorganic C contents of a sample (either soil or litter) and provides a wide range of parameters that can be used to evaluate OM composition and its thermal stability. ...
... The shape of this pyrogram is sample-specific and is indicative of the thermal stability of organic molecules in the sample. The area under the S2 pyrogram was subdivided into four sections (A1, A2, A3 and A4) using temperature cut-offs frequently used in the literature (Sebag et al., 2016): 200-340°C for A1, 340-400°C for A2, 400-460°C for A3 and 460-650°C for A4. Thermally labile organic molecules release high quantities of HC during the early stage of the pyrolysis process (i.e. ...
... large A3 and A4 areas). On this basis, the thermal stability of each sample was represented by two indices previously proposed by Sebag et al. (2016): the R-Index representing thermally refractory OM [R = (A3 + A4) / (A1 + A2 + A3 + A4)], and the I-Index representing thermally labile OM [I = log 10 (A1 + A2) / (A3)]. These two indices are negatively correlated and only the R-Index was retained as an indicator of OM thermal stability in the present study (Suppl. ...
Article
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Our understanding of mechanisms governing soil organic matter (OM) stability is evolving. It is gradually becoming accepted that soil OM stability is not primarily regulated by the molecular structure of plant inputs, but instead by the biotic and abiotic properties of the edaphic environment. Moreover, several experimental studies conducted in artificial systems have suggested that mechanisms regulating OM stability may differ with depth in the soil profile. Up to now however, there is very limited field-scale evidence regarding the hierarchy of controls on soil OM dynamics and their changes with soil depth. In this study, we take advantage of the high heterogeneity of ecological conditions occurring in the alpine belt to identify the major determinants of OM dynamics and how their significance varies with depth in the soil profile. Aboveground litter, mineral topsoil, and subsoil samples originating from 46 soil profiles spanning a wide range of soil and vegetation types were analysed. We used Rock-Eval pyrolysis, a technique that investigates the thermal stability of OM, as an indicator of OM dynamics. Our results show a clear divergence in predictors of OM thermal stability in the litter, topsoil, and subsoil layers. The composition of OM correlated with its thermal stability in the litter layer but not in mineral soil horizons, where the supply rate of fresh organic material and the physical and chemical characteristics of the pedogenic environment appeared important instead. This study offers direct confirmation that soil OM dynamics are influenced by different ecosystem properties in each soil layer. This has important implications for our understanding of carbon cycling in soils under a changing climate.
... As a basis for comparing the studied Arenosols to other situations, we used data published by Sebag et al. (2016) on Ferralsols (≤0−15 cm deep) from profiles of soils in Gabonese forests and savannahs that had not been disturbed by human activities. The Rock-Eval® signatures of the selected 87 samples, corresponding to organic layers (OL, OF, OH and Op, n = 25) and organo-mineral layers (A, Ah; n = 62), are representative of the diverse undisturbed situations reported in Sebag et al.'s (2016) worldwide dataset. ...
... The Rock-Eval® signatures of the selected 87 samples, corresponding to organic layers (OL, OF, OH and Op, n = 25) and organo-mineral layers (A, Ah; n = 62), are representative of the diverse undisturbed situations reported in Sebag et al.'s (2016) worldwide dataset. The signatures of these 87 samples were referred to as the "Humic trend" by Sebag et al. (2016), as described in more details in the following section. ...
... In this study, thermal status of SOM was characterized by combining two indexes (denoted R and I) calculated from five subdivided areas of the S2 thermograms related to HC (Disnar et al., 2003;Sebag et al., 2006Sebag et al., , 2016. Areas were calculated between the following bounds (or nodes): 200−340°C for A1, 340−400°C for A2, 400−460°C for A3, 460−520°C for A4, and 520−650°C for A5. ...
... This technique has been recommended for characterizing soil organic matter (Derenne and Quénéa, 2015;Disnar et al., 2003). In this study, soil organic matter dynamics was analyzed through the combination of two indices (I and R) calculated from five areas of the hydrocarbon thermograms, according to predefined temperature thresholds (Disnar et al., 2003;Sebag et al., 2016Sebag et al., , 2006. By construction, the R-index is related to the most thermally refractory fraction of organic matter, while the I-index is related to the most thermally labile one (see Sebag et al. (2016) for details). ...
... In this study, soil organic matter dynamics was analyzed through the combination of two indices (I and R) calculated from five areas of the hydrocarbon thermograms, according to predefined temperature thresholds (Disnar et al., 2003;Sebag et al., 2016Sebag et al., , 2006. By construction, the R-index is related to the most thermally refractory fraction of organic matter, while the I-index is related to the most thermally labile one (see Sebag et al. (2016) for details). Previous studies showed both indices are highly correlated along a constant line ("humic trend" in Sebag et al., 2016) reflecting a direct relation between mineralization of labile organic fractions and SOC stabilization processes in undisturbed soil profiles. ...
... By construction, the R-index is related to the most thermally refractory fraction of organic matter, while the I-index is related to the most thermally labile one (see Sebag et al. (2016) for details). Previous studies showed both indices are highly correlated along a constant line ("humic trend" in Sebag et al., 2016) reflecting a direct relation between mineralization of labile organic fractions and SOC stabilization processes in undisturbed soil profiles. Raw data are presented in Supplementary readings 3 & 4. In this study we focused on the delta-I as the distance (or residuals) from the "humic trend" (delta-I = I measured − I model ), which is considered as representing stabilization processes in undisturbed soil profiles. ...
Article
The assessment of the impacts of land-use and management on soil organic carbon (SOC) dynamics is a major environmental concern, as the soil carbon cycle underpins key ecosystem services. However, assessments based on short-term SOC dynamics face methodological and experimental difficulties. Hurisso et al. (2016) proposed a method to assess SOC dynamics by coupling two methods: Permanganate Oxidizable Carbon (POXC) and Basal Soil Respiration (BSR). This method has been used in laboratory on dried and re-wetted soil samples from temperate regions mainly. In our study, we adapted this method to the field and proposed a cost-effective in-field indicator combining the POXC and in situ Basal Soil Respiration (SituResp® method). We tested the indicator at four study sites (n = 169 points) within various tropical land-use and management contexts based on rubber, soybean and oil palm cropping systems respectively in Thailand, Cambodia and Indonesia. The results demonstrated the relevance, sensitivity and robustness of the POXC-SituResp® indicator to characterize the impact of a gradient of disturbance on SOC dynamics. The results also highlighted the potential of conservation agriculture (no-tillage and crops residues) and compost amendments to accumulate SOC. Rock-Eval® analysis showed that POXC-SituResp® indicator is negatively linked to excess of potentially mineralizable labile carbon. Carbon pools targeted by the POXC were specified by Rock-Eval® pyrolysis measurements to be a rather thermal resistant pool of SOC. Our study confirms that the integrated indicator based on POXC and BSR assess a relative carbon stabilization of SOC pools. This indicator can be measured in the field by a rapid and cost-effective method.
... The total OC of earthworm casts and control soil aggregates was determined using the Rock Eval (RE6) signals. The RE6 R-and I-index were calculated following Sebag et al. (2016). The Rindex corresponds to the relative contribution of thermally stable OM with refractory compounds released at higher temperatures; while the Iindex provides information on the pool of thermally labile and immature OM with compounds released at lower temperatures (Cécillon et al., 2018;Sebag et al., 2016). ...
... The RE6 R-and I-index were calculated following Sebag et al. (2016). The Rindex corresponds to the relative contribution of thermally stable OM with refractory compounds released at higher temperatures; while the Iindex provides information on the pool of thermally labile and immature OM with compounds released at lower temperatures (Cécillon et al., 2018;Sebag et al., 2016). ...
... The explicative variables used in the predictive model and their respective contributions are presented on the right part of the chart. all earthworm species exhibited higher OC content and higher I indices, which could indicate a higher contribution of thermally labile compounds Sebag et al., 2016). This is in agreement with previous studies indicating that earthworm casts are characterized by increased OC contents and higher contribution of fresh OM as compared to the surrounding soil aggregates Guggenberger et al., 1995;Le Mer et al., 2021;Lee, 1985). ...
Article
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The role of earthworms on biogeochemical carbon cycling is a major knowledge gap resulting from the difficulty of isolating and exploring the effects provided by the diversity of organisms. In this study, we investigated the effect of six earthworm species belonging to three ecological categories on soil organic carbon (SOC) mineralization. To this end, we produced casts in microcosms with the six species in the same soil and with the same litter material. The casts were subjected to laboratory ageing for 140 days. During this process, we monitored physicochemical parameters, CO2 emissions and determined the micro-scale organization of the casts’ particulate organic matter and pores using X-ray microtomography. Our results showed contrasting properties of fresh casts of the three ecological categories, in accordance with the earthworm species’ morphological or behavioral strategies, indicating that those were maintained in artificial environments. However, species-specific changes in cast properties throughout ageing increased intragroup variability among ecological categories. As a result we observed earthworm species-specific evolution of CO2 mineralization rates during casts ageing. We found that at least half of the variability in CO2 emissions was explained by cast microstructural changes, related to the spatial arrangement between particulate organic matter, porosity, and mineral particles. We conclude that earthworm species-specific traits may play a role in organic carbon protection through their impact on microstructural cast properties.
... bacterial community structure, bacterial diversity, fungi presence, and enzymatic activity influenced microbial community carbon use efficiency [6]. Here, we analyzed the formed SOM after four months of growth on cellobiose, using a method commonly used to quantify thermal stability and gradual stabilization of SOM [10]. The hydrocarbon compounds released at each temperature for each sample during the pyrolytic phase of Rock-Eval ® was used to calculate the Bray-Curtis-based chemical dissimilarity of the soil samples as a proxy for soil C composition, and the and the Rock-Eval ® thermal stability index (R-index) was calculated as a proxy for C persistence, as previously [10]. ...
... Here, we analyzed the formed SOM after four months of growth on cellobiose, using a method commonly used to quantify thermal stability and gradual stabilization of SOM [10]. The hydrocarbon compounds released at each temperature for each sample during the pyrolytic phase of Rock-Eval ® was used to calculate the Bray-Curtis-based chemical dissimilarity of the soil samples as a proxy for soil C composition, and the and the Rock-Eval ® thermal stability index (R-index) was calculated as a proxy for C persistence, as previously [10]. Bacterial or fungal diversity did not drive SOM composition. ...
... Moreover, the ordination first axis was strongly correlated with the Rock-Eval ® R-index (ρ = −0.95, P < 0.0001) which quantifies the relative contribution of thermally stable compounds [10] (i.e., compounds that require higher activation energy for thermal-decomposition). Thus, this suggests that distinct microbial communities produced SOM with different degrees of thermal stability. ...
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The largest terrestrial carbon sink on earth is soil carbon stocks. As the climate changes, the rate at which the Earth’s climate warms depends in part on the persistence of soil organic carbon. Microbial turnover forms the backbone of soil organic matter (SOM) formation and it has been recently proposed that SOM molecular complexity is a key driver of stability. Despite this, the links between microbial diversity, chemical complexity and biogeochemical nature of soil organic matter remain missing. Here we used a model soil system to test the hypothesis that more diverse microbial communities generate more stable soil organic matter. We inoculated microbial communities of varying diversities into an model soil matrix amended with simple carbon, and measured the thermal stability of the resultant soil organic matter. Using a novel data analysis approach with Rock-Eval ® ramped thermal analysis, we found that microbial community diversity drives the chemical fingerprint of soil organic matter. Bacteria-only and low diversity communities lead to less chemically-diverse and more thermally-labile soil carbon pools than highly diverse communities. Our results provide direct evidence for a link between microbial diversity, molecular complexity and SOM stability. This evidence demonstrates the benefits of managing soils for maximum biological diversity as a means of building persistent SOM stocks. Classification Biological Sciences: Ecology
... For the HC pyrolysis thermogram we also determined three parameters reflecting a proportion of thermally resistant or labile hydrocarbons: a parameter representing the proportion of hydrocarbons evolved between 200 °C and 450 °C (thermo-labile 20 hydrocarbons, TLHC-index, modified from Saenger et al., 2015), the I-index representing the preservation of thermally labile immature hydrocarbons (after Sebag et al., 2016), and the R-index representing the proportion of hydrocarbons thermally stable at 400°C (after Sebag et al., 2016). Those three RE6 parameters were calculated as follows: ...
