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... To obtain further proxies for the degree of decomposition of plant-derived OM toward more microbiallyprocessed OM, two additional methods were applied: (a) An assessment of the relative chemical composition of OM by diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) (Demyan et al., 2012), and (b) an assessment of thermal stability by Rock-Eval pyrolysis (Sebag et al., 2016) (Figure 2b). ...
... In parallel to the above, but on bulk soil only, Rock-Eval pyrolysis was performed to create a second and independent proxy for the degree of OM decomposition through microbial transformation related to its decomposition and stabilization (Sebag et al., 2016). For this, we used 0.1 g milled bulk soil samples, and conducted the following analyses on a pyrolyzer (Vinci Technologies, Rock-Eval 6, France). ...
... The applied protocol consisted of two phases: a pyrolysis in an inert N 2 atmosphere starting at a temperature of 200°C until 650°C, and a pyrolysis in an oxidized atmosphere between 400°C and 850°C, both with a heating rate of 25°C min 1 . Subsequently, the Rock-Eval I-Index for the degree of biological transformation of OM was calculated (Sebag et al., 2016). Briefly, areas under defined segments of the S2 curve (i.e., the hydrocarbons that form during thermal pyrolysis) were used as follows : Equation 3. ...
Organic matter accumulation in soil is understood as the result of the dynamics between mineral‐associated (more decomposed, microbial derived) organic matter and free particulate (less decomposed, plant derived) organic matter. However, from regional to global scales, patterns and drivers behind main soil organic carbon (SOC) fractions are not well understood and remain poorly linked to the pedogenetic variation across soil types. Here, we separated SOC associated with silt‐ and clay‐sized particles (S + C), stable aggregates (>63 μm, SA) and particulate organic matter (POM) from a diverse range of grassland topsoils sampled along a geoclimatic gradient. The relative contribution of the two mineral‐associated fractions (S + C & SA) to SOC differed significantly across the gradient, while POM was never the dominant SOC fraction. Stable aggregates (>63 μm) emerged as the major SOC fraction in carbon‐rich soils. The degree of decomposition of carbon in stable aggregates (>63 μm) was consistently between that of the S + C and POM fractions and did not change along the investigated gradient. In contrast, carbon associated with the S + C fraction was less microbially decomposed in carbon‐rich soils than in carbon‐poor soils. The amount of SOC in the S + C fraction was positively correlated to pedogenic oxide contents and texture, whereas the amount of SOC associated with stable aggregates (>63 μm) was positively correlated to pedogenic oxide contents and negatively to temperature. We present a conceptual summary of our findings, which integrates the role of stable aggregates (>63 μm) with other major SOC fractions and illustrates their changing importance across (soil‐)environmental gradients.
... It allowed us to highlight the link between the thermal properties of SOM and its stability in soils (Plante et al., 2009(Plante et al., , 2011. Since then, several studies have demonstrated the effectiveness of the method in measuring SOM content and distinguishing its labile, resistant, and refractory thermal pools (Albrecht et al., 2015;Saenger et al., 2013;Sebag et al., 2016;Soucémarianadin et al., 2018;Zhang et al., 2023). The links between the thermal stability of the different pools and their bioavailability to soil microorganisms were discussed in a few studies (Barré et al., 2016;Gregorich et al., 2015). ...
... In complement to these standard parameters, Sebag et al. (2006Sebag et al. ( , 2016 showed that the HC thermogram, denoted S2 in the literature, can be decomposed into five C thermal pools defined by their range of cracking temperature, by splitting the surface area of the thermogram in five areas based on cracking temperature thresholds. These pools were categorized from thermal most-labile pool (A1 between 200 and 340 • C), labile pool (A2 between 340 and 400 • C), resistant pool (A3 between 400 and 460 • C), refractory pool (A4 between 460 and 520 • C) and most-refractory pool (A5 between 520 and 650 • C) (Fig. 2). ...
... The negative correlation between OI and HI accords with previous observations (e.g. Disnar et al., 2003;Gregorich et al., 2015;Sebag et al., 2016;Soucémarianadin et al., 2018). ...
... To assess the relative degree of decomposition and thermal lability versus stability of MOM and implied biogeochemical stability in soil [68], we applied Rock-Eval 6 pyrolysis analysis. In addition to commonly reported OI and HI values representing the ratio of H:C and O:C in SOM [69], we calculated I-and R-indices ('immature' and 'refractory', respectively) designed for SOM comparison [70]. ...
... Rock-Eval 6 indices I (immature) and R (refractory) that better describe SOM were developed by Sebag et al. [70]. These I and R indices were calculated by comparing the relative areas of the pyrograms. ...
... Following the hypothesis that greater activation energy (E a ) corresponds to stability in soil, 'labile' forms of OC should be oxidized at lower temperature, decompose more easily, and be younger than the mean MOM 14 C age, while high E a OC should be older and more aromatic in nature [22]. This was generally observed across all samples ( figure 3), with the additional observation that the low E a OC was associated with pedogenic oxide minerals (SRO and CO), while high E a OC was associated with clay minerals, Figure 4. Results of Rock-Eval 6 SOM across samples calculated to assess the degree of decomposition using I-index (immaturity) and the thermal stability using the R-index (refractory) [70]. ...
Organic carbon (OC) association with soil minerals stabilizes OC on timescales reflecting the strength of mineral–C interactions. We applied ramped thermal oxidation to subsoil B horizons with different mineral–C associations to separate OC according to increasing temperature of oxidation, i.e. thermal activation energy. Generally, OC released at lower temperatures was richer in bioavailable forms like polysaccharides, while OC released at higher temperatures was more aromatic. Organic carbon associated with pedogenic oxides was released at lower temperatures and had a narrow range of ¹⁴C content. By contrast, N-rich compounds were released at higher temperatures from samples with 2 : 1 clays and short-range ordered (SRO) amorphous minerals. Temperatures of release overlapped for SRO minerals and crystalline oxides, although the mean age of OC released was older for the SRO. In soils with more mixed mineralogy, the added presence of older OC released at temperatures greater than 450°C from clays resulted in a broader distribution of OC ages within the sample, especially for soils rich in 2 : 1 layer expandable clays such as smectite. While pedogenic setting affects mineral stability and absolute OC age, mineralogy controls the structure of OC age distribution within a sample, which may provide insight into model structures and OC dynamics under changing conditions.
This article is part of the Theo Murphy meeting issue ‘Radiocarbon in the Anthropocene’.
... The thermal stability of OM in the GWS was determined by 1) deconvolution of temperature nodes of 200-340 • C (A1), 340-400 • C (A2), 400-460 • C (A3) and >460 • C (A4), 2) determination of the immature OM index (I-index) and the refractory OM index (R-index), I − index = Log 10 ((A1 + A2) / A3), and R − index = ((A3 +A4) /100) (Brown et al., 2023;Garcin et al., 2022;Sebag et al., 2016). ...
... The thermal stability of OM released in the pyrolysis cracking stages of the RE analysis can be explored using mathematical deconvolution of the S2 peak, either by calculating fractions using the base/tops of peaks or by focusing on peak areas in specified temperature ranges (Newell et al., 2016;Sebag et al., 2016;Ordoñez et al., 2019). Only Haeseler et al. (1999) has applied the deconvolution of S2 to GWS previously, but only into two fractions whereas recent studies use 4-5 temperature fractions. ...
... Only Haeseler et al. (1999) has applied the deconvolution of S2 to GWS previously, but only into two fractions whereas recent studies use 4-5 temperature fractions. This study explored the selected temperature ranges specified by Sebag et al. (2016), Ordoñez et al. (2019), Malou et al. (2020) and Haeseler et al. (1999) (SI Figure S5). A positive correlation (R 2 = 0.6245) between the log transformed ∑ PAH51 concentrations and the lowest temperature range fraction percentage indicated that hydrocarbons cracked at these lower temperature ranges (A1) (Fig. 2b) included PAHs. ...
... In addition to the above standard parameters, Disnar et al. (2003) proposed using the shape of thermograms to obtain additional information about OM quality. In the present study, we used the I index to quantify the preservation of the thermolabile C pool (Sebag et al., 2016). This index is derived from the integrated S2 areas in specific temperature ranges (200-400 • C, 400-460 • C, and > 460 • C), and is usually interpreted in terms of specific thresholds of the thermal stability of organic compounds, separating the thermolabile, thermoresistant, and thermorefractory C pools (Disnar et al., 2003;Sebag et al., 2006;Saenger et al., 2013). ...
... The decreasing I-index observed in the NGaoundaba peat deposit from 6.4 ka cal BP to the surface reflects the decrease of thermolabile compounds and the relative increase in thermostable compounds, reflecting the progressive decomposition of labile organic matter and the increase in thermal stability of organic matter from 6.4 ka cal BP until present (Sebag et al., 2016) (Fig. 5(D)). During this period, raw TOC increases, indicating an enrichment in thermostable C-rich compounds relatively to thermolabile compounds more rich in heteroatoms. ...
... The comparison between residual TOC and HI from the same peat core shows striking similarities ( Fig. 6(D) & (G)). The HI can be controlled by the degree of decomposition but can also reflect changes in the origin of the organic matter (Sebag et al., 2016). The higher values observed between 8 and 6 ka cal BP and at the bottom of the core tend to indicate well preserved terrestrial organic matter that is consistent with the TOC data and the wettest period of the African Humid Period. ...
