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... Full details on the Rock-EvalÒ methods and different indices can be found in the supplementary information, ''Materials and methods''. The S2 thermogram was split into 5 separate components (A1:A5) at fixed temperature bounds and then used to calculate the I and R Index scores according to Eqs. 1 and 2 (Malou et al. 2020;Sebag et al. 2016). The I and R Index scores from our samples were then compared to the negative linear trend (''humic'' trend) from geochemically- (Matteodo et al. 2018) and pedoclimatically-diverse datasets (Sebag et al. 2016). ...
... The I and R Index scores from our samples were then compared to the negative linear trend (''humic'' trend) from geochemically- (Matteodo et al. 2018) and pedoclimatically-diverse datasets (Sebag et al. 2016). This trend in thermal stability is commonly ascribed to changes in OM quality upon decomposition in soils (Malou et al. 2020;Thoumazeau et al. 2020). Fig. 1 Schematic showing the sequential sonication and density separation process. ...
... Based on our data, we can hypothesise that active decomposition and mineralisation processes were operant at the CaCO 3 -free site; which meant that OM composition was dominated either by relatively fresh plant material that had not yet fully entered the decomposition continuum or highly decomposed residues from active decomposition and mineralisation processes. This would result in a thermal signature with a relatively increased presence of thermally labile compounds (high I index), but also thermally stable compounds representing advanced decomposition residues (A5; Malou et al. 2020). This proposition is also consistent with the observation that the bulk SOC contents were lower and the d 13 C values were higher at the CaCO 3 -free site. ...
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Geochemical indicators are emerging as important predictors of soil organic carbon (SOC) dynamics, but evidence concerning the role of calcium (Ca) is scarce. This study investigates the role of Ca prevalence in SOC accumulation by comparing otherwise similar sites with (CaCO3-bearing) or without carbonates (CaCO3-free). We measured the SOC content and indicators of organic matter quality (C stable isotope composition, expressed as δ 13C values, and thermal stability) in bulk soil samples. We then used sequential sonication and density fractionation (DF) to separate two occluded pools from free and mineral-associated SOC. The SOC content, mass, and δ 13C values were determined in all the fractions. X-ray photoelectron spectroscopy was used to investigate the surface chemistry of selected fractions. Our hypothesis was that occlusion would be more prevalent at the CaCO3-bearing site due to the influence of Ca on aggregation, inhibiting oxidative transformation, and preserving lower δ 13C values. Bulk SOC content was twice as high in the CaCO3-bearing profiles, which also had lower bulk δ 13C values, and more occluded SOC. Yet, contrary to our hypothesis, occlusion only accounted for a small proportion of total SOC (< 10%). Instead, it was the heavy fraction (HF), containing mineral-associated organic C, which accounted for the majority of total SOC and for the lower bulk δ 13C values. Overall, an increased Ca prevalence was associated with a near-doubling of mineral-associated SOC content. Future investigations should now aim to isolate Ca-mediated complexation processes that increase organo-mineral association and preserve organic matter with lower δ 13C values. Supplementary information: The online version of this article (10.1007/s10533-021-00779-7) contains supplementary material, which is available to authorized users.
... The Mkhuze Wetland System lies within the summer rainfall zone of South Africa, with about 60 % of precipitation occurring during the austral summer months (November through March) in association with cold fronts moving northward along the coast (Watkeys et al., 1993). Precipitation gradually decreases from east to west (see Fig. 2; Van Heerden and Swart, 1986) from 1000 to 600 mm yr −1 (Hutchison, 1976;Maud, 1980). Flooding is highly variable and usually associated with cutoff-lowpressure systems that develop during December and January or with infrequent tropical cyclones. ...
... The relatively enriched δD values suggest that the plants producing them were exposed to relatively dry conditions during their growth phase. These dry conditions are met to a large extent in the hinterland, considering that the precipitation gradient across the Mkhuze Wetland System is nearly halved from 1000 mm yr −1 in the east near the coast to 600 mm yr −1 at the Lebombo Mountains (Maud, 1980; Fig. 2). ...