... For the HC pyrolysis thermogram we also determined three parameters reflecting a proportion of thermally resistant or labile hydrocarbons: a parameter representing the proportion of hydrocarbons evolved between 200 °C and 450 °C (thermo-labile 20 hydrocarbons, TLHC-index, modified from Saenger et al., 2015), the I-index representing the preservation of thermally labile immature hydrocarbons (after Sebag et al., 2016), and the R-index representing the proportion of hydrocarbons thermally stable at 400°C (after Sebag et al., 2016). Those three RE6 parameters were calculated as follows: ...
... Despite the fact that three RE6 parameters used here, i.e. the TLHC-index, the I-index, and the R-index, had originally been proposed as useful qualitative metrics of soil carbon dynamics, reflecting a proportion of thermally resistant or labile hydrocarbons (Disnar et al., 2003;Sebag et al., 2006;Saenger et al., 2013Saenger et al., , 2015Sebag et al., 2016), those parameters were weakly correlated (TLHC-index) or not correlated (I-index, R-index) to the CP SOC proportion. Furthermore, they also had a 15 weak importance in the random forests model predictions of the CP SOC proportion ( Table 2). ...
Article
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Changes in global soil carbon stocks have considerable potential to influence the course of future climate change. However, a portion of soil organic carbon (SOC) has a very long residence time (> 100 years) and may not contribute significantly to terrestrial greenhouse gas emissions during the next century. The size of this persistent SOC reservoir is presumed to be large. Consequently, it is a key parameter required for the initialization of SOC dynamics in ecosystem and Earth system models, but there is considerable uncertainty in the methods used to quantify it. Thermal analysis methods provide cost-effective information on SOC thermal stability that has been shown to be qualitatively related to SOC biogeochemical stability. The objective of this work was to build the first quantitative thermal analysis based model of the size of the centennially persistent SOC pool. We used a unique set of soil samples from four agronomic experiments in Northwestern Europe with long-term bare fallow and non-bare fallow treatments (e.g. manure amendment, cropland and grassland), as a sample set for which estimating the size of the centennially persistent SOC pool is relatively straightforward. At each experimental site, we estimated the average concentration of centennially persistent SOC and its uncertainty by applying a Bayesian curve fitting method on the observed declining SOC concentration over the duration of the long-term bare fallow treatment. Overall, the estimated concentrations of centennially persistent SOC ranged from 5 to 11 gC.kg−1 soil (lowest and highest boundaries of four 95 % confidence intervals). Then, by dividing site-specific concentrations of persistent SOC by the total SOC concentration of 118 archived soil samples from long-term bare fallow and non-bare fallow treatments, we could estimate the proportion of centennially persistent SOC in the samples and the associated uncertainty. The proportion of centennially persistent SOC ranged from 0.14 (standard deviation of 0.01) to 1 (standard deviation of 0.15). Samples were subjected to thermal analysis by Rock-Eval 6 that generated a series of 30 parameters reflecting their SOC thermal stability and bulk chemistry. The sample set was split into a calibration set (n = 88) and a validation set (n = 30). We trained a non-parametric machine learning algorithm (random forests multivariate regression model) that accurately predicted the size of the centennially persistent SOC pool using Rock-Eval 6 thermal parameters as predictors in the calibration set (pseudo-R² = 0.91, RMSEC = 0.06) and the validation set (R² = 0.91, RMSEP = 0.07). The uncertainty of the predictions obtained using the multivariate regression model was quantified by a Monte Carlo approach that produced conservative 95% prediction intervals across the 30 samples of the validation set. This model based on Rock-Eval 6 thermal analysis can thus be used to predict the proportion of centennially persistent SOC with known uncertainty in new soil samples from similar pedoclimates. Our study strengthens the evidence for a link between the thermal and biogeochemical stability of soil organic matter, and demonstrates that Rock-Eval 6 thermal analysis can be used to quantify the size of the centennially persistent organic carbon pool in temperate soils.
... Four palaeoenvironmental proxies in core indicate that the Ghost Interval consists of highly decomposed peat; that is, more decomposed than during earlier or later periods, consistent with contemporaneous (scenario 2) and/or secondary decomposition (scenario 3). First, I-index values from Rock-Eval pyrolysis, which describe the balance between thermally labile and resistant pools of organic matter [8][9][10] , indicate intense decomposition of the peat in the Ghost Interval and predominantly better preservation elsewhere in the peat column (Fig. 2b, Methods and Extended Data Fig. 1). ...
... Second, within the Ghost Interval peat, the lowest I-index values are associated with the highest total organic carbon (TOC) values (of up to approximately 65%, Fig. 2c). These values are similar to those measured in lignite 9 (Extended Data Fig. 1a) suggesting that the peat within the Ghost Interval consists of highly condensed, refractory organic matter. ...
... Two indices (R-index and I-index) represent the thermal status of organic matter. They are calculated from five subdivided areas of the S2 thermograms (Extended Data Fig. 1c) between the following bounds 8,9 : 200-340 °C for highly labile (A1), 340-400 °C for labile (A2), 400-460 °C for resistant (A3), 460-520 °C for refractory (A4) and 520-650 °C for highly refractory pool (A5). The R-index ((A3 + A4 + A5)/100) relates to the thermally resistant and refractory pools of organic matter, and the I-index (log 10 [(A1 + A2)/A3]) relates to the ratio between the thermally labile and resistant pools 9 (Extended Data Fig. 1b). ...
Article
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The forested swamps of the central Congo Basin store approximately 30 billion metric tonnes of carbon in peat1,2. Little is known about the vulnerability of these carbon stocks. Here we investigate this vulnerability using peat cores from a large interfluvial basin in the Republic of the Congo and palaeoenvironmental methods. We find that peat accumulation began at least at 17,500 calibrated years before present (cal. yr bp; taken as ad 1950). Our data show that the peat that accumulated between around 7,500 to around 2,000 cal. yr bp is much more decomposed compared with older and younger peat. Hydrogen isotopes of plant waxes indicate a drying trend, starting at approximately 5,000 cal. yr bp and culminating at approximately 2,000 cal. yr bp, coeval with a decline in dominant swamp forest taxa. The data imply that the drying climate probably resulted in a regional drop in the water table, which triggered peat decomposition, including the loss of peat carbon accumulated prior to the onset of the drier conditions. After approximately 2,000 cal. yr bp, our data show that the drying trend ceased, hydrologic conditions stabilized and peat accumulation resumed. This reversible accumulation–loss–accumulation pattern is consistent with other peat cores across the region, indicating that the carbon stocks of the central Congo peatlands may lie close to a climatically driven drought threshold. Further research should quantify the combination of peatland threshold behaviour and droughts driven by anthropogenic carbon emissions that may trigger this positive carbon cycle feedback in the Earth system.
... CO and CO2 were again monitored using the infrared detector during the oxidation phase (CO_OX and CO2_OX Rock-Eval® thermograms). The resulting five thermograms were processed using the Geoworks software (Vinci Technologies, Geoworks V1.6R2), except for the parameters R-index, I-index which were computed using homemade Python scripts 130 according to the formula proposed by Sebag et al. (2016). ...
... (respectively T70_CO_OX; °C) is the temperature at which 50 % (respectively 70 %) of the CO have been emitted during the oxidation phase (the integration ends at 850 °C). We also calculated two other parameters previously used in assessing the thermal stability of SOC: the I-index (related to the thermolabile organic carbon released as hydrocarbon effluents, Sebag et al., 2016;no unit) and the R-index (the proportion of thermostable organic carbon released as hydrocarbon effluents after 400 °C, Sebag et al., 2016;no unit). Finally, we calculated the three following Rock-Eval® parameters, related to the SOM 165 stoichiometry. ...
... (respectively T70_CO_OX; °C) is the temperature at which 50 % (respectively 70 %) of the CO have been emitted during the oxidation phase (the integration ends at 850 °C). We also calculated two other parameters previously used in assessing the thermal stability of SOC: the I-index (related to the thermolabile organic carbon released as hydrocarbon effluents, Sebag et al., 2016;no unit) and the R-index (the proportion of thermostable organic carbon released as hydrocarbon effluents after 400 °C, Sebag et al., 2016;no unit). Finally, we calculated the three following Rock-Eval® parameters, related to the SOM 165 stoichiometry. ...
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The quality and quantity of soil organic matter (SOM) are key elements of soil health and climate regulation by soils. The Rock-Eval® thermal analysis technique is increasingly used as it represents a powerful method for SOM characterization by providing insights on bulk SOM chemistry and thermal stability. In this study, we applied this technique on a large soil sample set from the first campaign (2000–2009) of the French monitoring network of soil quality: RMQS. Based on our analyses on ca. 2000 composite surface (0–30 cm) samples taken all over mainland France, we observed a significant impact of land cover on both SOM thermal stability and elemental stoichiometry. Cropland soils had a lower mean value of hydrogen index (a proxy for SOM H / C ratio) and a higher thermal stability than grasslands and forests. Regarding the oxygen index (a proxy for SOM O / C ratio), we observed significant differences in values for croplands, grasslands and forests. Positive correlations between the temperature parameters on the one hand and the clay content and pH on the other hand highlight the protective effect of clay on organic matter and the impact of pH on microorganisms mineralization activity. Surprisingly, we found weak effects of climatic parameters on the thermal stability and stoichiometry of SOM. Our data suggest that topsoil SOM is on average more oxidized and biogeochemically stable in croplands. More generally, the high number and even repartition of data on the whole French territory allow to build a national interpretative referential for these indicators in surface soils.
... Similarly to structuration patterns, OM dynamics and in particular the mechanisms of its mixing with mineral particles are strongly controlled by the heterogeneous layout of floodplain soils. The origin of OM in soil aggregates can however be distinguished through the analysis of its thermal stability by the way of indices representing immature OM (I-index) and OM thermal stability (R-index) (Albrecht et al., 2015;Sebag et al., 2016;Matteodo et al., 2018;Schomburg et al., 2018a). ...
... In this study, the area under the S2 curve was subdivided into to five areas (A1 to A5) with predefined temperature ranges: 200-340°C for A1 (labile biopolymers), 340-400°C for A2 (resistant biopolymers), 400-460°C for A3 (immature geopolymers), and A4 and A5, which stands for refractory geopolymers (> 460°C). These areas were used for the calculation of the R-index and I-index (Sebag et al., 2016). I-index was related to the thermally more labile fraction of OM and measures its degree of preservation as follows: ...
... Moreover, the R-Index plotted against I-index is an indicator for the OM stability in aggregates. This diagram is a useful tool to describe complex processes by taking into account only two synthetic parameters: it graphically represents the complex transformations of soil organic constituents, even if their chemical composition is not precisely known (Sebag et al., 2016). These authors calibrated > 1000 soil samples from different soil horizons and indicated a strong correlation between I-index and R-index in nondisturbed soils, where changes in labile OM fraction drive the bulk thermal stability. ...
Article
Soil structure formation in alluvial soils is a fundamental process in near-natural floodplains. A stable soil structure is essential for many ecosystem services and helps to prevent river bank erosion. Plants and earthworms are successful soil engineering organisms that improve the soil structural stability through the incorporation of mineral and organic matter into soil aggregates. However, the heterogeneous succession of different textured mineral and buried organic matter layers could impede the development of a stable soil structure. Our study aims at improving the current understanding of soil structure formation and organic matter dynamics in near natural alluvial soils. We investigate the effects of soil engineering organisms, the composition, and the superimposition of different alluvial deposits on the structuration patterns, the aggregate stability, and organic matter dynamics in in vitro soil columns, representing sediment deposition processes in alluvial soils. Two successions of three different deposits, silt–buried litter–sand, and the inverse, were set up in mesocosms and allocated to four different treatments, i.e. plants, earthworms, plants + earthworms, and a control. X-ray computed tomography was used to identify structuration patterns generated by ecosystem engineers, i.e. plant root galleries and earthworm tunnels. Organic matter dynamics in macro-aggregates were investigated by Rock-Eval pyrolysis. Plant roots only extended in the top layers, whereas earthworms preferentially selected the buried litter and the silt layers. Soil structural stability measured via water stable aggregates (%WSA) increased in the presence of plants and in aggregates recovered from the buried litter layer. Organic matter dynamics were controlled by a complex interplay between the type of engineer, the composition (silt, sand, buried litter) and the succession of the deposits in the mesocosm. Our results indicate that the progress and efficiency of soil structure formation in alluvial soils strongly depends on the textural sequences of alluvial deposits.