... Soil samples were analysed at IFPEN laboratory (Institut Français du Pétrole É nergies Nouvelles) with a Rock-Eval 6 device. This technique has been recommended for characterizing soil organic matter (Sebag et al., 2006;Derenne and Quéné, 2015;Disnar et al., 2003) and has proven relevance in various contexts (Malou et al., 2020;Romanens et al., 2019;Sebag et al., 2016;Thoumazeau et al., 2020), and for studying the impact of soil engineering organisms (Schomburg et al., 2019(Schomburg et al., , 2018Le Mer et al., 2020). The method uses ramped pyrolysis of organic matter under an artificial air supply (N 2 ) between 200 and 650 • C and then the combustion of residual carbon under oxidative conditions between 200 and 850 • C. The released gases are quantified using a flame ionization detector (FID) for hydrocarbon compounds (HC) and infrared detectors (IR) for CO and CO 2 . ...
... Soil organic matter quality was assessed with the hydrogen index (HI in mg HC/g TOC), calculated as the total amount of HC normalized to the TOC content. Soil organic matter thermal stability was assessed by combining two indices (denoted R-and I-index) calculated from five subdivided areas of the S2 thermograms related to HC (Sebag et al., 2016). By construction, the R-index relates to the thermally resistant and refractory pools of organic matter, while the I-index is related to the ratio between the thermally labile and resistant pools. ...
... As derived from a mathematical construct, these two indices may be inversely correlated when OM stabilization results from progressive decomposition of labile organic compounds and relative enrichment in refractory compounds. Then, in the I/R diagram, the same "decomposition line model" describes the decreasing labile pools and concomitant increase in more thermally stable pools, as observed in compost (Albrecht et al., 2015) and in soils (Sebag et al., 2016;Matteodo et al., 2018;Thoumazeau et al., 2020). I/R diagrams were used to calculate the deviation of I-index values from LM and TN to those predicted from the regression line of the control soil. ...
In Cambodia, termite mounds are commonly used by farmers as amendments to increase the fertility of their paddy fields. However, despite their utilization, their chemical and physical properties have not been described yet. Therefore, the aim of this study was to analyze the chemical and physical properties of two termite constructions commonly found in paddy fields: (a) termitaria built and occupied by the fungus-growing termite Macrotermes gilvus and (b) lenticular mounds that are initially built by termites but host a large diversity of other invertebrates and plants. This study shows that these biogenic structures have very specific properties. Termitaria were characterized by higher clay, phosphorus and electrical conductivity than the surrounding soil. However, their effect on carbon dynamics was limited to a modification of the interactions between soil organic matter and minerals and to the presence of carbonates. At the same time, lenticular mounds appeared as patches of nutrients in paddy fields because they were always enriched in carbon, nitrogen, and phosphorus in comparison with the surrounding cultivated soil. Lenticular mounds were also enriched in clay, although this effect was only measured when the sand content in the surrounding environment was >60%. Together with these changes, lenticular mounds were characterized by a lower bulk density, higher saturated hydraulic conductivity (Ksat), and higher water holding capacity. In conclusion, this study shows that termite constructions can be considered fertility and biogeochemical hotspots in paddy fields, thus explaining their use by farmers for improving the fertility of their lands.
... However, this index does not consider the quality and the complexity features of each organic fraction and other complementary protocols were thus investigated such as Rock-Eval® anaylsis (RE) (Albrecht et al., 2015;Barré et al., 2016;Jimenez et al., 2015;David Sebag et al., 2006;Sebag et al., 2022aSebag et al., , 2022b. Rock-Eval® thermal analysis was originally designed for petroleum evaluation (Espitalie et al., 1986) and is a tool that can also be used for soil issues (Gregorich et al., 2015;Saenger et al., 2015;Sebag et al., 2016Sebag et al., , 2022aSebag et al., , 2022bSoucémarianadin et al., 2018). With this method, the quantity of organic and inorganic carbon in a product is estimated, but its quality in terms of labile and resistant or refractory organic carbon is also defined and its degree of thermal stability is evaluated. ...
... To characterize OM quality defined by the thermal stability and described by the thermolabile and thermostable carbon pools of our samples, we focused on the S2 peak. This S2 peak was divided into five thermal ranges (A1: 200-340 • C, A2: 340-400 • C, A3: 400-460 • C, A4: 460-520 • C and A5: 520-650 • C) according to Sebag et al. (2016), characterizing three thermal SOC pools: labile (A1 and A2) and stable (resistant (A3) and refractory (A4 and A5). Based on the integration of these five S2 sub-signals, Sebag et al. (2016) proposed two new indices: the I-index reflecting the amplitude of mineralization (I index = log (A1 + A2/A3) and the Rindex reflecting the carbon stability (R index = (A3 + A4 + A5)/100). ...
... This S2 peak was divided into five thermal ranges (A1: 200-340 • C, A2: 340-400 • C, A3: 400-460 • C, A4: 460-520 • C and A5: 520-650 • C) according to Sebag et al. (2016), characterizing three thermal SOC pools: labile (A1 and A2) and stable (resistant (A3) and refractory (A4 and A5). Based on the integration of these five S2 sub-signals, Sebag et al. (2016) proposed two new indices: the I-index reflecting the amplitude of mineralization (I index = log (A1 + A2/A3) and the Rindex reflecting the carbon stability (R index = (A3 + A4 + A5)/100). Fig. 1 summarizes the RE principle. ...
Organic fraction of municipal solid waste can be recovered using a variety of processes including composting,
anaerobic digestion and vermicomposting. The impact of these products was evaluated using an innovative
approach combining quantitative and qualitative analyses. Chemical analysis showed that the organic matter
content of dry compost, digestate and vermicompost was 76 %, 67.2 % and 41.7 %, respectively. Index of
Recalcitrant Organic Carbon (IROC) was higher for vermicompost (87.2 %) than for compost (77.9 %) or digestate
(66.9 %). The Rock-Eval® approach was correlated to the IROC with its R-index (R2 = 0.97). Transmission
electron microscopy was used to describe the microbial activity, the decomposition state and we observed more
advanced maturity from vermicompost, compost to digestate. Finally, the digestate is predicted to have a
fertilizing effect whereas the compost should have more an amending effect in the medium/long term. Due to a
higher degree of stability, the vermicompost could have both a fertilizing and amending effect.
... In addition to the above standard parameters, Disnar et al. (2003) proposed using the shape of thermograms to obtain additional information about OM quality. In the present study, we used the I index to quantify the preservation of the thermolabile C pool (Sebag et al., 2016). This index is derived from the integrated S2 areas in specific temperature ranges (200-400 • C, 400-460 • C, and > 460 • C), and is usually interpreted in terms of specific thresholds of the thermal stability of organic compounds, separating the thermolabile, thermoresistant, and thermorefractory C pools (Disnar et al., 2003;Sebag et al., 2006;Saenger et al., 2013). ...
... The decreasing I-index observed in the NGaoundaba peat deposit from 6.4 ka cal BP to the surface reflects the decrease of thermolabile compounds and the relative increase in thermostable compounds, reflecting the progressive decomposition of labile organic matter and the increase in thermal stability of organic matter from 6.4 ka cal BP until present (Sebag et al., 2016) (Fig. 5(D)). During this period, raw TOC increases, indicating an enrichment in thermostable C-rich compounds relatively to thermolabile compounds more rich in heteroatoms. ...
... The comparison between residual TOC and HI from the same peat core shows striking similarities ( Fig. 6(D) & (G)). The HI can be controlled by the degree of decomposition but can also reflect changes in the origin of the organic matter (Sebag et al., 2016). The higher values observed between 8 and 6 ka cal BP and at the bottom of the core tend to indicate well preserved terrestrial organic matter that is consistent with the TOC data and the wettest period of the African Humid Period. ...
... 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. ...
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.
... 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. ...
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.
... 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). ...
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.
... 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. ...
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
... 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). ...
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.
... The biological impact of ERW was assessed by looking at whether rock powder addition had led to changes in the abundance and stability of SOM and in the biological activity of soil organisms at the microbial, meso-and macrofauna levels. Soil organic C content and thermostability were measured using Rock-Eval pyrolysis according to Sebag et al. (2016). The relative importance of thermally labile and highly thermostable organic matter fractions was calculated using Rock-Eval indices, respectively named I and R, based on the approach proposed by Sebag et al. (2016). ...
... Soil organic C content and thermostability were measured using Rock-Eval pyrolysis according to Sebag et al. (2016). The relative importance of thermally labile and highly thermostable organic matter fractions was calculated using Rock-Eval indices, respectively named I and R, based on the approach proposed by Sebag et al. (2016). Total nitrogen content was quantified by automated dry combustion to calculate the C:N ratio used here as a proxy of SOM degradability (Bengtsson et al., 2003). ...