... Since cracking temperature of organic compounds depends on their structural stability, the thermal status of OM was characterized by combining the R index (i.e., relative contribution of most thermally stable HC pools) and I index (i.e., ratio between thermally labile and resistant HC pools; details are in Sebag et al., 2016). As it is derived from a mathematical construct, if the gradual decomposition of labile compounds is its main driver, OM composition can be described as a continuum from biological tissues to a mixture of organic constituents derived from OM decomposition and plotted along a linear regression line (called the "decomposition line"; Malou et al., 2020) in the diagram of the I index vs. R index (called hereafter I /R diagram; Albrecht et al., 2015). ...
... origin of the sedimentary organic matter: red denotes upper reach; orange denotes floodplain; green denotes swamp; yellow denotes delta; blue denotes lake. The grey-shaded area bordered by dashed grey lines refers to the linear regression describing the continuum from biological tissue to a mixture of decomposition constituents (decomposition line; Malou et al., 2020). Green circles and squares denote splitting up of swamp samples into groups of differing degradation state, while grey-bordered ellipses indicate more general splitting of the data set into groups regardless of their depositional origin. ...
Article
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.
... Since cracking temperature of organic compounds depends on their structural stability, the thermal status of OM was characterized by combining R-index (i.e., relative 170 contribution of most thermally stable HC pools) and I-index (i.e., ratio between thermally labile and resistant HC pools; details in Sebag et al., 2016). As derived from a mathematical construct, if the gradual decomposition of labile compounds is its main driver, OM composition can be described as a continuum from biological tissues to a mixture of organic constituents derived from OM decomposition and plotted along a linear regression line (called "Decomposition line"; Malou et al., 2020) in the I-index vs R-index diagram (called thereafter I/R diagram; Albrecht et al., 2015). However, situations with OM mixture from 175 different sources or where decomposition is so intense that it even affects the more thermally stable pools may generate a distribution diverging from the "Decomposition line". ...
... origin of the sedimentary organic matter: red = upper reach, orange = floodplain, green = swamp, yellow = delta, and blue = lake. The grey shaded area refers to the linear regression describing the continuum from biological tissue to a mixture of decomposition constituents ("decomposition line",Malou et al., 2020). ...
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Sedimentary organic matter (OM) analyses along a 130 km-long transect of the Mkhuze River from the Lebombo Mountains to its outlet into Lake St. Lucia, Africa’s most extensive estuarine system, revealed the present active trapping function of a terminal freshwater wetland. A combination of organic bulk parameters, thermal analyses, and determination of plant waxes, and their corresponding stable carbon (δ13C) and hydrogen (δD) isotopic signatures in surface sediments and local plant species enabled characterization and comparison of sedimentary OM in terms of stability, degradation status, sources, and sinks within and among the respective sub-environments of the Mkhuze Wetland System. This approach showed that fluvial sedimentary OM originating from inland areas is mainly deposited on the floodplain and Mkhuze Swamps. In contrast to samples from upstream areas, a distinctly less degraded signature characterizes the sedimentary OM in the northern section of Lake St. Lucia. Although lake sedimentary plant waxes are similar in the observed wax distribution pattern and δ13C values, they exhibit considerably higher δD values. This offset in δD indicates that lakeshore vegetation dominates plant-derived sedimentary OM in the lake, elucidating the effective capturing of OM and its fate in a sub-tropical coastal freshwater wetland. These findings raise important constraints for environmental studies assuming watershed-integrated signals in sedimentary archives retrieved from downstream lakes or offshore.
... Finally, as shown in previous studies (Malou et al. 2020;Thoumazeau et al., 2020), a Delta I-index can be calculated: it refers to the difference between the I-index value of each sample and the I-index value calculated with the "humic trend" equation (in bold in Fig. 3b), calculated starting from study area sample data, at the R-index value of each sample. ...