... In summary, the sector with HI:OI > 1.25 indicates aquatic OM and biological tissues rich in lipids (Algae in Fig. 5), the sector with 1.25 > HI:OI > 0.5 indicates biogenic OM from terrestrial organic sources, such as fresh plant tissues and soil litters (Litter in Fig. 5), and the sector with HI:OI <0.5 indicates detrital soil OM from organomineral and mineral horizons (Soil in Fig. 5). These observations are in line with other published dataset (e.g., Disnar et al., 2003;Boussafir et al., 2012;Saenger et al., 2013;Sebag et al., 2013;Mabicka Obame et al., 2014;Debret et al., 2014;Sebag et al., 2016) and support the use of the Van Krevelen pseudo-diagram for the interpretation of the siderite-effect-free results from Lake Barombi sediments. ...
... This early diagenetic "shift" in the Van Krevelen pseudo-diagram derives from the differential decay of organic components according to their intrinsic resistance (De Leeuw and Largeau, 1993) and their interactions with the mineral matrix (Keil et al., 1994). Such a "decaying shift" associated with a decrease in HI and a relative increase in OI (Disnar et al., 2003) is also evident in the continuum of plants, litters, humus, and soils (Sebag et al., 2016). The sediments of the Lake Barombi show a similar decaying trend in the Van Krevelen pseudo-diagram, and this decaying trend is also seen in the data when plotted through time: the younger samples (i.e. ...
Article
Originally developed for use in the petroleum industry, Rock-Eval pyrolysis is a technique commonly applied to lake sediments to infer paleoenvironmental reconstructions. The standard Rock-Eval parameters provide information on the amount of total organic and inorganic carbon (TOC and MinC, respectively), and are usually interpreted as proxies for the source (aquatic or terrestrial) of the primary production of organic matter (Hydrogen Index vs Oxygen Index). Although this method usually provides valuable evidence, the common presence of siderite in tropical lake sediments can alter the primary signal of the sedimentary organic matter (SOM). Indeed, the CO2 and CO released by the pyrolysis of siderite are integral to the calculation of the SOM-related standard Rock-Eval parameters. In this study, we analyze sediments from a core collected in the Lake Barombi (southwest Cameroon) and describe the impact of siderite on standard Rock-Eval parameters. We propose a workflow that allows standard Rock-Eval parameters to be corrected, based on the analysis of thermograms. The proposed corrections provide siderite-effect-free parameters, accurately reflecting the changes in sedimentary organic matter composition.
... Rock-Eval 6 pyrolysis has been used to investigate carbon dynamics within a range of systems e.g. mangrove , marine sediments (Hare et al., 2014), freshwater and saltmarsh peats (Engelhart et al., 2013;Newell et al., 2016;Kemp et al., 2017), as well as trends in SOM dynamics through soil profiles (Sebag et al., 2006;Delarue et al., 2013;Biester et al., 2014;Sebag et al., 2016), providing a powerful tool for rapid assessment of shifts in peat organic geochemistry among vegetation types and depth. ...
... The decline in OI and HI with depth indicate SOM maturation and persistence of woody materials, which are richer in lignin, and have a high resistance to anaerobic decay (Disnar et al., 2003, Hetényi et al., 2005, Saenger et al., 2013 but note that lignin can by depleted rapidly by fungal decay under aerobic conditions . At greater depth HI values continue to decrease, reflecting degraded plant material and increased humification of SOM, including greater aromaticity and dehydrogenation ( Fig. 3; Hetényi et al., 2006, Carrie et al., 2012, Sebag et al., 2016. Increasing humification with depth is also supported by the depletion of carbohydrates down core, as demonstrated by the FTIR spectra and ratios (Fig. 6), and the gradual decline in C l and concurrent increase in C p (Fig. 8). ...
Article
Tropical peatlands hold large amounts of carbon but the influence of litter inputs and variation in peat properties with depth on carbon storage are poorly understood. Here we present a stratigraphy of peatland carbon stocks and accumulation through the peat profile in a tropical ombrotrophic wetland and assess shifts in vegetation inputs and organic matter degradation using n-alkane distributions and Rock-Eval 6 pyrolysis. Mixed forest (including canopy palms and tropical hardwood trees) contained the greatest total carbon stock in the soil (1884 Mg C ha⁻¹), followed by Rhizophora mangle (mangrove, 1771 Mg C ha⁻¹), Campnosperma panamensis (hardwood, 1694 Mg C ha⁻¹) and Cyperus (sawgrass) bog plain (1488 Mg C ha⁻¹). The long-term apparent rate of carbon accumulation, determined by ¹⁴C dating of the carbon stored in different layers in the peat profile, decreased from the edge to the interior of the peatland, with the highest accumulation rate in at the Rhizophora site (102.2 g C m⁻² y⁻¹) and the lowest in the deeper peat layers at the Cyperus site (45.6 g C m⁻² y⁻¹). High molecular weight n-alkanes dominated in surface peat in all four phasic communities, while deeper in the peat profile n-alkane profiles differed more among sites, suggesting contrasting litter inputs or decomposition environments. Deeper peat was depleted in carbohydrates and had a relatively larger thermostable C pool. Taken together our findings show (i) that different forest types hold varying C stocks and have different peat accumulation rates, even over relatively small distances, and (ii) progressive depletion of carbohydrates and thermolabile compounds with depth, despite strong variation in litter inputs throughout the peat profile.
... Soil organic carbon quality was assessed with the R-index calculated as the relative HC contribution of the most thermally resistant organic fraction (i.e. thermal cracking above 400°C; see Disnar et al., 2003;Sebag et al., 2016Sebag et al., , 2006. The higher the R-index, the more organic fractions are thermally resistant. ...
... Asterisks below the boxplots indicate differences between the plantations and the forest ("***", p < 0.001, "**", p < 0.01, "*", p < 0.05); p-values above the boxplots indicate differences between rotations. Coloured bands correspond to Rock-Eval references for organic layer (green) and top soil (brown) presented in Sebag et al. (2016). ...
Article
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Rubber tree plantations (Hevea brasiliensis) cover large areas in the tropics. In historical producing regions like South Thailand, rubber has been planted by smallholders for three successive rotations lasting a total of 75 years. Despite possible consequences on topsoil, the long-term impacts of repeated rubber plantations on soil quality remain unknown. This study aims to better understand how various factors linked to long-term rubber land use and land use change affect topsoil physico-chemical properties and soil organic carbon (SOC) thermal stability. We focus on the effects of three factors: i. deforestation (change from forest to first rubber plantation); ii. the age of the rubber stand (immature vs mature); and iii. Long-term rubber cultivation (first, second or third successive rotation) over a chronosequence in farmers plots. Our results show that soil was deeply degraded after deforestation to a rubber plantation. Long-term rubber cultivation is also detrimental for the soil and has a more negative impact on soil physico-chemical properties and carbon dynamics, than the age of the rubber stand (e.g. on average, decrease of 50% of SOC content between forest and third rotation). At the third rotation, after 50 years of rubber cultivation, the quality of the 0–10 cm soil layer was very low, with an increase in SOC thermal stability. At this stage, logging practices upset the sustainability of the system. These impacts could be limited by less destructive practices during planting. This article is protected by copyright. All rights reserved.
... Total Organic Carbon (TOC in wt-%), Mineral Carbon (MinC in wt-%), Hydrogen Index (HI in mg HC/g TOC −1 ) and Oxygen Index (OI in mg CO 2 /g TOC −1 ) were calculated by integrating the amounts of HC, CO, and CO 2 produced during thermal cracking and combustion of OM ort thermal decomposition of carbonates between defined temperature limits (Behar et al., 2001;Lafargue et al., 1998). Since cracking temperature of organic compounds depends on their structural stability, the thermal status of OM was characterized by combining R-index (i.e., relative 170 contribution of most thermally stable HC pools) and I-index (i.e., ratio between thermally labile and resistant HC pools; details in Sebag et al., 2016). As derived from a mathematical construct, if the gradual decomposition of labile compounds is its main driver, OM composition can be described as a continuum from biological tissues to a mixture of organic constituents derived from OM decomposition and plotted along a linear regression line (called "Decomposition line"; Malou et al., 2020) in the I-index vs R-index diagram (called thereafter I/R diagram; Albrecht et al., 2015). ...
... Lake and delta samples contain the most labile OM (R < 0.57 and I > 0.25, Figure 7) while upper reach and floodplain 265 samples contain the most stable OM (R > 0.62 and I < 0.25, Figure 7) with swamp samples falling in between these extremes (Figure 7). It is important to highlight that floodplain and swamp samples display high dispersion around the "decomposition line" in comparison with the strong correlation (R 2 > 0.9) usually observed for composts, litters, and topsoils(Albrecht et al., 2015;Sebag et al., 2016). ...
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Sedimentary organic matter (OM) analyses along a 130 km-long transect of the Mkhuze River from the Lebombo Mountains to its outlet into Lake St. Lucia, Africa’s most extensive estuarine system, revealed the present active trapping function of a terminal freshwater wetland. A combination of organic bulk parameters, thermal analyses, and determination of plant waxes, and their corresponding stable carbon (δ13C) and hydrogen (δD) isotopic signatures in surface sediments and local plant species enabled characterization and comparison of sedimentary OM in terms of stability, degradation status, sources, and sinks within and among the respective sub-environments of the Mkhuze Wetland System. This approach showed that fluvial sedimentary OM originating from inland areas is mainly deposited on the floodplain and Mkhuze Swamps. In contrast to samples from upstream areas, a distinctly less degraded signature characterizes the sedimentary OM in the northern section of Lake St. Lucia. Although lake sedimentary plant waxes are similar in the observed wax distribution pattern and δ13C values, they exhibit considerably higher δD values. This offset in δD indicates that lakeshore vegetation dominates plant-derived sedimentary OM in the lake, elucidating the effective capturing of OM and its fate in a sub-tropical coastal freshwater wetland. These findings raise important constraints for environmental studies assuming watershed-integrated signals in sedimentary archives retrieved from downstream lakes or offshore.
... This correction can yield some negative values for the CO_PYR and CO2_PYR thermograms of soil samples with very low SOC content (data not shown). For the HC_PYR thermogram we also determined three parameters reflecting a proportion of thermally resistant or labile hydrocarbons: a parameter representing the proportion of hydrocarbons evolved between 200 and 450 • C (thermolabile hydrocarbons, TLHC index, unitless; modified from Saenger et al., 2013Saenger et al., , 2015, as described by Cécillon et al. (2018); a parameter representing the preservation of thermally labile hydrocarbons (I index, unitless; after Sebag et al., 2016); and a parameter representing the proportion of hydrocarbons thermally stable at 400 • C (R index, unitless; after Sebag et al., 2016). We also considered the hydrogen index (HI, mg HC g −1 C) and oxygen index (OI RE6 , mg O 2 g −1 C) that respectively describe the relative elemental hydrogen and oxygen enrichment of soil organic matter (see e.g. ...