Terrestrial enhanced rock weathering (ERW) is a promising carbon dioxide removal technology that consists in applying ground silicate rock such as basalt on agricultural soils. On top of carbon sequestration, ERW has the potential to raise the soil pH and release nutrients, thereby improving soil fertility. Despite these possible co-benefits, concerns such as heavy metal pollution or soil structure damage have also been raised. To our knowledge, these contrasted potential effects of ERW on soil fertility have not yet been simultaneously investigated. This field trial aimed at assessing the impact of ERW on biological, physical, and chemical soil properties in a temperate agricultural context. To do so, three vineyard fields in Switzerland were selected for their distinct geochemical properties and were amended with basaltic rock powder at a dose of 20 tons per hectare (2 kg.m−2). On each field, basaltic rock powder was either applied one year before the sampling campaign, one month before the sampling campaign, or not applied (control) for a total of 27 plots with 9 repetitions of each level. Overall, basaltic rock powder addition had a predominantly positive to neutral effect on soil fertility. Most soil properties showed no significant change either 1 month or 1 year post application. Nevertheless, our study highlighted a significant increase in earthworm abundance (+71 % on average), soil respiration (+50 %) and extractable sodium concentration (+23 %) as early as 1 month post application. The higher soil respiration raises the question of CO2 losses from organic matter mineralization that could limit ERW's efficiency. The increase in sodium raises concerns about a sodification risk potentially damaging soil fertility. These elements now require further investigation before enhanced rock weathering can be considered a viable and secure carbon dioxide removal technology.
... In addition to the soil C content, we evaluated the quality of the rhizospheric soil organic carbon (SOC) with the rock-eval ramped thermal analysis which allows the SOC to be divided into thermally labile and thermally stable fractions (I index and R index, respectively) 41 . Ramped thermal analyses of soil samples have been shown to be a promising technique to disentangle distinct organic matter (OM) compounds differing in the energy needed for thermal decomposition [42][43][44] and has been shown to be informative of microbial activity and functioning in soils 45 . ...
... This method allows us to obtain the soil total organic carbon percent (TOC) in these soils. Additionally, the resultant thermogram also allow us to calculate the I index ("labile carbon fraction") and R index ("recalcitrant carbon fraction") as previously 41 . ...
Expanding and intensifying agriculture has led to a loss of soil carbon. As agroecosystems cover over 40% of Earth’s land surface, they must be part of the solution put in action to mitigate climate change. Development of efficient management practices to maximize soil carbon retention is currently limited, in part, by a poor understanding of how plants, which input carbon to soil, and microbes, which determine its fate there, interact. Here we implement a diversity gradient by intercropping undersown species with barley in a large field trial, ranging from one to eight undersown species. We find that increasing plant diversity strengthens positive associations within the rhizosphere soil microbial community in relation to negative associations. These associations, in turn, enhance community carbon use efficiency. Jointly, our results highlight how increasing plant diversity in agriculture can be used as a management strategy to enhance carbon retention potential in agricultural soils.
... RockEval pyrolysis was done to assess thermostability of SOC as in Sebag et al. (2016). Milled soil samples were pyrolyzed on a Rock-Eval 6 device (Vinci Technologies, France), first in an N 2 atmosphere between 200 and 650°C, and second in an oxidized atmosphere between 400 and 850°C, both with a heating rate of 25°C min −1 . ...
... Subsequently, the Rock-Eval I-Index was calculated exactly as in Sebag et al. (2016) to estimate the degree of biological transformation of SOC. The higher the I-index value, the less biologically transformed is the bulk SOC. ...
Soil microbial traits and functions play a central role in soil organic carbon (SOC) dynamics. However, at the macroscale (regional to global) it is still unresolved whether (i) specific environmental attributes (e.g., climate, geology, soil types) or (ii) microbial community composition drive key microbial traits and functions directly. To address this knowledge gap, we used 33 grassland topsoils (0–10 cm) from a geoclimatic gradient in Chile. First, we incubated the soils for 1 week in favorable standardized conditions and quantified a wide range of soil microbial traits and functions such as microbial biomass carbon (MBC), enzyme kinetics, microbial respiration, growth rates as well as carbon use efficiency (CUE). Second, we characterized climatic and physicochemical properties as well as bacterial and fungal community composition of the soils. We then applied regression analysis to investigate how strongly the measured microbial traits and functions were linked with the environmental setting versus microbial community composition. We show that environmental attributes (predominantly the amount of soil organic matter) determined patterns of MBC along the gradient, which in turn explained microbial respiration and growth rates. However, respiration and growth normalized for MBC (i.e., specific respiration and growth) were more linked to microbial community composition than environmental attributes. Notably, both specific respiration and growth followed distinct trends and were related to different parts of the microbial community, which in turn resulted in strong effects on microbial CUE. We conclude that even at the macroscale, CUE is the result of physiologically decoupled aspects of microbial metabolism, which in turn is partially determined by microbial community composition. The environmental setting and microbial community composition affect different microbial traits and functions, and therefore both factors need to be considered in the context of macroscale SOC dynamics.
... Rock-Eval® pyrolysis is a rapid and cost-effective method for determining the organic matter (OM) content of samples, including both organic and inorganic carbon fractions. The temperature-programmed pyrolyzer identifies OM sources, maturation types, and stages in diverse environments, such as soil (e.g., Sebag et al., 2016;Le Meur et al., 2021), recent lake sediments (e.g., Boussafir et al., 2012;Zhang et al., 2023), marine sediments (e.g., Baudin et al., 2015 and references therein), and mangrove sediments (e.g., Duan et al., 2020;Marchand et al., 2008). Rock-Eval can serve as a simulation of the natural OM decomposition during early diagenesis in sediments (Disnar, 1994;Williams and Rosenheim, 2015) and facilitates the determination of variations in OM stability over depth, time, and across different sites. ...
... The S2 pyrogram often does not follow a Gaussian distribution in immature samples such as mangrove sediments. Instead, it has multiple peaks or shoulders, indicating that the samples consist of a mixture of different fractions or clusters of organic components undergoing thermal degradation at various specific temperatures (e.g., Disnar and Trichet, 1984;Disnar et al., 2003;Sebag et al., 2016). To identify these distinct clusters from the multilobed S2 signal, mathematical deconvolution was used to isolate five elemental Gaussian distributions with peaks at well-defined temperatures of 300 • C, 360 • C, 415 • C, 470 • C, and 560 • C (±15 • C) (Table S1; Figs S1 to S4 in Supplementary Material). ...
Mangroves are one of the most Blue Carbon-rich ecosystems worldwide, as they are highly efficient at storing and sequestering a large amount of organic carbon (C org). The degradation of C org inventories in mangrove sediments could cause carbon dioxide (CO 2) emissions, contributing to atmospheric warming. In this study, we used Rock-Eval pyrolysis and palynofacies identification to explore the composition and sources of organic matter (OM) and the quantity and liability of C org in four 210 Pb-dated sediment cores from contrasting Mexican mangrove areas. The composition of terrestrial and refractory OM was similar in all cores, with variations attributed to the influence of the local river discharges on OM inputs and preservation. A progressive decrease in C org quantity and lability from 2021 to 1990 in some cores was attributed to early diagenesis. Past precipitation and river discharge events appeared to have influenced carbon accumulation and preservation: increased influx and preservation of labile C org in the sediments occurred during low river discharge and precipitation, whereas larger inputs and oxidation of refractory C org occurred during high river discharge and precipitation. Sedimentary C org stocks, assessed for 1921-2021, were primarily composed of refractory organic components, with degradation of allochthonous and autochthonous C org mainly occurring before sediment burial. Sediments acted as efficient and long-term sinks for the C org supplied to these mangroves, particularly in the context of increasing C org inputs caused by an acceleration since the 1950s in continental erosion.
... This technique initially developed for the analysis of petroleum source rocks (Espitalié et al., 1977(Espitalié et al., , 1985a(Espitalié et al., ,b, 1986Lafargue et al., 1998;Behar et al., 2001), has proven to be effective in measuring the organic and mineral carbon content in soils while characterising SOM quality (Di-Giovanni et al., 1998;Disnar et al., 2003;Sebag et al., 2006;Barré et al., 2016). A RE analysis provides a wide bench of indicators related to the stability of the SOM (e.g., Albrecht et al., 2015;Sebag et al., 2016;Soucémarianadin et al., 2018Soucémarianadin et al., , 2019Malou et al., 2023). Moreover, RE thermal analysis coupled with a machine-learning approach lead to the development of a model, named PartySOC, which predicts the fraction of SOC with long turnover time (>100 years, stable carbon) versus the fraction of SOC with a decadal turnover time (active carbon) for European soils (Cécillon et al., , 2021Kanari et al., 2022). ...
... Moreover, RE thermal analysis coupled with a machine-learning approach lead to the development of a model, named PartySOC, which predicts the fraction of SOC with long turnover time (>100 years, stable carbon) versus the fraction of SOC with a decadal turnover time (active carbon) for European soils (Cécillon et al., , 2021Kanari et al., 2022). RE is relatively quick (ca. 1 h/sample) and has already been used to characterise large sample sets (>2000 samples) (Sebag et al., 2016;Delahaie et al., 2022). ...
Rock-Eval® (RE) is a thermal analysis technique increasingly used to characterise soil organic matter. To interpret the results, particularly when investigating differences between samples, it is necessary to know the expected ranges of analytical error associated with the RE measurements. Moreover, the RE analyzer is now at its seventh version (RE7) while most literature results were produced using the previous version (RE6). Thus, a characterization of the reproducibility of RE measurements is necessary. We measured the reproducibility of RE measurements using fifteen samples from French croplands and forests that were analysed on five different RE instruments, located in different laboratories and belonging to both generations RE6 and RE7. From each RE analysis, we extracted RE parameters commonly used for soil organic matter characterization and we performed the prediction of the active and stable soil organic carbon fractions using a machine learning model (PartySOC) that uses RE parameters. We obtained a measure of the expected relative errors in RE parameters and PartySOC predictions per instrument, across instruments of the same generation and across generations. We found that the parameters total organic carbon (TOC), mineral carbon (MinC) and R-index are well reproducible, even across the RE6 and RE7 generations. Instead, the hydrogen index (HI) and oxygen index (OI) are more sensitive to signal variations, even within the same generation, especially when TOC is low. The PartySOC predictions were well reproducible across RE6 instruments but not across RE generations. In the future, the results of this study will help discriminate relevant differences between soil samples characterised using RE thermal analysis.
... Alongside HI and OI, the immature organic matter index (I-index) and the refractory organic matter index (R-index) are used in this study as measures of the intrinsic organic matter stability of the peat (Sebag et al., 2016). The I-index is calculated using the lower temperature zones representing more labile organic matter compounds. ...
... The decrease in HI and OI values with depth reflected more degraded plant material and increased humification of soil organic matter (Hetényi et al., 2006;Carrie et al., 2012;Sebag et al., 2016). ...
Tropical peatland condition across southeast Asia is deteriorating as a result of conversion to agriculture and urban zones. Conversion begins by lowering the water table, which leads to peat decomposition, subsidence and increased risk of large-scale forest fires. Associated changes in mechanical peat properties impact the magnitude and timing of changes in peatland surface motion, making them a potential proxy for peatland condition. However, such a relationship is yet to be observed in a tropical peatland setting. This study aimed to establish whether patterns of tropical peatland surface motion were a function of peat condition at North Selangor Peat Swamp Forest in Selangor, Malaysia. Results showed that subsidence was greatest at fire-affected scrubland sites, whilst the lowest mean water table levels were found at smallholder oil palm sites. Peat condition and magnitude of tropical peat surface oscillation were significantly different between peat condition classes, whilst peat condition differed with depth. More degraded tropical peats with high bulk density throughout the peat profile due to high surface loading and low mean water table levels showed greater surface oscillation magnitudes. The dominant peat surface oscillation mechanisms present at all sites were compression and shrinkage from changes in water table level. Mean water table level and subsidence rate were related to surface oscillation magnitude. However further work towards measuring surface and within-water table range bulk densities and surface loading is required to better understand the controls on surface oscillation magnitudes.
... To correct for light scatter, spectra were resampled to a range of 4000-600 cm − 1 with duplicates averaged and normalized using the normal variate method available within the R packages. Then, based on published information 62 , the following six wavenumber ranges were assigned to three types of functional groups: In parallel to the above, but on bulk soil only, Rock-Eval pyrolysis was performed to create a second and independent proxy for the degree of OM transformation related to its decomposition and stabilization 59 . For descriptive statistics summarizing all quantitative and qualitative SOC data of the fractions, see Table S8. ...
... The applied protocol consisted of two phases: a pyrolysis in an inert N 2 atmosphere starting at a temperature of 200°C until 650°C, and a pyrolysis in an oxidized atmosphere between 400°C and 850°C, both with a heating rate of 25°C min − 1 . Subsequently, the Rock-Eval I-Index for the degree of biological transformation of OM was calculated59 . Brie y, areas under de ned segments of the S2 curve (i.e., the hydrocarbons that form during thermal pyrolysis) were used following Eq. ...
Organic matter accumulation in soil is understood as the result of the dynamics between mineral-associated (often more decomposed, microbial derived) organic matter and free particulate (often less decomposed, plant derived) organic matter. However, at global scales, the patterns and drivers behind main SOC reservoirs are not well understood and remain poorly linked to the pedogenetic variation across soil types that may impact SOC stabilization. Here, we separated soil organic carbon (SOC) associated with silt- and clay-sized particles (S + C), stable microaggregates (> 63 µm, SA) and free particulate organic matter (POM) from a diverse range of grassland topsoils sampled along a geo-climatic gradient. The relative contribution of the two predominantly mineral-associated fractions (S + C & SA) differed significantly across the gradient while free POM was never the dominant SOC reservoir. Rather, stable microaggregates emerged as the major SOC reservoir in soils with high SOC content. The SOC content in the two mineral-associated reservoirs was related to distinct climatic and mineralogic proxies that followed predictable patterns across the gradient. Furthermore, carbon quality in stable microaggregates was clearly distinct from carbon associated with silt- and clay-sized particles and free particulate organic matter. We summarize our findings in a conceptual framework, which integrates the role of stable microaggregates with other major SOC reservoirs and illustrates their changing importance across (soil) environmental gradients.
... Tmax ( • C) related to the pyrolysis temperature where the greatest amount of bound hydrocarbons were released during the cycle. The hydrogen index (HI) (expressed as mg HC g TOC − 1 ) corresponded to the amount of bound hydrocarbons released relative to the TOC, while the oxygen index (OI) (expressed as mg CO 2 g TOC − 1 ) referred to the quantity of oxygen released as CO and CO 2 relative to the TOC (Sebag et al., 2016;Brown et al., 2023). ...
... These are examples of the methods utilizing the thermolability of organic and inorganic compounds [10,48,102]. The combination of various methods for thermal analysis (thermogravimetry, differential scanning calorimetry, and gas emission analysis) in a single approach makes them a useful tool for soil research [85]. ...
... Methods based on SOC and SIC thermal lability could serve as examples (Wang et al., 2012;Apesteguia et al., 2018;Gomez et al., 2020). The combination of various thermal analysis methods (thermogravimetry, differential scanning calorimetry, gas emission analysis) in a single approach makes them a useful tool in soil studies (Sebag et al., 2016). The potential of the method is based on the exothermic SOC combustion reaction proceeding at a lower temperature compared to the thermal decomposition of SIC (Vuong et al., 2013). ...
... The organic geochemistry data were further compared with the previously acquired I-index, which is derived from Rock-Eval® analysis (Garcin et al., 2022). The I-index relates to the ratio between the thermally labile and resistant pools of organic matter (Sebag et al., 2016). Further methodological details are given in Appendix B. ...
... This method identifies different thermal pools of SOC (Disnar et al., 2003) and the thermal stability of SOC can be compared to its biogeochemical stability Plante et al., 2011). In sandy soils in the Senegalese Groundnut Basin, Malou et al. (2020) documented a depletion of organic compounds with intermediate thermal stability, which tend to accumulate in other environments (Sebag et al., 2016). The thermal stability and biogeochemical stability of SOC seem to be specific to the pedoclimatic conditions. ...
Soil organic carbon (SOC) contributes to agrosystem productivity. Understanding how farming practices implemented by smallholders affect the levels and distribution of SOC in carbon (C) pools with different stabilities is essential in sub-Saharan Arenosols where SOC mineralization is intense. The stability of SOC was studied by thermal (Therm-C), physical (particulate organic matter >50 μm, POM-C and fine soil fractions <50 μm, FF-C), chemical (permanganate-oxidizable carbon, POX-C) and biological (mineralizable C, Min-C) approaches. Soil samples were collected at depths of 0–10 and 10–30 cm in cultivated fields (out- or home-fields) without any input, with millet residues, amended with manure, or with household organic wastes. Globally, average SOC contents were low (<6 g C kg-1). The variability in SOC and C pool contents was sensitive to field management. The different approaches to measuring the stability of SOC did not measure the same fraction of SOC. POM-C and Therm-C were correlated and both explained Min-C similarly, thus suggesting that in these sandy soils, POM-C or Therm-C probably measured comparable properties of the stability of C. The lack of relationships between POX-C and other pools suggested that POX-C encompassed a different nature of SOC while providing complementary information on the biogeochemical stability of SOC.
... Specifically, the I-index quantifies the degree of transformation of fractions originating from biological material, while the R-index qualifies the thermally stable (i.e., refractory) fraction. From an I-Index vs R-Index diagram, Sebag et al. (2016) define three linear trends corresponding to the decomposition of biogenic compounds line ("humic"), to the "inherited" contributions from non-pedological processes, and a specific "spodic" trend coming from podzolization such as fulvic acids and aluminium. Besides, along with the decomposition line, they identify three areas corresponding to biological tissue as fresh plants remain, litter and organic horizons (i.e., poorly decomposed organic matter), and organo-mineral or mineral peat samples (i.e. ...
Peatlands are permanent wetlands recognized for ecosystem services such as biodiversity conservation and carbon storage capacity. Little information is available about their response to global change, the reason why most Earth system climate models consider a linear increase in the release of greenhouse gases (GHG), such as CO2, with increasing temperatures. Nevertheless, numerous studies suggest that an increase in the temperature may not imply a decrease in photosynthesis and carbon storage rates if water availability is sufficient, the latter being under the control of local hydrology mechanisms. Mediterranean peatlands well illustrate this fact. Since they are groundwater-dependent, they are hydrologically resilient to the strong seasonality of hydroclimatic conditions, especially during the summer drought. In the present study, we demonstrate that, even if such peatlands release CO2 into the atmosphere, they can maintain a carbon storage capacity. To this end, a geochemical study disentangles the origin and fate of carbon within a Mediterranean peatland at the watershed scale. Field parameters, major ions, dissolved organic and inorganic carbon content and associated δ13C values allow for characterizing the seasonality of hydrochemical mechanisms and carbon input from an alluvial aquifer (where rain, river, shallow, and deep groundwater flows are mixing) to the peatland. The inorganic and organic content of peat soil and δ13C values of total organic matter and CO2 complete the dataset, making it possible to provide arguments in favour of lower organic matter oxidation compared to primary production. Overall, this study highlights the groundwater role in the fluxes of CO2 at the peatland-atmosphere interface, and more broadly the need to understand the interactions between the water and carbon cycles to build better models of the future evolution of the global climate.