... This correction can yield some negative values for the CO_PYR and CO2_PYR thermograms of soil samples with very low SOC content (data not shown). For the HC_PYR thermogram we also determined three parameters reflecting a proportion of thermally resistant or labile hydrocarbons: a parameter representing the proportion of hydrocarbons evolved between 200 and 450 • C (thermolabile hydrocarbons, TLHC index, unitless; modified from Saenger et al., 2013Saenger et al., , 2015, as described by Cécillon et al. (2018); a parameter representing the preservation of thermally labile hydrocarbons (I index, unitless; after Sebag et al., 2016); and a parameter representing the proportion of hydrocarbons thermally stable at 400 • C (R index, unitless; after Sebag et al., 2016). We also considered the hydrogen index (HI, mg HC g −1 C) and oxygen index (OI RE6 , mg O 2 g −1 C) that respectively describe the relative elemental hydrogen and oxygen enrichment of soil organic matter (see e.g. ...
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Partitioning soil organic carbon (SOC) into two kinetically different fractions that are stable or active on a century scale is key for an improved monitoring of soil health and for more accurate models of the carbon cycle. However, all existing SOC fractionation methods isolate SOC fractions that are mixtures of centennially stable and active SOC. If the stable SOC fraction cannot be isolated, it has specific chemical and thermal characteristics that are quickly (ca. 1 h per sample) measurable using Rock-Eval® thermal analysis. An alternative would thus be to (1) train a machine-learning model on the Rock-Eval® thermal analysis data for soil samples from long-term experiments for which the size of the centennially stable and active SOC fractions can be estimated and (2) apply this model to the Rock-Eval® data for unknown soils to partition SOC into its centennially stable and active fractions. Here, we significantly extend the validity range of a previously published machine-learning model (Cécillon et al., 2018) that is built upon this strategy. The second version of this model, which we propose to name PARTYSOC, uses six European long-term agricultural sites including a bare fallow treatment and one South American vegetation change (C4 to C3 plants) site as reference sites. The European version of the model (PARTYSOCv2.0EU) predicts the proportion of the centennially stable SOC fraction with a root mean square error of 0.15 (relative root mean square error of 0.27) at six independent validation sites. More specifically, our results show that PARTYSOCv2.0EU reliably partitions SOC kinetic fractions at its northwestern European validation sites on Cambisols and Luvisols, which are the two dominant soil groups in this region. We plan future developments of the PARTYSOC global model using additional reference soils developed under diverse pedoclimates and ecosystems to further expand its domain of application while reducing its prediction error.
... Notre étude se réfère à la méthode développée par Disnar et al. (2003) pour estimer la contribution des pools de constituants organiques définis par leur température de craquage spécifique. Ainsi, selon les travaux d' Albrecht et al. (2015), les courbes S2 ont été décomposées en intervalles (Sebag et al., 2007 ;Albrecht et al., 2015 ;Sebag et al., 2016). I-index représente la contribution de la matière organique immature, tandis que R-index représente celle de la matière organique mature. ...
... Notre étude se réfère à la méthode développée par Disnar et al. (2003) pour estimer la contribution des pools de constituants organiques définis par leur température de craquage spécifique. Ainsi, selon les travaux d' Albrecht et al. (2015), les courbes S2 ont été décomposées en intervalles (Sebag et al., 2007 ;Albrecht et al., 2015 ;Sebag et al., 2016). I-index représente la contribution de la matière organique immature, tandis que R-index représente celle de la matière organique mature. . ...
... Thermal analysis techniques have also been used to characterize soil organic matter (SOM) stability (e.g., Plante et al. 2009). Among them, Rock-Eval 6 (RE6) analysis has shown promising results in the determination of SOM biogeochemical stability (Barré et al. 2016;Gregorich et al. 2015;Saenger et al. 2015;Sebag et al. 2016). RE6-derived parameters are reliable indicators of the stable SOC pool (Barré et al. 2016;Cécillon et al. 2018) and can be a useful complement to the aforementioned usual indicators of the labile SOC pool . ...
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AimsIn temperate forests, soils contain a large part of the ecosystem carbon that can be partially lost or gained upon global change. Our aim was to identify the factors controlling soil organic carbon (SOC) stability in a wide part of French forests. Methods Using a set of soils from 53 French forest sites, we assessed the effects of depth (up to 1 m), soil class (dystric Cambisol; eutric Cambisol; entic Podzol), vegetation types (deciduous; coniferous) and climate (continental influence; oceanic influence; mountainous influence) on SOC stability using indicators derived from laboratory incubation, physical fractionation and thermal analysis. ResultsLabile SOC pools decreased while stable SOC pool increased with depth. Soil class also significantly influenced SOC stability. Eutric Cambisols had less labile SOC in surface layers but had more labile SOC at depth (> 40 cm) than the other soil classes. Vegetation influenced thermal indicators of SOC pools mainly in topsoils (0–10 cm). Mountainous climate forest soils had a low thermal SOC stability. Conclusions On top of the expected effect of depth, this study also illustrates the noticeable effect of soil class on SOC stability. It suggests that environmental variables should be included when mapping climate regulation soil service.
... Although previous studies have shown that Ca may promote the adsorption of OM onto mineral surfaces, associated changes (if any) in the thermal stability of the adsorbed organics have not been determined. Thermal stability is of interest, because it may serve as a proxy of the binding strength of organo-mineral associations and as such provide a measure of chemical and biological C availability and SOM stability (Demyan et al., 2013;Johnson et al., 2015;Kučerík et al., 2018;Leifeld et al., 2006;Plante et al., 2011Plante et al., , 2009Sanderman and Grandy, 2020;Sebag et al., 2016). Here, we combined thermal analyses with batch sorption experiments and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopic measurements to generate an integrated dataset of the effects of Ca on the extent, stability, and chemical speciation of OC sorption in a synthetic clay-size fraction mimicking the mineralogy of highly weathered tropical soils (oxisols/ferralsol). ...
Article
Increasing organic matter storage in highly weathered soils is an important strategy to improve soil fertility and to help mitigate rising atmospheric carbon dioxide levels that may cause climate change. High calcium (Ca) availability in soils resulting from lime and gypsum application may increase soil organic matter contents, and this is frequently attributed to increased adsorption of organic molecules by soil minerals. Here, we evaluated the effects of Ca on the thermal stability of organo-mineral associations in a synthetic mineral mixture of kaolinite, goethite, and aluminum (Al)-oxides mimicking the mineralogy of the clay-size fraction of weathered tropical soils. Sorption experiments were carried out with humic acid (HA) at pH 6.5 and 4.5, using Ca concentrations ranging from 0.0 to 0.4 mM. Adsorption of HA increased with decreasing pH. The presence of 0.4 mM Ca increased HA adsorption by a factor of approximately 2.4 at pH 6.5 and 1.6 at pH 4.5. The thermal stability of HA increased upon Ca addition, suggesting that Ca strengthens HA-mineral bonding. UV–Vis analyses indicated that HA is fractionated during sorption in all treatments, and that fractionation may increase in the presence of Ca, possibly by enhanced preferential adsorption of HA compounds of relatively low molecular weight. Attenuated total reflectance–Fourier transform infrared spectroscopic results indicated that HA adsorbed primarily through the carboxylate moieties under all conditions. Our combined results show that dissolved Ca not only increases the extent of HA adsorption by soil minerals, but may also enhance the chemical stability and change the physical-chemical properties of sorbed HA compounds. A stronger association of organic compounds with mineral surfaces in the presence of Ca may be a significant contributing factor to Ca-promoted OM accumulation in soils.
... Open-system programmed-pyrolysis techniques such as Rock-Eval, have found increased application for geochemical screening of hydrocarbon-plays (Espitalié et al., 1977;Peters and Cassa, 1994;Peters et al., 2005;Hazra et al., 2019a). The Rock-Eval technique, apart from finding its usage in hydrocarbon-plays, has been used for characterizing organic matter in soil and recent sediments (Sanei et al., 2005;Sebag et al., 2016Sebag et al., 2018Malou et al., 2020), soil contamination studies (Poot et al., 2009), etc. The reason for the extensive usage of Rock-Eval by researchers working in different fields is due to its rapid analysis technique and generation of reliable data. ...
Article
The Rock-Eval pyrolysis-stage derived parameters such as free hydrocarbons (S1), heavier pyrolysis-hydrocarbons (S2), pyrolyzable carbon (PC) and pyrolysis Tmax (from S2 curve) have received considerable interest for source-rock screening and thermal maturity assessment. On the other hand, the Rock-Eval oxidation-stage S4CO2 curve, which gives the amount of residual carbon (RC), only recently has received some interest. While the pyrolysis-stage S2 temperature-peak (Tmax) is conventionally used as a maturity proxy, in this work we show that the temperature-peak of S4CO2 curve (S4Tmax) can also be used as a thermal maturity proxy for shales. For overmature and low-TOC shale samples, showing asymmetric S2 shape and concomitantly producing doubtful Tmax, the S4 curves showed symmetric nature and consequently the S4Tmax was observed to be a reliable thermal maturity estimate. While the S4Tmax clearly resolved immature and overmature shales, for the early mature and peak mature shales the S4Tmax showed overlapping values. S4Tmax of pre-pyrolyzed and pyrolyzed masses showed good positive correlation with differential scanning calorimetry temperature-peak (DSCTpeak), and consequently indicated its applicability as a thermal maturity proxy. When early mature pre-pyrolyzed samples were directly analyzed using the Rock-Eval oxidation stage, the S4 curves showed formation of two sub-peaks, and consequently the Tmax was observed to decrease. It is recommended that analysts and interpreters should thoroughly cross-check S2 curves before reporting data, and in case of asymmetric or unreliable S2 curves, the S4Tmax can be used as a maturity proxy.
... The cation exchange capacity (CEC) was were extracted by neutral 1 N ammonium acetate (Chapman, 1965) and analysed by flame photometer (Sherwood model 420) for K and Ca and by Atomic Absorption spectrophotometer (Shimadzu AA 6200) for Mg. Carbon content was analysed using a Rock-Eval 6 pyrolyzer (Vinci Technologies) at Lausanne University (Disnar et al., 2003;Sebag et al., 2016). Analyses were carried out with 30 to 70 mg of powder samples. ...
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The rapid expansion of perennial crops is a major threat to biodiversity in Southeast Asia. The biodiversity losses related to the conversion of forest lands to oil palm or rubber plantations (RP) are well documented by recent studies. However, the impact of the conversion from intensively managed annual crops to perennial crops on soil biodiversity has not yet been addressed. This study aims at assessing the impact on soil biodiversity of a) the shortterm effect of land use conversion from cassava crop to RP, and b) the long-term effect of RP ageing. Soil biodiversity (bacterial, fungal and macrofaunal), microbial activities and pedoclimatic characteristics were measured over a chronosequence of 1–25 years old of RP compared to cassava fields, the former crop, in Thailand. The conversion from cassava to young RP (1–3 yr) had a significant effect on microbial biomass and activities and fungal composition, but did not impact the bacterial and macrofaunal diversity. This effect of land use conversion could be explained by the change in land management due to the cultivation of pineapple in the inter-row of the young RP. Canopy closure appeared to be the main driver of soil biota shifts, as most of the biotic parameters, composition, abundance and activities were significantly modified after 7 years of RP. The changes of composition in older rubber plantations originated from the dominance of Trichoderma (fungi), Firmicutes (bacteria), and earthworms. Old rubber plantations (23–25 yr) harboured the highest microbial and macrofaunal biomass; however, they were also characterised by a significant decrease in bacterial richness. The change in pedoclimatic conditions across the rubber chronosequence, i.e. increase in soil moisture, litter and organic carbon content, was a stronger driver of soil biota evolution than land use conversion. The macrofaunal composition was more resistant to land use conversion than the bacterial composition, whereas the microbial biomass was sensitive to land use conversion, but showed resilience after 20 years. However, bacterial, fungal and macrofaunal diversity, macrofaunal and microbial biomass and microbial activities were all sensitive to RP ageing.
... jusqu'ici, la prise en compte des paramètres (autres que le TOC) mesurés lors de l'analyse Rock-Eval des sédiments pourrait peut-être aider à distinguer les deux types de sédiments par la nature de leur matière organique. Plusieurs ratios Rock-Eval classique ont été tracés et sont disponibles en annexe (Fig. A3) : le pseudo-diagramme de Van Krevelen (HI versus OI ; Sebag et al., 2016), HI versus 1/TOC (Simonneau et al., 2014), RC/TOC versus S2/S3 (Carrie et al., 2012 ;Copard et al., 2015) et HI versus Tmax (Copard et al., 2015). Les valeurs de RC/TOC sont globalement comprises entre 0,7 et 0,8 pour les deux types de sédiments, ce qui correspond classiquement à une matière organique (MO) caractéristique d'horizons organominéraux (Copard et al., 2015). ...
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Si des interactions complexes entre les hommes et les rivières existent depuis plusieurs millénaires, les pressions anthropiques exercées sur les hydrosystèmes ont fortement augmenté au cours des deux derniers siècles. Il est donc essentiel de mieux appréhender l’évolution récente des corridors fluviaux en lien avec les activités humaines qui s’y sont développées, et l’investigation du compartiment sédimentaire est particulièrement appropriée pour ce type d’approche rétrospective. Cette thèse s’appuie sur l’étude du Rhône, un fleuve européen majeur soumis à de fortes pressions anthropiques, et s’intéresse particulièrement à deux de ces pressions : la contamination et l’aménagement du cours d’eau. Plus précisément, elle vise à 1) prouver l’existence de sédiments hérités des aménagements mis en place sur le Rhône, et les caractériser ; 2) développer une méthodologie pour l’interprétation d’archives sédimentaires issues d’un milieu fluvial fortement anthropisé, dans le cadre des contaminations historiques et émergentes ; 3) démontrer l’intérêt du concept de sédiments hérités dans un contexte européen et en lien avec la mise en place d’infrastructures fluviales. La méthodologie développée dans ce travail combine une analyse diachronique des sites d’étude, leur investigation à l’aide de sondages géophysiques et le prélèvement de carottes sédimentaires permettant la quantification de nombreux paramètres sédimentaires (e.g., granulométrie, carbone organique total, teneur en contaminants et radionucléides). Son implémentation a permis de démontrer l’existence de deux types de sédiments hérités sur le Rhône, en lien avec les deux phases d’aménagement auxquelles le fleuve a été soumis (correction du tracé et aménagements hydroélectriques). Dans les deux cas, les sédiments hérités résultent d’un processus de déconnexion entre le chenal principal et les environnements de dépôt concernés, et la granulométrie est le paramètre principal permettant de les distinguer des dépôts sous-jacents. Une évolution de la définition de sédiments hérités est donc proposée afin d’y inclure ces nouvelles caractéristiques. Il est également prouvé que la capacité d’enregistrement des sédiments fluviaux est fortement influencée par l’environnement de dépôt étudié et par les modifications de leurs conditions hydrodynamiques induites par les différents aménagements. La méthodologie proposée permet néanmoins d’interpréter de façon robuste ces archives et de reconstruire la trajectoire récente du Rhône et les tendances temporelles de contamination de polluants historiques (éléments métalliques, polychlorobiphényles) ou émergents (retardateurs de flammes bromés) à partir de carottes issues de divers environnements. Une telle approche rétrospective est essentielle afin de mieux appréhender l’ensemble des impacts anthropiques en lien avec les enjeux actuels de gestion des cours d’eau, et de proposer des politiques publiques et des mesures de suivi et/ou de restauration cohérentes avec l’évolution à long terme du fleuve.
... Estimates of the amount of hydrogen (HI) and oxygen (OI) are compared to established reference samples and or published criteria to infer OM (kerogen)-type (Emmings et al., 2017;Hennissen et al., 2017;Slowakiewicz et al., 2015). Similarly, recent environmental studies have utilised variations in the profile of the bound hydrocarbons (S2), that broadly corresponds to strength and extent of intra and inter polymer bonding, to identify changes in OM that were used to understand hydrological-climate effects on peatland carbon, variable OM preservation in soils and historical oil spills in urban waterway sediments (Cooper et al., 2021;Girkin et al., 2018;Newell et al., 2016;Sebag et al., 2016;Thomas et al., 2019). ...
Article
The UN Sustainable Development Goals highlight the myriad of socio-economic and environmental challenges occurring as a result of anthropogenic chemical pollution. Urban sediments from informal settlements (slums) on the Nairobi, Ngong and Mathare Rivers (n = 25), were evaluated for sediment quality. Microtox bioassay identified 8 sites as toxic, 9 as moderately toxic and 8 as non-toxic. Slum sediments were characterised by high total organic carbon and Rock-Eval pyrolysis revealed bound carbon from a mix of raw sewage and domestic refuse. Sediments from Kiambio, Kibera, Mathare and Kawangware slums contained high coprostanol at 55–298 μg/g and epicoprostanol at 3.2–21.7 μg/g confirming appreciable incorporation of untreated human faeces. Hormones, antianalgeiscs, antiinflamatories, antiepileptics and antibiotics most affected Mathare > Kiambio > Kibera > Mukuru > Kawangware slums. Carbamazepine, ibuprofen, diclofenac and acetaminophen concentrations are amongst the highest reported in Kenyan river sediments and were positively correlated with faecal steroids (sewage). Common persistent organic pollutants, such as organochlorine insecticides ΣDDT 1–59 μg/kg, mean 21.2 μg/kg, Σ¹⁶PAH 182–2218 μg/kg, mean 822 μg/kg and Σ³⁰ PCB 3.1–157.1 μg/kg, mean of 21.4 μg/kg were between probable effect likely and unlikely sediment quality guidelines (SQG). PAH source ratios and parent to alkyl-PAH distribution suggested vehicle exhaust, power stations (heavy oil), kerosene (cooking oil) and other pollution sources. Trace metal concentrations As, Cd, Cr, Hg and Ni were below SQG whereas Pb exceeded the SQG. This multi-contaminant characterisation of sediment quality in Nairobi supports the development and implementation of policies to improve urban infrastructure to protect ecological and human health. It demonstrates the need for environmental geochemists to engage in the science-policy interface associated with both global and national development frameworks, with particular reference to the Sustainable Development Goals, New Urban Agenda, and Kenya’s Vision 2030.
... Based on the data from this programmed-pyrolysis technique, several 'classic' guidelines have been put forward by some workers to aid geochemical interpretations (see Espitalié et al., 1977;Peters, 1986;Peters and Cassa, 1994). Recent years have also seen the application of this technique in other areas such as for typing organic-matter in soils and recent sediments, and for soil contamination studies etc. (Poot et al., 2009;Sebag et al., 2016). The technique is simple, quick, requiring small sample quantities and producing reliable data such as amount of free hydrocarbons present within the sample (S1 curve), hydrocarbons generated from cracking of kerogen (S2 curve), T max (thermal maturity proxy, calculated from temperature-peak of S2 curve), total organic carbon (TOC), hydrogen index (HI; (S2/TOC)*100; indicates the type of kerogen present within the sample), etc. ...
Article
Rock-Eval S2 and S4 curves are commonly used to interpret active source rocks. In this work we report the formation of unusual, uneven, spiky S2 pyrograms from Rock-Eval of manually isolated vitrain bands from Mvb and Lvb coals. The spikes and unevenness in the S2 curves for coking-vitrains are interpreted to form from the release of gases or bubbles bursting from the melt during the pyrolysis stage. Crucible swelling numbers measured for these vitrains were observed to be 3 to 4 times higher than the parent coal from which they were separated. Field Emission Scanning Electron Microscope (FE-SEM) images also document formation of three dimensional bubbles due to bursting of gaseous bubbles, in the pyrolysis-residue (using Rock-Eval) of coking-vitrains. For bulk coal samples and vitrains isolated from HvbA coals, the S2 pyrograms were observed to be smooth. At higher sample weights, the formation of spikes for the vitrains from Mvb and Lvb was observed to increase, irrespective of the heating-rates used, owing to the release of greater amounts of gases or bubbles bursting from the melt. The results indicate the importance of cross-examining S2 pyrograms as it offers clues towards the behavior of coals during pyrolysis, and for better interpretation of results.
... Other works reported that the amounts of SOC associated with the coarse fractions (F > 50 μm) in sandy soils, mostly comprising particulate organic matter (Christensen 2001), are an important part of the total SOC contents and contribute to SOC storage after a change in cropland management (Fujisaki et al. 2018a;Manlay et al. 2002;Zinn et al. 2007). Moreover, in a complementary investigation using a Rock-Eval method to assess the thermal stability of soil organic matter (SOM), in the same territories and management practices as in the present study, Malou et al. (2020) revealed that SOM of these soils was dominated by thermally labile forms that mineralize quickly (Sebag et al. 2016). It is especially important for such Arenosols, as crop production relies on fast cycling organic matter (Wood et al. 2016). ...
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Soil organic carbon (SOC) is essential for the productivity of agroecosystems and for mitigating climate change. Because the SOC contents of sandy soils are usually small, the effects of agricultural management upon SOC stocks in such soils have been insufficiently studied. In West sub-arid Africa, the coarse-textured soils (mostly Arenosols) are diversely managed by smallholders. In this study, we aimed to quantify SOC stocks in cultivated soils of that region, in a context where agricultural practices rely mainly upon organic inputs derived from various integrated crop-livestock systems. SOC stocks were estimated for the 0–30 cm depth in 1,813 plots in Senegal’s groundnut basin. We found that SOC stocks in farmers’ fields varied between 2.3 and 59.8 Mg C ha-1 (mean ± standard deviation, 14.6 ± 0.14 Mg C ha-1). SOC stocks were influenced slightly by soil type, but were only weakly correlated to soils’ clay and silt contents. SOC stocks differed significantly among the three studied village territories due to contrasting livestock-raising systems. Average stocks were significantly higher in plots close to housings (home-fields), which receive larger amounts of organic inputs, than in plots farther from the village (out-fields). Thus, the organic inputs to home-fields improves soil C stocks of these sandy soils in the short term. Innovative agricultural practices in the studied area probably need to target options for managing all fields optimally. Those options will require continuous application of organic products—a measure that will in turn require solutions for improving availability or management of local organic resources.
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Microbes are responsible for cycling carbon (C) through soils, and the predictions of how soil C stocks change with warming are highly sensitive to the assumptions made about the mechanisms controlling the microbial physiology response to climate warming. Two mechanisms, microbial thermal-acclimation and changes in the quantity and quality of substrates available for microbial metabolism have been suggested to explain the long-term warming impact on microbial physiology. Yet studies disentangling these two mechanisms are lacking. To resolve the drivers of changes in microbial physiology in response to long-term warming, we sampled soils from 13- and 28-year old soil warming experiments in different seasons. We performed short-term laboratory incubations across a range of temperatures to measure the relationship between temperature sensitivity of physiology (growth, respiration, carbon use efficiency and extracellular enzyme activity) and the chemical composition of soil organic matter. We observed apparent thermal acclimation in microbial processes important for C cycling, but only when warming had exacerbated the seasonally induced, already small soil organic matter pools. Irrespective of warming, greater quantity and quality of soil carbon enhanced the extracellular enzymatic pool and its temperature sensitivity. We suggest that fresh litter input into the system seasonally cancels apparent thermal acclimation of C-cycling processes. Our findings reveal that long-term warming has indirectly affected microbial physiology via reduced C availability in this system, implying that earth system models including these negative feedbacks may be best suited to describe long-term warming impact in soils.
Article
In the present study, shale samples from Rajmahal Basin, India, were analysed in terms of their source rock properties using an open-system programmed pyrolysis instrument (Rock–Eval 6). Comprehensive analysis of the different Rock–Eval graphics was conducted for the samples under consideration. Construction of S2 (mg HC/g rock) vs. total organic carbon (wt%) cross-plot using iso-HI (iso-hydrogen index) lines classified the studied suit of samples into three zones with increasing hydrogen index (Zone A < Zone B < Zone C). Analysis of S2 curves of samples from different zones revealed distinctive features attributed to the variable nature of kerogen present within the sample as well as the levels of S2 and HI. S2 curves of sample with higher Tmax were observed to be asymmetric, broad, and marked by lower flame ionization detector signals. However, for those samples, the S4 Tpeak was observed to be similar to that of the other samples. On the other hand, the higher S2 Tmax of some samples coincided with higher S4 Tpeak indicating the samples to be more mature. Additionally, samples with higher levels of oxygen index (OI), and siderite content, were observed to have S3′ curve marked by pronounced release of CO2 above 400 °C, whereas the samples with higher OI but without any presence of siderite were marked by noisy curves due to low IR CO2 signal. The results reiterated the importance of careful monitoring of the different curves obtained during the pyrolysis and oxidation stage so that erroneous characterization of the samples could be avoided.