... The use of these techniques, and other emerging ones such as Rock-Eval pyrolysis, coupled to different strategies of soil and organic matter fractionation, is a promising perspective for a better understanding of the relationship between SOC and SIC cycling and the possible consequences of agricultural management on it. In particular, Rock-Eval pyrolysis allows using the pyrograms obtained to identify components that can be associated with different constituents of the organic fraction of the soil (Sebag et al., 2006(Sebag et al., , 2016 and their 'energy signature' (Barré et al., 2016). It can also be used to study the level of interaction of organic compounds with the mineral fraction (Cécillon et al., 2018;Poeplau et al., 2019). ...
Carbonate-rich soils are common in many arid and semiarid areas. Many of them are cultivated, and agriculture is expanding by the spreading of irrigation. Although the soil mineral fraction has been usually considered little or not affected by agricultural management in the short term, increasing evidence suggests that this is not the case for carbonates in the tilled layer. The consequences of management can be intense and depend on the modifications induced in inorganic carbon cycling, which can result in gains or losses of total inorganic soil C, and in changes in its type (pedogenic vs. lithogenic), size and distribution. Net soil CO2 emissions and interactions with organic C cycle can be affected by these changes, thereby altering soil organic matter dynamics. This chapter summarizes the major observed effects of agricultural practices potentially altering soil carbonates, includes a case study in a newly irrigated area, and identifies the most important knowledge gaps and research perspectives.
... 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). ...
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.
... In recent years, the technique of Rock-Eval has emerged as a vital cog for assessment of unconventional shale and coal reservoir rocks, in addition to its widespread application for conventional hydrocarbon reservoirs (Peters 1986;Sykes and Snowdon 2002;Petersen 2006;Jarvie 2012a, b;Carvajal-Ortiz and Gentzis 2015;Hazra et al. 2017Hazra et al. , 2019aHazra et al. , b, 2020aKarayigit et al. 2018). Generation of simple, reliable and reproducible data has led to its application in Belds beyond source rock assessment, viz., soil contamination, recent organic matter characterization, identiBcation of black carbon in soil, etc. Poot et al. 2009;Sebag et al. 2016). Hazra et al. (2020bHazra et al. ( , 2021a used the S4 T peak (temperaturepeak of S4 organic-CO 2 curve) for predicting the combustion behaviour of few coals and as thermal maturity estimator for shales. ...
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.
Partitioning soil organic carbon (SOC) in fractions with different biogeochemical stability is useful to better understand and predict SOC dynamics and provide information related to soil health. Multiple SOC partition schemes exist, but few of them can be implemented on large sample sets and therefore be considered relevant options for soil monitoring. The well-established particulate organic carbon (POC) vs. mineral-associated organic carbon (MAOC) physical fractionation scheme is one of them. Introduced more recently, Rock-Eval® thermal analysis coupled with the PARTYSOC machine learning model can also fractionate SOC into active (Ca) and stable SOC (Cs). A debate is emerging as to which of these methods should be recommended for soil monitoring. To investigate the complementarity or redundancy of these two fractionation schemes, we compared the quantity and environmental drivers of SOC fractions obtained on an unprecedented dataset from mainland France. About 2000 topsoil samples were recovered all over the country, presenting contrasting land cover and pedoclimatic characteristics, and analysed. We found that the environmental drivers of the fractions were clearly different, the more stable MAOC and Cs fractions being mainly driven by soil characteristics, whereas land cover and climate had a greater influence on more labile POC and Ca fractions. The stable and labile SOC fractions provided by the two methods strongly differed in quantity (MAOC/Cs=1.88± 0.46 and POC/Ca=0.36± 0.17; n=843) and drivers, suggesting that they correspond to fractions with different biogeochemical stability. We argue that, at this stage, both methods can be seen as complementary and potentially relevant for soil monitoring. As future developments, we recommend comparing how they relate to indicators of soil health such as nutrient availability or soil structural stability and how their measurements can improve the accuracy of SOC dynamics models.
В мировой практике измерение массовой доли углерода органических соединений (С орг ) в почвах, содержащих карбонаты, выполняют различными способами. Проведен анализ методов, позволяющих решить данную задачу, включая новейшие подходы: термогравиметрия, дифференциальная сканирующая калориметрия, спектроскопия. Показано, что присутствие CaCO 3 не препятствует применению дихроматометрического метода (Тюрин, Уолкли—Блэк) определения C орг . Недостатки метода сводятся к трудоемкости анализа, необходимости постоянного присутствия оператора, неполному окислению органических соединений и загрязнению окружающей среды. Метод измерения потерь массы почвы при прокаливании (ППП) экономичен и экспресен, однако дает завышенное содержание С орг , что связано с неадекватностью пересчетного коэффициента 1.724, наличием адсорбированной и химически связанной воды, а также минеральных компонентов, разлагающихся при T = 105–550°С. Наиболее актуальным решением нахождения С орг в карбонатных почвах является использование анализатора и кальциметра, хотя точность измерений С орг при наличии карбонатов существенно снижается из-за квадратичного суммирования погрешностей двух методов. Высокая стоимость прибора, обслуживания, поверки и ремонта ограничивает его массовое использование в почвенных лабораториях. Для измерения содержания карбонатов почв возможно применение и гравиметрического (ППП), и волюмометрического (кальциметр) методов. Использование последнего предпочтительно для почв с преобладанием CaCO 3 в карбонатном составе. Предварительное удаление карбонатов из образцов почв трудоемко, а также может приводить к частичной потере С орг вследствие кислотной экстракции. Высокая стоимость приборов и отсутствие библиотек спектров почв сдерживают развитие vis-NIR и MIR спектроскопии как альтернативы методам “мокрой” химии. Продолжение сравнительных исследований улучшит понимание пространственных закономерностей распределения углерода органических соединений почв.
Soil is the basis for life on Earth as we know it. Healthy and stable soil is a prerequisite for well-functioning terrestrial ecosystems and has, thus, been proposed to play a key role in plant diversity–ecosystem functioning relationships. The overall objective of this sub-project is to study multidimensional soil stability as affected by plant diversity in a long-term plant diversity experiment. We designed three coordinated work packages (WPs) to comprehensively assess soil multistability to environmental fluctuations and climate extremes by considering the biological, chemical and physical dimensions that are key for soil functioning. We will use all unique facilities and approaches of the Jena Experiment Research Unit by combining synthesis of long-term data in the Main Experiment and the ΔBEF Experiment with performing new soil analyses in the DrY Experiment, the ResCUE Experiment and a joint CoMic Experiment, to gain a better mechanistic understanding of plant diversity–ecosystem functioning relationships. In close collaboration with other sub-projects, we will assess biological, chemical and physical soil properties and stability indicators that will be used to calculate soil multifunctionality and multistability indices. In WP1, we will build on three unique datasets to explore short-term and long-term effects of plant diversity on the stability of soil (microbial) properties. In WP2, we will combine different datasets and approaches to explore if plant diversity effects on the magnitude and stability of soil properties increase with abiotic and biotic stresses. In WP3, we will combine measurements of the above-mentioned dimensions of soil stability to explore if plant diversity increases the stability of multiple soil properties under hot drought. This sub-project is at the heart of the Research Unit by testing the overarching hypotheses outlined in the Coordination Proposal of the Jena Experiment, contributing to all main experiments, sharing data and performing joint sampling campaigns with all sub-projects and, at the same time, introducing a novel concept of soil multistability as affected by plant diversity and climate extremes. We propose to use a combination of simple, high-throughput (e.g. bait-lamina test) and more sophisticated methods (e.g. extracellular polymeric substances analyses) to be able to investigate temporal dynamics of soil processes and their mechanistic basis. Taken together, the results of the three WPs will provide new insights into the stabilising mechanisms of soil properties in the long term and in relation to climate extremes through plant diversity.
Partitioning soil organic carbon (SOC) in fractions with different biogeochemical stability is useful to better understand and predict SOC dynamics, and provide information related to soil health. Multiple SOC partition schemes exist but few of them can be implemented on large sample sets and therefore be considered as relevant options for soil monitoring. The well-established particulate- (POC) vs. mineral-associated organic carbon (MAOC) physical fractionation scheme is one of them. Introduced more recently, Rock-Eval® thermal analysis coupled with the PARTYSOC machine-learning model can also fractionate SOC into active (Ca) and stable SOC (Cs). A debate is emerging as to which of these methods should be recommended for soil monitoring. To investigate the complementarity or redundancy of these two fractionation schemes, we compared the quantity and environmental drivers of SOC fractions obtained on an unprecedented dataset from mainland France. About 2,000 topsoil samples were recovered all over the country, presenting contrasting land covers and pedoclimatic characteristics, and analysed. We found that the environmental drivers of the fractions were clearly different, the more stable MAOC and Cs fractions being mainly driven by soil characteristics, whereas land cover and climate had a greater influence on more labile POC and Ca fractions. The stable and labile SOC fractions provided by the two methods strongly differed in quantity (MAOC/Cs = 1.88 ± 0.46 and POC/Ca = 0.36 ± 0.17; n = 843) and drivers, suggesting that they correspond to fractions with different biogeochemical stability. We argue that, at this stage, both methods can be seen as complementary and potentially relevant for soil monitoring. As future developments, we recommend comparing how they relate to indicators of soil health such as nutrient availability or soil structural stability, and how their measurements can improve the accuracy of SOC dynamics models.