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Changes in global soil carbon stocks have considerable potential to influence the course of future climate change. However, a portion of soil organic carbon (SOC) has a very long residence time ( > 100 years) and may not contribute significantly to terrestrial greenhouse gas emissions during the next century. The size of this persistent SOC reservoir is presumed to be large. Consequently, it is a key parameter required for the initialization of SOC dynamics in ecosystem and Earth system models, but there is considerable uncertainty in the methods used to quantify it. Thermal analysis methods provide cost-effective information on SOC thermal stability that has been shown to be qualitatively related to SOC biogeochemical stability. The objective of this work was to build the first quantitative model of the size of the centennially persistent SOC pool based on thermal analysis. We used a unique set of 118 archived soil samples from four agronomic experiments in northwestern Europe with long-term bare fallow and non-bare fallow treatments (e.g., manure amendment, cropland and grassland) as a sample set for which estimating the size of the centennially persistent SOC pool is relatively straightforward. At each experimental site, we estimated the average concentration of centennially persistent SOC and its uncertainty by applying a Bayesian curve-fitting method to the observed declining SOC concentration over the duration of the long-term bare fallow treatment. Overall, the estimated concentrations of centennially persistent SOC ranged from 5 to 11 g C kg⁻¹ of soil (lowest and highest boundaries of four 95 % confidence intervals). Then, by dividing the site-specific concentrations of persistent SOC by the total SOC concentration, we could estimate the proportion of centennially persistent SOC in the 118 archived soil samples and the associated uncertainty. The proportion of centennially persistent SOC ranged from 0.14 (standard deviation of 0.01) to 1 (standard deviation of 0.15). Samples were subjected to thermal analysis by Rock-Eval 6 that generated a series of 30 parameters reflecting their SOC thermal stability and bulk chemistry. We trained a nonparametric machine-learning algorithm (random forests multivariate regression model) to predict the proportion of centennially persistent SOC in new soils using Rock-Eval 6 thermal parameters as predictors. We evaluated the model predictive performance with two different strategies. We first used a calibration set (n = 88) and a validation set (n = 30) with soils from all sites. Second, to test the sensitivity of the model to pedoclimate, we built a calibration set with soil samples from three out of the four sites (n = 84). The multivariate regression model accurately predicted the proportion of centennially persistent SOC in the validation set composed of soils from all sites (R² = 0.92, RMSEP = 0.07, n = 30). The uncertainty of the model predictions was quantified by a Monte Carlo approach that produced conservative 95 % prediction intervals across the validation set. The predictive performance of the model decreased when predicting the proportion of centennially persistent SOC in soils from one fully independent site with a different pedoclimate, yet the mean error of prediction only slightly increased (R² = 0.53, RMSEP = 0.10, n = 34). This model based on Rock-Eval 6 thermal analysis can thus be used to predict the proportion of centennially persistent SOC with known uncertainty in new soil samples from different pedoclimates, at least for sites that have similar Rock-Eval 6 thermal characteristics to those included in the calibration set. Our study reinforces the evidence that there is a link between the thermal and biogeochemical stability of soil organic matter and demonstrates that Rock-Eval 6 thermal analysis can be used to quantify the size of the centennially persistent organic carbon pool in temperate soils.
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Sedimentary organic matter (OM) analyses along a 130 km long transect of the Mkhuze River from the Lebombo Mountains to its outlet into Lake St Lucia, Africa's most extensive estuarine system, revealed the present active trapping function of a terminal freshwater wetland. Combining bulk OM analyses, such as Rock-Eval®, and source-specific biomarker analyses of plant-wax n-alkanes and their stable carbon (δ13C) and hydrogen (δD) isotopic composition showed that fluvial sedimentary OM originating from inland areas is mainly deposited in the floodplain and swamp area of the wetland system but not in the downstream lake area. A distinctly less degraded OM signature, i.e., a considerably lower degree of transformation of unstable components (higher I index) and lower contribution of refractory and persistent fractions (lower R index) as well as recognizably higher δD values compared to samples from upstream sub-environments, characterizes surface sediments of Lake St Lucia. The offset in δD indicates that the contributing vegetation, although similar to upstream vegetation inputs in terms of photosynthetic pathway (δ13C) and alkane distribution pattern, experienced different hydrological growth conditions. The results suggest that under current conditions hinterland sedimentary OM is deposited throughout the wetland system up to the Mkhuze Swamps, which ultimately captures the transported OM. Consequently, samples from the downstream located Lake St Lucia show locally derived signals instead of integrated signals encompassing the river catchment. This finding raises important constraints for future environmental studies as the assumption of watershed-integrated signals in sedimentary archives retrieved from downstream lakes or offshore might not hold true in certain settings.
Article
Vitrain is one of the most important lithotype for the end utilization of humic coals in relevant industries. In this work, we examine the thermal, structural and pyrolysis properties of vitrains, manually isolated from coals of three distinct thermal maturity levels (rank). The high volatile bituminous (HvbA) vitrain showed highest moisture content and reactivity during combustion, while showing least Rock-Eval S2 Tmax and S4 Tpeak. On the other hand, the low volatile (Lvb) sample showed properties exact opposite to that of the HvbA sample. Thus, the rank of the vitrains was observed to directly control their behaviour. Owing to their inherently lower ash content, all the vitrains during thermogravimetric analysis developed smooth thermograms, indicating easy burning. Interlayer spacing (d002), obtained from XRD displayed a strong decrease with increasing coal rank, indicating formation of condensed stacking structures with increasing rank. On the other hand, other XRD parameters, viz., the crystallite height (Lc) and the crystallite diameter (La) were not correlated with the rank of the samples. Rock-Eval S2 pyrograms depicted distinctive responses for the vitrains. While smooth curves were observed for the HvbA vitrain, the pyrograms showed unevenness and spikes for the Mvb and Lvb vitrains. We interpret these spikes or ruggedness to be caused due to melt formation during pyrolysis of Lvb and Mvb vitrains, and concomitant gas/bubble-bursting. We back our results with distinctive observations from the field emission scanning electron microscope (FE-SEM) of the pyrolysis-residues of the vitrains. We interpret that the distinctive S2 signatures shown by coals can be useful in predicting their end usage.
Thesis
Les dynamiques de constitution des stocks de matière organique des sols, leur variabilité et leurs relations avec les autres composantes biotiques et abiotiques des écosystèmes sont insuffisamment connues à l’échelle du paysage. Or cette compréhension des interdépendances entre le système pédologique et les autres compartiments environnementaux est cruciale pour prévoir les changements globaux et mesurer l’impact des activités anthropiques. Les milieux de montagnes présentent dans ce contexte des spécificités fortes dues au fait de leur importante variabilité spatiale, des stocks élevés de matière organique contenus dans leurs sols et de leur vulnérabilité aux changements globaux. Par ailleurs, ils subissent intensément les effets du changement climatique. Pour toutes ces raisons, ils offrent un grand intérêt pour l’étude temporelle des dynamiques écosystémiques.En étudiant des sols de montagne aux situations environnementales contrastées, nous avons tenté de comprendre quelles étaient les dynamiques qualitative et quantitative de la matière organique au cours du temps. Pour cela, cette thèse s’est appuyée sur (i) l’étude de sols récemment formés le long de chronoséquences, (ii) de climatoséquences pour l’examen de la variabilité de la matière organique à l’échelle du paysage dans des sols ayant suivis des trajectoires différentes et enfin (iii) sur des simulations expérimentales de changement pédoclimatique pour analyser la réactivité du carbone organique stocké à la surface de sols de prairies de haute altitude.Nos résultats ont mis en évidence, indépendamment des conditions locales de chaque site, un schéma commun d’accumulation de la matière organique lors des phases initiales de formation d’un nouvel écosystème terrestre après retrait glaciaire en différents points du monde. Cette accumulation est affectée par le temps et accélérée par un climat plus chaud. La végétation contribue alors largement à l’incorporation de matière organique relativement labile dans ces sols nouvellement formés. Dans les écosystèmes plus évolués, la stabilisation environnementale par le climat structure en partie la variabilité quantitative et qualitative de la matière organique des sols en montagne à l’échelle du paysage. L’importance de cette structuration par le climat est plus forte dans les horizons de surface des sols qu’en profondeur. La stabilisation environnementale par le climat maintient dans les sols de montagne du carbone organique particulièrement réactif en préservant de la dégradation une matière organique vulnérable à haute altitude. En simulant un réchauffement de 3°C par transplantation d’un sol alpin à l’étage subalpin, nous avons effectivement démontré le rapide relargage de ce carbone organique labile.Ces résultats offrent un éclairage nouveau, à la fois sur la nature de la matière organique des sols de montagne, et aussi sur sa dynamique à-travers le temps et l’espace. La stabilité de la matière organique dans les sols est bien une fonction de l’écosystème.
Article
Pyrolysis is a promising treatment for soil remediation for rapidity and fertility preservation. But it is difficult to establish the relationship between pyrolysis behaviors and soil organic matter (SOM) structures, for SOM is a mixture of heterogeneous compounds. HA sub-fractions from the same soil source may provide a series of promising objects to understand SOM at molecular level and the resulting patterns in SOM pyrolysis. We first propose a novel insight into pyrolysis mechanism response to molecular signatures using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with thermogravimetric analysis (TGA) to study six humic acid (HA) sub-fractions extracted from a forest soil. The findings indicate that decomposition of soil HA occurs systematically due to molecular signatures. The decomposition can be categorized as carboxyl controlled (below 280 °C), lipid-dominated (280–450 °C) and condensed aromatics-dominated processes (450–700 °C). Predominant reaction mechanism of all HA sub-fractions was random nucleation (α > 0.25). Lipid in HA tend to initiate multiple nuclei in thermal degradation, while condensed aromatics tend to initiate and grow centering single random point in higher conversion rate (α > 0.75). Bridging the molecular signature and thermogravimetry reveals that the pyrolysis stage below 350 °C should be divided into two distinct processes related to the carboxylic group and lipid compounds, although this stage has conventionally been considered as a single process. The N element of HA was mostly preserved in the condensed aromatics which was mainly pyrolyzed above 450 °C, suggesting that pyrolysis below 450 °C is a preferable remediation treatment considering nitrogen fertility preservation. The observed molecular-level pyrolysis patterns can be applied as a targeted remediation procedure for contaminated soils and can improve the understanding of SOM thermal behaviors at the molecular level.
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Tropical peatland ecosystems are a significant component of the global carbon cycle and feature a range of distinct vegetation types, but the extent of links between contrasting plant species, peat biogeochemistry and greenhouse gas fluxes remains unclear. Here we assessed how vegetation affects small scale variation of tropical peatland carbon dynamics by quantifying in situ greenhouse gas emissions over 1 month using the closed chamber technique, and peat organic matter properties using Rock-Eval 6 pyrolysis within the rooting zones of canopy palms and broadleaved evergreen trees. Mean methane fluxes ranged from 0.56 to 1.2 mg m⁻² h⁻¹ and were significantly greater closer to plant stems. In addition, pH, ranging from 3.95 to 4.16, was significantly greater closer to stems. A three pool model of organic matter thermal stability (labile, intermediate and passive pools) indicated a large labile pool in surface peat (35–42%), with equivalent carbon stocks of 2236–3065 g m⁻². Methane fluxes were driven by overall substrate availability rather than any specific carbon pool. No peat properties correlated with carbon dioxide fluxes, suggesting a significant role for root respiration, aerobic decomposition and/or methane oxidation. These results demonstrate how vegetation type and inputs, and peat organic matter properties are important determinants of small scale spatial variation of methane fluxes in tropical peatlands that are affected by climate and land use change. Electronic supplementary material The online version of this article (10.1007/s10533-018-0531-1) contains supplementary material, which is available to authorized users.