Quantifying both soil organic carbon (SOC) and soil inorganic carbon (SIC) is essential to understand carbon (C) dynamics and to assess the atmospheric C sequestration potential in calcareous soils. The procedures usually used to quantify SOC and SIC involve pretreatments (decarbonation, carbonate removal) and calculations of the difference between C contents estimated by elemental analysis on raw and pretreated aliquots. These procedures lead to analytical bias associated with pretreatments, measurement deviations associated with sample heterogeneity, and cumulative errors associated with calculations. The Rock-Eval® analysis is a ramped thermal analysis that has been used in soil sciences since the 2000s, consisting of pyrolysis of the sample followed by oxidation of the residue. A single Rock-Eval® analysis on non-pretreated aliquots provides two parameters estimating the organic (TOC) and inorganic (MinC) C contents of the samples. Nevertheless, the Rock-Eval® protocol was standardised in the 1970s by IFP Energies Nouvelles for studying oil-bearing rocks and is thus not perfectly suited for soil study. Previous studies have suggested statistical corrections of the standard parameters to improve their estimations of C contents assessed by elemental analysis, but only a few of them have focused on the estimation of inorganic C content using the MinC parameter. Moreover, none of them have suggested adjustments to the standard Rock-Eval® protocol. This study proposes to adapt this protocol to optimise SOC and SIC quantifications in soil samples. Comparisons between SOC and SIC quantifications by elemental analysis and by Rock-Eval®, with and without statistical corrections of the standard TOC and MinC parameters, were carried out on 30 agricultural topsoils with a wide range of SOC and SIC contents. The results show that the standard Rock-Eval® protocol can properly estimate SOC contents once the TOC parameter is corrected. However, it cannot achieve a complete thermal breakdown of SIC amounts > 4 mg, leading to an underestimation of high SIC contents by the MinC parameter, even after correcting for this. Thus, the final oxidation isotherm is extended to 7 min to complete the thermal breakdown of SIC before the end of the analysis. This work is a methodological step to measure SOC and SIC contents in a single analytical run on a non-pretreated aliquot. More work is needed (i) on a wider range of soil samples with differing land use and other forms of carbonate mineral and sampling depths and (ii) to avoid the use of statistical corrections of the TOC and MinC parameters.
In this study, we combined Rock-Eval® analysis, analytical pyrolysis, and wet-chemical extractions, assisted by Fourier transform infrared spectroscopy (FTIR), and measurements of soil heterotrophic respiration. Our objective was to assess the biological and thermal stability of mixed-nature soil organic matter (SOM) derived from grass litter and kerogen. We studied a Technosol constructed with Ca, Mg and kerogen-rich waste rock (black shales), which has been under pasture cultivation for 20 years. We compared the results with those from black shales and an adjacent natural soil under long-term pasture cultivation, both used as endmembers representing kerogen and plant-derived SOM, respectively. Analytical pyrolysis and FTIR analyses revealed a mixed composition of SOM in the Technosol. Predominantly, polysaccharides, lignin, lipids and N-compounds were originated from plant-derived SOM, while (poly)aromatic and most aliphatic compounds were traced back to kerogen. Rock-Eval analysis showed that 58% of SOM in Technosol was kerogen-derived, which was poorly accessible to soil microbiota, as evidenced by heterotrophic respiration. In addition, an important portion of plant-derived SOM (>70%) was only released during the Rock-Eval oxidation stage. The impact of chemical recalcitrance of kerogen compounds on short-term biological stability was remarkable and demonstrated a strong correlation with the thermal indices derived from the Rock-Eval pyrolysis stage. Conversely, the parameters from Rock-Eval oxidation stage showed a positive correlation with the amount of SOM involved in mineral-organic associations, particularly with Ca2+ and Mg2+ (i.e., cation bridging). Thus, to disentangle the contribution of chemical recalcitrance and mineral-organic associations to SOM stability, we recommend the assessment of all Rock-Eval thermograms (from pyrolysis and oxidation stages) in combination with chemical and biological assessments, especially when studying soils containing mixed-nature SOM.
Quantifying both soil organic and inorganic carbon (SOC & SIC) is essential to understand carbon (C) dynamics and to assess the atmospheric C sequestration potential in calcareous soils. The procedures usually used to quantify SOC and SIC involve pretreatments (decarbonation, decarbonatation) and calculation of the difference between C contents estimated by elemental analysis on raw and pretreated aliquots. These procedures lead to analytical bias associated to pretreatments, measurement deviations associated to the sample heterogeneity, and cumulative errors associated to calculations. The Rock-Eval® thermal analysis, used in soil sciences since the 2000s, provides two parameters estimating the organic (TOC) and inorganic (MinC) C contents of a non-pretreated aliquot with a single analysis. Nevertheless, the Rock-Eval® protocol has been standardized in the 70s by IFP Energies Nouvelles for studying oil bearing rocks and is, thus, not perfectly suited for soil study. Previous studies suggested statistical corrections of the standard parameters to improve their estimations of C contents assessed by elemental analysis but only few of them focused on the estimation of inorganic C content using the MinC parameter. Moreover, none of them suggested adjustments of the standard Rock-Eval® protocol. This study proposes to adapt this protocol to optimize SOC and SIC quantifications in soil samples. Comparisons between SOC and SIC quantifications by elemental analysis and by Rock-Eval®, with and without statistical corrections of the standard TOC and MinC parameters, were carried out on a soil panel with a wide range of SOC and SIC contents. The results show that the standard Rock-Eval® protocol properly estimates SOC contents once the TOC parameter is corrected. However, it cannot achieve a complete thermal breakdown of SIC amounts > 4 mg leading to an underestimation of high SIC contents by the MinC parameter, even after correcting it. Thus, the final oxidation isotherm is extended to 7 min to complete the thermal breakdown of SIC before the end of the analysis.
Tropical peatlands are important carbon stores that are vulnerable to drainage and conversion to agriculture. Protection and restoration of peatlands are increasingly recognised as key nature based solutions that can be implemented as part of climate change mitigation. Identification of peatland areas that are important for protection and restauration with regards to the state of their carbon stocks, are therefore vital for policy makers. In this paper we combined organic geochemical analysis by Rock-Eval (6) pyrolysis of peat collected from sites with different land management history and optical remote sensing products to assess if remotely sensed data could be used to predict peat conditions and carbon storage. The study used the North Selangor Peat Swamp forest, Malaysia, as the model system. Across the sampling sites the carbon stocks in the below ground peat was ca 12 times higher than the forest (median carbon stock held in ground vegetation 114.70 Mg ha⁻¹ and peat soil 1401.51 Mg ha⁻¹). Peat core sub-samples and litter collected from Fire Affected, Disturbed Forest, and Managed Recovery locations (i.e. disturbed sites) had different decomposition profiles than Central Forest sites. The Rock-Eval pyrolysis of the upper peat profiles showed that surface peat layers at Fire Affected, Disturbed Forest, and Managed Recovery locations had lower immature organic matter index (I-index) values (average I-index range in upper section 0.15 to -0.06) and higher refractory organic matter index (R -index) (average R-index range in upper section 0.51 to 0.65) compared to Central Forest sites indicating enhanced decomposition of the surface peat. In the top 50 cm section of the peat profile, carbon stocks were negatively related to the normalised burns ratio (NBR) (a satellite derived parameter) (Spearman’s rho = -0.664, S = 366, p-value = <0.05) while there was a positive relationship between the hydrogen index and the normalised burns ratio profile (Spearman’s rho = 0.7, S = 66, p-value = <0.05) suggesting that this remotely sensed product is able to detect degradation of peat in the upper peat profile. We conclude that the NBR can be used to identify degraded peatland areas and to support identification of areas for conversation and restoration.
Unconventional shale petroleum systems, owing to their petroleum generation and storage properties have gained tremendous exploitation and research interest in recent years. Moreover, their emergence as potential atmospheric carbon dioxide sinks has further warranted detailed examination of their multiple properties. Laboratory geochemical screening and geomechanical investigations provide valuable information for classifying these reservoirs. Although some guidelines exist for conducting such analyses, the analytical methods and sampling techniques applied do influence the quality of the derived measurements. For laboratory geochemical screening using the Rock-Eval technique, various factors including sample grain size, type of kerogen, sample quantity, nature of S2 and S4 curves, all substantially influence the quality of the data generated. In this work, we summarize the different factors that influence the data generated from Rock-Eval analysis and recommend a method that involves optimization of sample weight and sizes for generation of reliable geochemical data. New emerging technique in the field of organic petrography for simultaneous characterization of organic and inorganic phases in shales has also been discussed. Once source-rock potential of the shale formation is ascertained, the next step is selection of suitable target shale reservoir zones, and designing successful hydraulic fracturing programs for the exploitation of the reservoir. For this purpose, detailed knowledge of geomechanical properties is essential. However, quantity of intact shale-core samples recovered from coring operations is typically insufficient for reliably analyzing geomechanical properties applying the established standards. A comparison is made between the uniaxial compressive strength and Young’s modulus measurements on shale specimens of different length to diameter ratios. It reveals that specimens smaller than the recommended standard exhibit unacceptable variations in the values of strength and elastic parameters they generate. To overcome this, it is justified to use alternative techniques suited to the small sample sizes typically recovered in borehole shale cores. For instance, a punching tool and nanoindentation, which require small sample sizes, can both be used to reliably analyze geomechanical properties in circumstances where larger shale samples are not available from borehole zones of interest.