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Partitioning soil organic carbon (SOC) into two kinetically different fractions that are centennially stable or active is key information for an improved monitoring of soil health and for a more accurate modelling of the carbon cycle. However, all existing SOC fractionation methods isolate SOC fractions that are mixtures of centennially stable and active SOC. If the stable SOC fraction cannot be isolated, it has specific chemical and thermal characteristics that are quickly (ca. 1 h per sample) measureable using Rock-Eval® thermal analysis. An alternative would thus be to (1) train a machine-learning model on the Rock-Eval® thermal analysis data of soil samples from long-term experiments where the size of the centennially stable and active SOC fractions can be estimated, and (2) apply this model on the Rock-Eval® data of unknown soils, to partition SOC into its centennially stable and active fractions. Here, we significantly extend the validity range of the machine-learning model published by Cécillon et al. [Biogeosciences, 15, 2835–2849, 2018, https://doi.org/10.5194/bg-15-2835-2018], and built upon this strategy. The second version of this statistical model, which we propose to name PARTYSOC, uses six European long-term agricultural sites including a bare fallow treatment and one South American vegetation change (C4 to C3 plants) site as reference sites. The European version of the model (PARTYSOCv2.0EU) predicts the proportion of the centennially stable SOC fraction with a conservative root-mean-square error of 0.15 (relative root-mean-square error of 0.27) in a wide range of agricultural topsoils from Northwestern Europe. We plan future expansions of the PARTYSOC global model using additional reference soils developed under diverse pedoclimates and ecosystems, and we already recommend the application of PARTYsocv2.0eu in European agricultural topsoils to provide accurate information on SOC kinetic pools partitioning that may improve the simulations of simple models of SOC dynamics.
Thesis
Le sol est un compartiment clé dans le cycle global du carbone, à la fois par son réservoir, 2 à 3 fois supérieur à celui de l’atmosphère, et par ses flux entre terres émergées et atmosphère. Dans le cadre de la mise en place de politiques d’atténuation du changement climatique, il est nécessaire de mieux connaitre les stocks de carbone organique du sol (COS) et leurs variations lors de changements d’usages. L’évaluation du stock du carbone organique du sol à l’échelle territoriale étant un enjeu méthodologique, il est proposé une méthodologie simple et à bas coût de mesure de la teneur en COS, de la densité apparente et du stock de COS par spectroscopie proche et moyen infrarouge in situ et au laboratoire. Afin de cartographier et quantifier les stocks de carbone organique des sols agricoles de La Réunion, une méthodologie de stratification du territoire en unités pédoclimatiques a été développée. La spectroscopie moyen infrarouge, couplée à une classification non supervisée, a montré son efficacité pour définir des unités pédologiques homogènes (discriminantes) selon un gradient d’altération caractéristique de la pédogenèse en zone volcanique tropicale de La Réunion. En s’appuyant sur les résultats de la stratification du territoire, le calcul des stocks de COS a montré des stocks de carbone organique très élevés pour les sols agricoles de La Réunion (sous canne à sucre, culture majoritaire de l’île, le stock moyen de COS est de 131 MgC ha-1) mais vulnérables aux changements d’usages, notamment lors de changement d’usage de la canne à sucre vers une culture maraichère ou une culture d’ananas (déstockage de COS allant de -14 à -41 % du stock initial de COS). Afin d’évaluer les bilans de gaz à effet de serre (GES) de différents scénarios de changement d’usage des terres agricoles, les variations de stocks de carbone organique du sol selon les changements d’usages agricoles ont été calculées par unité pédoclimatique puis utilisés comme facteur d’émissions Tier 2 dans le calculateur EX-ACT. Les résultats des bilans de GES ont montré qu’une approche spatiale était indispensable pour évaluer les variations de stocks de carbone organique du sol selon les changements d’usages agricoles afin de ne pas modifier l’ampleur et/ou le sens des émissions de GES, notamment pour le compartiment sol. Il est ainsi démontré que la mise en place de systèmes de suivi (Mesure, Rapport, Vérification) fiables pour le carbone organique du sol nécessite de prendre en compte l’hétérogénéité spatiale des stocks de COS et de leurs déterminants.
Article
Soil organic matter (OM) is a complex heterogeneous mixture: resulting from decomposition and organo-mineral interactions, it challenges characterization in terms of composition and biogeochemical stability. From this perspective, the Rock-Eval® method is a rapid and efficient thermal analysis, which combines quantitative and qualitative information on soil OM, including several parameters related to thermal stability. This approach has already been used to monitor changes in OM properties at landscape, cropland, and profile scales. This study aims to assess the stability of soil organic matter pools by characterizing grain-size fractions from forest litters and topsoils using Rock-Eval® thermal analyses. Organic and topsoil samples were selected from a beech forest located in Normandy (France), whose management has been documented for the last 200 years. Fractionation by wet sieving was used to separate large debris (>2000 μm), coarse (200–2000 μm), and fine particulate organic matter (50–200 μm) in organic samples, and coarse (200–2000 μm), medium (50–200 μm), and fine (<50 μm) fractions in topsoils. Rock-Eval® was able to provide thermal parameters sensitive enough to study fine-scale soil processes. In organic layers, quantitative and qualitative changes are all explained by progressive decomposition of labile organic compounds from plant debris to the finest organic particles. On the other hand, the grain size fractions of the topsoil display different characteristics: indeed, the coarse organo-mineral fractions show high C contents, but with a different composition and a higher thermal stability and degree of decomposition than the plant debris forming the organic layers. These results are consistent with previous studies concluding that the microbial activity is more effective in this fraction. The finest fractions of topsoil reveal low C contents and the highest thermal stability, but also a low degree of decomposition, which can be explained by stronger interactions with the mineral matrix. Therefore, it is suggested that the dynamics of OM present in the different size fractions be interpreted in the light of a plant-microbes-soil continuum. Finally, three distinct thermal stability C pools are highlighted through the grain-size heterogeneity of soil OM: free-coarse organic matter (large debris, coarse and fine particles), weakly-protected organic matter in (bio)aggregates (coarse fraction of topsoil), and stabilized organic matter in fine fractions of topsoil, the latter resulting from interactions inside organo-mineral complexes. These results allow Rock-Eval® thermal parameters to be used in order to empirically illustrate the conceptual models emphasizing the role of drivers played by the gradual decomposition and protection of the most thermally labile organic constituents.
Article
Both biotic and abiotic components, characterizing the mountain treeline ecotone, respond differently to climate variations. This study aims at reconstructing climate-driven changes by analyzing soil evolution in the late Holocene and by assessing the climatic trends for the last centuries and years in a key high-altitude climatic treeline (2515 m a.s.l.) on the SW slope of the Becca di Viou mountain (Aosta Valley Region, Italy). This approach is based on soil science and dendrochronological techniques, together with daily air/soil temperature monitoring of four recent growing seasons. Direct measurements show that the ongoing soil temperatures during the growing season, at the treeline and above, are higher than the predicted reference values for the Alpine treeline. Thus, they do not represent a limiting factor for tree establishment and growth, including at the highest altitudes of the potential treeline (2625 m a.s.l.). Dendrochronological evidences show a marked sensitivity of tree-ring growth to early-summer temperatures. During the recent 10-year period 2006–2015, trees at around 2300 m a.s.l. have grown at a rate that is approximately 1.9 times higher than during the 10-year period 1810–1819, one of the coolest periods of the Little Ice Age. On the other hand, soils show only an incipient response to the ongoing climate warming, likely because of its resilience regarding the changeable environmental conditions and the different factors influencing the soil development. The rising air temperature, and the consequent treeline upward shift, could be the cause of a shift from Regosol to soil with more marked Umbric characteristics, but only for soil profiles located on the N facing slopes. Overall, the results of this integrated approach permitted a quantification of the different responses in abiotic and biotic components through time, emphasizing the influence of local station conditions in responding to the past and ongoing climate change.
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Tropical peatlands are a globally important source of methane, a potent greenhouse gas. Vegetation is critical in regulating fluxes, providing a conduit for emissions and regular carbon inputs. However, plant roots also release oxygen, which might mitigate methane efflux through oxidation prior to emission from the peat surface. Here we show, using in situ mesocosms, that root inputs of oxygen reduce methane fluxes by up to 92% depending on species. Methanotroph abundance decreased with reduced oxygen input, demonstrating a likely mechanism for the observed oxidation. These first methane oxidation estimates for a tropical peatland demonstrate that although plants provide an important pathway for methane loss, this is balanced by the influence of root oxygen inputs that mitigate peat surface methane emissions.
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Geochemical indicators are emerging as important predictors of soil organic carbon (SOC) dynamics, but evidence concerning the role of calcium (Ca) is scarce. This study investigates the role of Ca prevalence in SOC accumulation by comparing otherwise similar sites with (CaCO3-bearing) or without carbonates (CaCO3-free). We measured the SOC content and indicators of organic matter quality (C stable isotope composition, expressed as δ 13C values, and thermal stability) in bulk soil samples. We then used sequential sonication and density fractionation (DF) to separate two occluded pools from free and mineral-associated SOC. The SOC content, mass, and δ 13C values were determined in all the fractions. X-ray photoelectron spectroscopy was used to investigate the surface chemistry of selected fractions. Our hypothesis was that occlusion would be more prevalent at the CaCO3-bearing site due to the influence of Ca on aggregation, inhibiting oxidative transformation, and preserving lower δ 13C values. Bulk SOC content was twice as high in the CaCO3-bearing profiles, which also had lower bulk δ 13C values, and more occluded SOC. Yet, contrary to our hypothesis, occlusion only accounted for a small proportion of total SOC (< 10%). Instead, it was the heavy fraction (HF), containing mineral-associated organic C, which accounted for the majority of total SOC and for the lower bulk δ 13C values. Overall, an increased Ca prevalence was associated with a near-doubling of mineral-associated SOC content. Future investigations should now aim to isolate Ca-mediated complexation processes that increase organo-mineral association and preserve organic matter with lower δ 13C values. Supplementary information: The online version of this article (10.1007/s10533-021-00779-7) contains supplementary material, which is available to authorized users.
Article
The fate of organic matter (OM) occluded in earthworm casts during the casts' lifetime is poorly known. We collected casts produced by the anecic earthworm Amynthas khami in tropical woodland in Northern Vietnam at different times after their production. The aim of this study was to assess the (1) the effect of earthworms on biogeochemical OM properties compared to control aggregates, separated from the surrounding soil and (2) the fate of OM during cast ageing. We analyzed five cast ageing stages for elemental content, OM chemical composition by analytical pyrolysis and stability by Rock-Eval thermal analysis. Moreover, we assessed the distribution of organic carbon (OC) in density fractions. Compared to control aggregates, fresh casts had higher OC values (5.3 vs. 2.6%) and increased OC in all density fractions. Cast OM had more lignin (1.1 vs.<0.1%), similar polysaccharide and N-compound contribution and was more thermally labile than those of control aggregates. Changes in OM composition and content during cast ageing appeared at the end of the casts’ lifetime, when they were broken into small sized aggregates (10–13 mm). At these latest stages, OC content decreased (1.5-fold); along with OM in macro-aggregates (2.2-fold) and microaggregates (6.8-fold). Reduced mineral-associated OC was also recorded (1.3-fold). During ageing, the percentage of lignin decreased (5.5-fold) while the thermal stability of OM increased. Most properties were still significantly different between the oldest ageing stage and control aggregates. This study highlights that the impact of earthworms on OM storage goes beyond cast disintegration and that there is a need to better understand their influence on soil organic matter stabilization processes and in particular the formation and stability of organo-mineral associations.
Article
Mountain grasslands contain large stocks of soil organic carbon (SOC), of which a good part is in labile particulate form. This labile SOC may be protected by cold climate that limits microbial activity. Strong climate change in mountain regions threatens to destabilize these SOC stocks. However, so far the climate response of SOC stocks in mountain grasslands remains highly uncertain, under either warming or cooling conditions. To overcome this knowledge gap, we studied the effect of pedoclimatic regime changes on topsoil (0–10 cm) SOC in two complementary experiments: 3 °C of warming or cooling by reciprocal transplanting to an alpine (2450 m a.s.l.) and a subalpine (1950 m a.s.l.) grassland and 1 °C of warming by open-top chambers in the same grasslands. Topsoil SOC stocks were higher at the alpine site than at the subalpine site, and the biogeochemical signature of the soil organic matter (SOM) also differed between the two study sites. SOM was O-enriched, H-depleted, and more thermally stable at the warmer subalpine site. After three years, abrupt warming by transplanting tended to decrease topsoil SOC content. The remaining SOC was characterized by a more thermostable signature. This result suggests the preferential depletion of labile SOC upon experimental topsoil warming. Cooling did not modify overall SOC content but uphill transplanted topsoils showed a more thermolabile biogeochemical signature. In contrast, open-top chamber warming of alpine and subalpine topsoils caused limited changes to SOC stocks and SOM biogeochemical signature, possibly because the induced pedoclimatic change was more limited and more gradual compared to the warming by transplantating which reduced the annual snow cover period by around 60 days and increased cumulative degree days by a factor of ten as compared to the OTC-induced warming. Gradual temperature changes may take longer to become effective than a shock transplant treatment. We conclude that SOC in mountain grassland topsoils can be highly reactive to climate shocks.
Thesis
This work focuses on the characterization of sewage sludges (DSS) and explores several bioremediation strategies. The objective is to find a process to reuse the nutrients (mostly P and N) contained in sewage sludges in agriculture while limiting negative externalities. The first strategy aims to rid the sewage sludges of heavy metals by using the mobilization, translocation and immobilization capacity of certain fungi which, under certain conditions, precipitate oxalates crystals of different metals. The second strategy attempts to explore the abundantly documented ability of basidiomycetes to concentrate heavy metals in there carpophores can be used in bioremediation. The third strategy explores the possibility of recovering nutrients, here phosphorus, from sludges by using the ability of certain bacteria to solubilize phosphates. The various analyzes (XRF, MEB-EDX and Rock-Eval) showed a high content of calcium, silicon, iron, phosphorus in the major elements, a strong presence of zinc, copper, chromium, lead and cadmium in the trace elements, as well as a high content of labile and refractory organic compounds. SEM-EDX analyzes showed a correlation between the presence of iron and phosphorus within the same aggregates, confirming that the phosphorus in the sewage sludges is probably mainly present as iron phosphate. The first strategy test showed a correlation between the presence of sludges and the precipitation of oxalate crystals, but these containing mostly calcium and only a few low detectable traces of other metallic elements. The second test remained in the protocol state although many preliminary steps have already been completed. These preliminary tests verify the selected basidiomycetes optimal conditions for fruiting, which are two strains of Pleurotus ostreatus and a strain of Agaricus bisporus, as well as the affinity of the strains with sewage sludges The tests relating to the third strategy revealed a good phosphates solubilizing capacity in sludge for two strains of bacteria : a Bacillus strain, already known for its capacity to solubilize tricalcium phosphate, and a paenibacillus polymyxa strain, isolated from sewage sludges. The third strategy seems promisingand would allow extraction of the solubilized nutrients by simple phases separation.
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An African oxalogenic tree, the iroko tree (Milicia excelsa), has the property to enhance carbonate precipitation in tropical oxisols, where such accumulations are not expected due to the theoretical acidic conditions of these soils. This uncommon process is linked to the oxalate-carbonate pathway, which increases soil pH through oxalate oxidation. In order to investigate the oxalate-carbonate pathway in the iroko system, fluxes of matter have been identified, described, and evaluated from field to microscopic scales. In the first centimeters of the soil profile, decaying of the organic matter allows the release of whewellite crystals, mainly due to the action of termites and saprophytic fungi. Regarding the carbonate flux, another direct consequence of wood feeding is a concomitant flux of carbonate formed in wood tissues, which is not consumed by termites. Nevertheless, calcite biomineralization of the tree is not a consequence of in situ oxalate consumption, but rather related to the oxalate oxidation inside the upper part of the soil. The consequence of this oxidation is the presence of carbonate ions in the soil solution pumped through the roots, leading to preferential mineralization of the roots and the trunk base. An ideal scenario for the iroko biomineralization and soil carbonate accumulation starts with oxalatization: as the iroko tree grows, the organic matter flux to the soil constitutes the litter. Therefore, an oxalate pool is formed on the forest ground. Then, wood rotting gents (mainly termites, fungi, and bacteria) release significant amounts of oxalate crystals from decaying plant tissues. In addition some of these gents are themselves producers of oxalate (fungi). Both processes contribute to a soil pool of "available" oxalate crystals. Oxalate consumption by oxalotrophic bacteria can start. Carbonate and calcium ions present in the soil solution represent the end products of the oxalate-carbonate pathway. The solution is pumped through the roots, leading to carbonate precipitation. The main pools of carbon are clearly identified as the organic matter (the tree and its organic products), the oxalate crystals, and the various carbonate features. A functional model based on field observations and diagenetic investigations with δ <sup>13</sup>C signatures of the various compartments involved in the local carbon cycle is proposed. It suggests that the iroko ecosystem can act as a long-term carbon sink, as long as the calcium source is related to non-carbonate rocks. Consequently, this carbon sink, driven by the oxalate carbonate pathway around an iroko tree, constitutes a true carbon trapping ecosystem as define by the ecological theory.
Article
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In mine soil, quantification of soil organic carbon (OC) derived recently from biomass decomposition is complicated by the presence of fossil (geogenic) C derived from coal, oil shale, or similar material in the overburden. The only reliable method for such measurement is 14C analysis (i.e. radiocarbon dating) using instrumentation such as accelerator mass spectrometry, which is too expensive for routine laboratory analysis. We tested two previously used and two new methods for recent C quantification and compared them with 14C AMS radiocarbon dating as a reference using a set of soil samples (n = 14) from Sokolov, Czech Republic: (i) 13C isotope ratio composition, (ii) cross polarization magic angle spinning 13C nuclear magnetic resonance (CPMAS 13C NMR) spectroscopy, (iii) near infrared spectroscopy (NIRS) coupled with partial least squares regression and (iv) Rock–Eval pyrolysis. Conventional methods for OC determination (dry combustion, wet dichromate oxidation, loss-on-ignition) were also compared to quantify any bias connected with their use. All the methods provided acceptable recent carbon estimates in the presence of mostly aliphatic fossil C from kerogen. However, the most accurate predictions were obtained with two approaches using Rock–Eval pyrolysis parameters as predictors, namely (i) S2 curve components and (ii) oxygen index (OI). The S2 curve approach is based on the lower thermal stability of recent vs. fossil organic matter. The OI approach corresponded well with 13C NMR spectra, which showed that samples rich in recent C were richer in carboxyl C and O-alkyl C. These two methods showed the greatest potential as routine methods for recent C quantification.
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Rock-Eval 6 Technology: Performances and Developments-The Rock-Eval 6 apparatus is the latest version of the Rock-Eval product line, commercialized since 1996 by Vinci Technologies. The present work describes the methodology developed at IFP for reliable data acquisition and endorses the quality of geochemical parameters acquired with Rock-Eval 6. Data were obtained on 147 source rocks from various sedimentary basins, of different organic matter types and maturity stages. Intrinsic correlations for two different Rock-Eval 6 apparatus were performed and the obtained data set shows an excellent consistency and good reproducibility conditions for the whole set of Rock-Eval parameters. Complete recovery of total carbon (TC) by Rock-Eval 6 was confirmed by comparison with elemental analysis. In order to check the carbon partition (mineral vs. organic) determined by Rock-Eval 6, measurements of mineral carbon (MinC) and total organic carbon (TOC) were performed by alternative techniques. TOC measured by Rock-Eval 6 was compared to that obtained either by: the Leco apparatus for bulk rocks: elemental analysis for kerogens; and calculation from the mass balance determined after destruction of mineral matrix and the carbon concentration determined by elemental analysis on recovered kerogens for bulk rocks. The results display a good correlation for the whole concentration range (0-90 wt% TOC), when comparing elemental analyses and Rock-Eval 6 for source rocks and kerogens. However, comparison of Rock-Eval 6 with Leco data leads to larger deviations while correlation factors are still good. For a subset of kerogen samples, preparative pyrolysis was performed in order to confirm the value of 83 wt% for the organic carbon of the total S2 peak for any rock with any organic type and to check the absolute value of the S2 peak by gas chromatography analysis of pyrolysis by-product. MinC measured with Rock-Eval 6 was compared to that determined by: weight loss after HCI treatment: the acidimetry technique; and calculation after TC, mass balance from kerogen isolation and organic carbon measurement on kerogen by elemental analysis. The results display a good correlation for the whole concentration range (0-12 wt% MinC), when comparing elemental analyses and Rock-Eval 6. However, comparison of Rock-Eval 6 with acidimetry data leads to larger deviations while correlation factors are still good while comparison with weight loss is poor. As a whole an excellent reliability of TOC and MinC obtained by Rock-Eval 6 was demonstrated, and consequently, it is now possible to get at once the total organic and mineral carbon mass balance for a given rock. Recommendations are proposed regarding the standard samples and analytical methods selected for calibrating the Rock-Eval 6 over a large mineral and organic carbon range. Consistency between S2 and Tmax measured by Rock-Eval 2 and Rock-Eval 6 for Types I and II bulk rocks was also checked. A good correlation was obtained for S2, even though S2 values are slightly higher when measured with Rock-Eval 2. It was demonstrated that this is due to carrier gas (nitrogen vs. helium) by running measurements with a Rock-Eval 6 under helium, the difference ranging from 5 to 10 relative wt% for most studied samples. For Tmax correlation, data are much more scattered and as a general trend Tmax obtained by Rock-Eval 6 are higher than Tmax obtained by Rock-Eval 2 and the difference increases with Tmax: this is due to the fact that the probe measuring the temperature in the Rock-Eval 2 is located in the oven wall, consequently Tmax determination is highly dependent on the setup and calibration of the apparatus. A special attention was given for temperature measurement in the Rock-Eval 6, where the probe is in contact with the crucible containing the sample, leading to much more reliable data.
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Organic matter (OM) is a key component of soils but information on its chemistry and behavior in soils is still incomplete. Numerous methods are commonly used to characterize and monitor OM dynamics, but only a few include the qualities required to become routine techniques i.e. simple, rapid, accurate and at low cost. Rock–Eval pyrolysis (RE pyrolysis) is a good candidate, as it provides an overview of OM properties by monitoring four com- ponents related to the main major classes of organic constituents (from A1 for the labile biological constit- uents to A4 for the mature refractory fraction). However, a question is still pending: do these four major classes used in the literature reflect a pertinent compositional chemical counterpart? 13C Nuclear Magnetic Resonance Spectroscopy in the solid state (13C CPMAS NMR) has been used to answer this question by collecting information on structural and conformational characteristics of OM. Moreover, in order to avoid the blurring effect of pedogenesis on OM dynamics, a ‘‘less complex OM’’ source, i.e. compost samples, has been used. Results showed significant and high determination coefficients between classes, indi- ces (of transformation of plant biopolymers, humifi- cation...) from RE pyrolysis, and the main classes of OM characterized by 13C NMR, e.g. A1 & A2 with labile/easily degradable components (alkyl C et O-alkyl C), A3 & A4 with humified OM (with aromatic C and phenolic C). The R index (contribution of bio- macromolecules) is correlated with phenolic and aromatic C, whereas the I index (related to immature OM) refers to labile––easily degradable components (alkyl C et O-alkyl C). The results confirm the pertinence of RE pyrolysis to monitor OM dynamics.