Keywords: shale assessment; Rock-Eval geochemical screening; geomechanics measurement; analytical techniques; precision; sample size considerations.
The effect of forest management practices on carbon quality is poorly documented. To assess changes in the quality and stability of soil organic carbon (SOC) of a temperate forest upon human activities, we investigated soil from forests (i) developed following natural regeneration after clearcutting 20 and 40 years ago, (ii) developed following afforestation on an abandoned crop area 40 years ago and (iii) in an area where regular clear-cut (with wood residues input) was conducted 40 years ago. Topsoil and subsoil layers were collected (0–20 cm and 50–80 cm). Soil organic matter (OM) was characterized by elemental analysis (total carbon and total nitrogen), thermal analysis (Rock-Eval®) and thermochemolysis (i.e., Py-GC/MS in the presence of tetramethylammonium hydroxide (TMAH)). In addition, a size fractionation to separate the labile coarse fraction (50–2000 μm) from the fine fraction (<50 µm) was performed. These fractions were analyzed by thermal analysis.
Despite no measurable differences in carbon and nitrogen contents, the characterization of the OM by thermal analysis, and the relative quantification of OM compounds revealed differences in the composition in OM for the topsoil layers. The thermal analysis clearly distinguished sites with inputs of woody residues (higher HI) with a higher relative contribution of lignin and cutin/suberin compounds. However, the OM thermal stability seems mainly controlled by the organo-mineral interactions rather than chemical composition. Combination of Rock-Eval® thermal analysis and Py-GC/MS suggests that thermal stability cannot be used as an indicator of stability in specific contexts where pedogenetic processes are deeply modified by regular and extensive anthropogenic inputs of woody residues.
Les Technosols sont des sols fortement impactés par les activités humaines et dont la couverture n'a de cesse d'augmenter depuis le début de l'Anthropocène. Ils sont caractérisés par leurs concentrations notables en artéfacts (i.e. matériaux parents d'origine anthropique) dont l'origine, la nature et la réactivité sont extrêmement variables. Les artéfacts influencent largement les propriétés et la pédogenèse des Technosols, et donc aussi la dynamique du carbone dans ces sols. Néanmoins, le caractère récent de l'étude des Technosols et leur forte hétérogénéité impliquent que le niveau actuel de connaissances sur le stockage de carbone est très limitée. Les recherches conduites ont tout d'abord permis de constituer la première base de données sur les stocks de carbone dans les Technosols. Il en ressort que les Technosols figurent parmi les sols du Monde avec les stocks de carbone les plus élevés, même si ceux-ci se caractérisent par une très forte variabilité, sans équivalent pour les autres sols. La base de données permet également de mettre en évidence l'influence de facteurs tels que le climat et le mode d'occupation des sols. Dans un second volet, une expérimentation en conditions contrôlées a permis d'évaluer le potentiel de minéralisation d'une gamme d'artéfacts organiques seuls ou en présence de matière organique naturelle. Les résultats soulignent la récalcitrance de certains artéfacts fréquemment rencontrés dans les Technosols (coke, charbon, biochar). Ces artéfacts interagissent également avec la matière organique naturelle et peuvent limiter sa minéralisation. Enfin, le suivi et la caractérisation des stocks de carbone de Technosols en conditions réelles ont été effectués. Alors que certains stocks restent pseudo-stables au cours du temps, d'autres diminuent de manière continue et l'un des Technosols présente une cinétique en deux phases : une courte décroissance, suivie d'une augmentation continue du stock. En lien avec les résultats précédents, la nature des artéfacts et les processus associés apparaissent essentiels dans cette dynamique. Il en ressort aussi que le mode de gestion de la biomasse est un facteur contrôlant l'évolution du stock.
Microbes are responsible for cycling carbon (C) through soils, and predicted changes in soil C stocks under climate change are highly sensitive to shifts in the mechanisms assumed to control the microbial physiological response to warming. Two mechanisms have been suggested to explain the long‐term warming impact on microbial physiology: microbial thermal‐acclimation and changes in the quantity and quality of substrates available for microbial metabolism. 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 relationships 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 of microbial respiration, but only in summer, when warming had exacerbated the seasonally‐induced, already small dissolved organic matter pools. Irrespective of warming, greater quantity and quality of soil carbon increased the extracellular enzymatic pool and its temperature sensitivity. We propose that fresh litter input into the system seasonally cancels apparent thermal acclimation of C‐cycling processes to decadal warming. 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 these soils.
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.
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.
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.
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.
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 13C 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.
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.
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.
The response of soil carbon dynamics to climate and land-use change will affect both the future climate and the quality of ecosystems. Deep soil carbon (> 20cm) is the primary component of the soil carbon pool, but the dynamics of deep soil carbon remains poorly understood. Therefore, radiocarbon activity (ΔD(14) C), which is a function of the age of carbon, may help to understand rates of soil carbon biodegradation and stabilization. We analyzed the published (14) C contents in 122 profiles of mineral soil that were well distributed in most of the large world biomes, except for the boreal zone. With a multivariate extension of a linear mixed effects model whose inference was based on the parallel combination of two algorithms, the Expectation-Maximization (EM) and the Metropolis-Hasting algorithms, we expressed soil Δ(14) C profiles as a four-parameter function of depth. The four-parameter model produced insightful predictions of soil Δ(14) C as dependent on depth, soil type, climate, vegetation, land-use and date of sampling (R2 = 0.68). Further analysis with the model showed that the age of topsoil carbon was primarily affected by climate and cultivation. By contrast, the age of deep soil carbon was affected more by soil taxa than by climate and thus illustrated the strong dependence of soil carbon dynamics on other pedologic traits such as clay content and mineralogy. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
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 analysis 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: weight loss after HCl treatment; the acidimetry technique; and calculation after TC, mass balance from kerogen isolation and organic carbon measurement on kerogen by elemental analysis. The results displays 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 while 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 T-max 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 in the Rock-Eval 6, where the probe is in contact with the crucible containing the sample, leading to much more reliable data.
Introduction Although fungi are generally disregarded in the biogeochemical literature, they undoubtedly constitute crucial biogeochemical factors in many elemental cycles. This fact, combined with their abundance in the soil warrants greater detailed study into their geoecological impact. The network formed by fungal filaments can represent 10 000 km of thread-like mycelia in 1 m 2 of fertile soil. Their mass is evaluated at 3500 kg ha À1 at a depth of 20 cm in an average continental soil, i.e. taking into account all the different terrestrial environments on the Earth (Gobat et al., 2004). In comparison, bacteria and algae would represent 1500 and 10–1000 kg ha À1 respectively, in the same virtual average soil. Fungi are not only biologically important as saprophytes in the recycling of organic matter, but also play a geological role by excreting notable amounts of organic acids, among which oxalic acid is particularly important (Gadd, 1999), contributing to continental weathering as well as to mineral neogenesis (Verrecchia & Dumont, 1996; Verrecchia, 2000; Burford et al., 2003 a, b). The first fossil fungi have been identified in rocks dated from the Ordovician, i.e. 460 to 455 Ma ago (Redecker et al., 2000). However, molecular clock estimates for the evolution of fungi have suggested a Late Precambrian (600 Ma) colonization on land (Berbee & Taylor 2000). Recent molecular studies, based on protein sequence analysis, indicate that fungi were present on continents 1 billion years ago and possibly affected (together with plants) the evolution of Earth's atmosphere and climate since 700 Ma (Heckman et al., 2001). Therefore, if fungi have been present on the Earth's surface for such a long time, producing large amounts of oxalic acid able to precipitate as metal oxalates, why is there no evidence of oxalate accumulation in paleosols?
Variations in the abundance of soil organic matter (SOM) constituents with different stability have a major impact on important environmental processes, e.g., carbon dioxide (CO2) fluxes between the soil and the atmosphere. Recently, besides the bulk Rock-Eval (RE) data, the mathematical deconvolution of the signals derived from hydrocarbon-like compounds released by thermal cracking of SOM during RE pyrolysis has been increasingly used to estimate the relative contribution of the major SOM classes differing in origin and preservation. This study applied the mathematical deconvolution of the S3 and S4 signals of carbon monoxide (CO) and CO2, produced both by the pyrolysis of the oxygen-containing moieties and by the oxidation of the residual highly resistant organic matter, to characterize the stability of these components. Our results suggested that the stability of the oxygen-containing moieties was controlled by the precursor material and was strongly affected by the land use and the presence of humic substances in the surface horizon of some main soil types in Hungary. In consistence with the bulk RE data, results of the mathematical deconvolution also proved to be diagnostic markers for discriminating the aquatic or terrigenous plants as the main sources of SOM. The mathematical deconvolution of S4 signals derived from the highly resistant SOM fraction allowed us to quantify the contribution of constituents with different stability. Furthermore, the results of this study displayed that the stability of this highly abundant SOM fraction in the surface soil samples depended on source biomass and intensity of leaching.
This paper reviews the role of alluvial soils in vegetated gravelly river braid plains. When considering decadal time scales of river evolution, we argue that it becomes vital to consider soil development as an emergent property of the developing ecosystem. Soil processes have been relatively overlooked in accounts of the interactions between braided river processes and vegetation, although soils have been observed on vegetated fluvial landforms. We hypothesise that soil development plays a major role in the transition (speed and pathway) from a fresh sediment deposit to a vegetated soil-covered landform. Disturbance (erosion and/or deposition), vertical sediment structure (process history), vegetation succession, biological activity and water table fluctuation are seen as the main controls on early alluvial soil evolution. Erosion and deposition processes may not only act as soil disturbing agents, but also as suppliers of ecosystem resources, because of their role in delivering and changing access (e.g. through avulsion) to fluxes of water, fine sediments and organic matter. In turn, the associated initial ecosystem may influence further fluvial landform development, such as through the trapping of fine-grained sediments (e.g. sand) by the engineering action of vegetation and the deposit stabilisation by the developing above and belowground biomass. This may create a strong feedback between geomorphological processes, vegetation succession and soil evolution which we summarise in a conceptual model. We illustrate this model by an example from the Allondon River (CH) and identify the research questions that follow. This article is protected by copyright. All rights reserved.
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.
Successful petroleum exploration relies on detailed analysis of the petroleum system in a given area. Identification of potential source rocks, their maturity and kinetic parameters, and their regional distribution are best accomplished by rapid screening of rock samples (cores and/or cuttings) using the Rock-Eval apparatus. The technique has been routinely used for about fifteen years and has become a standard tool for hydrocarbon exploration. This paper describes how the new functions of the latest version of Rock-Eval (Rock-Eval 6) have expanded applications of the method in petroleum geoscience. Examples of new applications are illustrated for source rock characterization, reservoir geochemistry, and environmental studies, including quantification and typing of hydrocarbons in contaminated soils.
We estimate the intensity of Late-glacial and Holocene methane emissions from peatlands based on their paleo net primary production (PNPP). The PNPP is derived from the carbon accumulation rates of the studied bog profile (Etang de la Gruère, Switzerland), which are corrected for the degree of peat degradation. The obtained PNPP curve is taken as a proxy for methane emissions. It shows relatively high values (90 g C m− 2 yr− 1) early in the Bolling/Allerod and drops to low values (40 g C m− 2 yr− 1) during the Younger Dryas cold period. With the onset of the Holocene the PNPP increases strongly up to 150 g C m− 2 yr− 1 around ca. 10,000 Cal. yr BP. This is followed by a decline to minimum values (30 to 40 g C m−2 yr−1) between 6500 and 4000 Cal. yr BP. Thereafter, the PNPP starts to increase again to reach its highest value (175 g C m− 2 yr− 1) around 1000 Cal. yr BP.
The PNPP curve correlates well with the evolution of the atmospheric methane concentrations as derived from Greenland ice- cores. For example, minima in atmospheric methane reported during the Younger Dryas and around 5200 Cal. yr BP are coinciding with the lowest values of PNPP and the negative atmospheric methane peak at 8200 Cal. yr BP corresponds to a marked decrease in PNPP.
Our PNPP curve suggests that the methane emissions from northern peatlands evolved similar to those of low latitude wetlands and together they largely determined the evolution of atmospheric methane throughout the Late-glacial and the Holocene. The abruptness of the rise of atmospheric methane at the end of the Younger Dryas probably points to an additional source (e.g. marine gas hydrates), but very early in the Holocene the peatlands have likely become the dominant source of atmospheric methane.
In recent years, Nguène Lake and Kamalété Lake (Gabon, West Central Africa) have been studied repeatedly, providing comprehensive reconstructions of environmental changes over the last millennia. Both lakes are in different geomorphological and environmental settings. They are therefore excellent sites to test new methodological approaches. Indeed, the sedimentary cores provide various facies, and the previous studies provide references for calibrating the results of new methods. In this methodological issue, the present study aims to evaluate the potential of spectrophotometric and Rock-Eval coupled analysis to describe the Holocene lake and marsh deposits from tropical moist forests. This assessment is carried out on samples taken from two well-documented reference cores. The spectrophotometric analysis provides reproducible colour measurements, which inform about the nature of the main colour-bearing constituents. Coupled with Rock-Eval pyrolysis, this technique can be used to describe lithological changes and identify the probable source of sedimentary organic matter. In the studied cases, this approach identified the facies dominated by detrital terrigenous inputs ("iron bearing" signature and high OI values) and those associated with a more abundant primary production ("chlorophyll" signature, low OI and high HI), providing a distinction between palustrine and lacustrine dynamics. However, although the facies are comparable, sedimentary dynamics and sediment sources may vary depending on geomorphological and climatic contexts.
a b s t r a c t Mountain soils store huge amounts of carbon which may be highly vulnerable to the strong land use and climate changes that mountain areas currently experience worldwide. Here, we tested the Rock–Eval (RE) pyrolysis as a proxy technique to (i) quantify soil organic carbon (SOC) stocks, (ii) bring insights into SOC bulk chemistry and (iii) investigate biogeochemical stability at the landscape scale in a mountain area of the French calcareous Prealps. A total of 109 soils from 11 eco-units representing the variety of ecosys-tems of the study area were analyzed with RE pyrolysis. RE pyrolysis showed an excellent predictive per-formance (R 2 = 0.99) for SOC content even in calcareous soils. The technique revealed specific chemical fingerprints for some eco-units and soil types, with decreasing hydrogen index values from Anthroposols (425 ± 62 mg HC/g SOC) to Umbrisols, Leptosols (311 ± 49 mg HC/g SOC) and to Cambisols (278 ± 35 mg HC/g SOC), associated with an increase in SOC maturation. Newly developed RE pyrolysis indices revealed the high stability of SOC in most eco-units developed on Cambisols (acidic grasslands, alpine meadows, bushy facies) and a significantly lower stability of SOC in mountain ridges, sheepfold areas and coniferous forest soils. The persistence of SOC in this mosaic of ecosystems may depend not only on its chemistry or thermal stability, but also on local environmental factors such as climatic conditions or pH, especially for high altitude soils. Overall, RE pyrolysis appears as an appropriate tool for landscape scale carbon inven-tories and could become a standardized proxy for assessing the vulnerability of SOC stocks.
At first sight nothing seems more obvious than that everything has a beginning and an end and that everything can be subdivided into smaller parts. Nevertheless, for entirely speculative reasons the philosophers of Antiquity, especially the Stoics, concluded this concept to be quite unnecessary. The prodigious development of physics has now reached the same conclusion as those philosophers, Empedocles and Democritus in particular, who lived around 500 BCE and for whom even ancient man had a lively admiration. (Svante Arrhenius, Nobel Lecture, 1903).
The role of forest age as a potential driver of intraspecific variation in leaf litter quality, that is a key plant trait determining ecosystem functioning, has largely been neglected. Using a set of fully replicated pure beech (Fagus sylvatica) forest stands differing in age (15, 65, 95 and 130 years), we quantified the forest stand age related variability of twelve leaf litter quality traits. Litter Mg, N and K showed significantly higher concentrations in litter from 15-yrs-old stands and decreased with increasing stand age. Mn was the only nutrient analyzed that was highest in the oldest stands. Hemicellulose and cellulose were lowest, and lignin and lignin/N ratio were highest in stands of intermediate age. The amount of N within the litter lignin fraction was highest in the 95-yrs-old stands (51% of total N) and lowest in the oldest stands (34% of total N). The amount of N associated within the hemicellulose fraction (<3%) showed the opposite pattern along the forest stand age gradient compared to lignin. Using Partial Least Squares regressions, we showed that litter N, C/N, lignin/N, K, Mn and Mg were the most important predictors of litter decomposition along the chronosequence. In contrast the proportions of C fractions and the amount of N within these C fractions were the most significant variables explaining the variation in final litter N content after one year of decomposition. N mineralization in ground litter was highly related to the proportion of total N within lignin and humus N mineralization was mostly explained by Mn and the lignin/N ratio. We showed that forest age is an important driver of litter quality variation and contributed considerably to the overall variation of F. sylvatica leaf litter quality traits observed from a reviewed data of published studies conducted at the continental scale. Furthermore, intraspecific litter quality variation greatly impacted belowground processes. Accounting for forest age related litter trait variation, and for the crucial role of the distribution of N within different litter C fractions, may improve the mechanistic understanding of ecosystem functioning.
Presents a soil organic matter size fractionation method for the study of poor sandy soils. By means of successive dry and wet sieving at 2000, 200 and 50 micrometres, soil is fractionated into organic, mineral and organo-mineral fractions. -from English summary
The exchange of nutrients, energy and carbon between soil organic matter, the soil environment, aquatic systems and the atmosphere is important for agricultural productivity, water quality and climate. Long-standing theory suggests that soil organic matter is composed of inherently stable and chemically unique compounds. Here we argue that the available evidence does not support the formation of large-molecular-size and persistent 'humic substances' in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds. We discuss implications of this view of the nature of soil organic matter for aquatic health, soil carbon-climate interactions and land management.