... Catena 197 (2021) 104951 the relatively low contribution of resistant pool compared to the most labile pools. Similar results have been obtained when advanced decomposition of OM affects both thermally labile and resistant pools, as in Arenosols for example (Malou et al. 2020;Romanens et al., 2019). In the context of the present study, the decomposition of thermally resistant pools can be explained by an earlier phase of pedogenesis and this peculiarity of the trend supports the hypothesis that these soils located along the trend are paleosols. ...
Article
Complex sequences of paleosols are often formed by the interaction between pedogenesis and geomorphological evolution. Their study, particularly in mountain areas, is useful to reconstruct past environmental conditions as well as climate shifts, and to gather information on the morphodynamical processes affecting the landscape through time. Since the combined role that all different factors can play in the soil formation and evolution through time and space influences the formation and evolution of those complex paleosol sequences, a multidisciplinary study was conducted at the NW slope of Mt. Cusna (Northern Apennines, Italy). This work aims to reconstruct and to evaluate how the interactions between the geomorphological context, the Holocene climate variations, and the modification of the vegetation cover and composition influence the soil development of this area. A combination of routine soil analyses (i.e., grain-size distributions, total organic carbon, total nitrogen, pH, and Fe/Al extractions), soil micromorphology and the Rock-Eval® pyrolysis allowed to characterize and to correlate the different soil units constituting a toposequence of six soil profiles. The presence of different pedological units that can be correlated along the slope underlines the occurrence of separate events of pedogenesis, spatio-temporally linked to recognizable stability phases at slope scale. These phases of biostasy, characterized by vegetation cover and soil development, alternate to phases of rhexistasy, characterized mainly by slope instability (i.e., aggradation/degradation). In detail, in the Mt. Cusna toposequence three different soil units, linked to three different stability phases, have been identified: the earliest stability phase, characterized by the presence of well-developed Luvisols, the subsequent stability phase typified by less expressed Luvisols, and the ongoing stability phase with Leptosols. This latter pedogenetic phase, in some cases, is superimposed to the previous one, so affecting the exhumed paleosols. In this light, the Mt. Cusna toposequence characterization allowed to enlighten the complexity of soil polygenesis in higher detail than the previous studies, not only reconstructing the past environmental conditions but also inferring the succession of phases of slope stability and phases characterized by erosion and deposition processes.
... The higher the R-index, the more organic fractions are thermally resistant. Recent application of the R-index proved its interest in agricultural management systems in tropical areas (Malou et al., 2020). ...
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Rubber tree plantations (Hevea brasiliensis) cover large areas in the tropics. In historical producing regions like South Thailand, rubber has been planted by smallholders for three successive rotations lasting a total of 75 years. Despite possible consequences on topsoil, the long-term impacts of repeated rubber plantations on soil quality remain unknown. This study aims to better understand how various factors linked to long-term rubber land use and land use change affect topsoil physico-chemical properties and soil organic carbon (SOC) thermal stability. We focus on the effects of three factors: i. deforestation (change from forest to first rubber plantation); ii. the age of the rubber stand (immature vs mature); and iii. Long-term rubber cultivation (first, second or third successive rotation) over a chronosequence in farmers plots. Our results show that soil was deeply degraded after deforestation to a rubber plantation. Long-term rubber cultivation is also detrimental for the soil and has a more negative impact on soil physico-chemical properties and carbon dynamics, than the age of the rubber stand (e.g. on average, decrease of 50% of SOC content between forest and third rotation). At the third rotation, after 50 years of rubber cultivation, the quality of the 0–10 cm soil layer was very low, with an increase in SOC thermal stability. At this stage, logging practices upset the sustainability of the system. These impacts could be limited by less destructive practices during planting. This article is protected by copyright. All rights reserved.
... Studies have shown that climate change will have a significant effect on the soil and water availability and the biodiversity of rangelands (Zhang et al., 2012;Mondal et al., 2019). Previous researches (Li et al., 2018;Dignac et al., 2017;Malou et al., 2020) have shown that farming activities and the transfer of rangelands to croplands will lead to pure liberation of C into the environment and a decline of C contained in soils that have dramatically impacted ionospheric CO 2 and steady global C amounts during the last decades. As a result, the transfer of these lands over the past 100 years has affected all sections of society, especially farmers around the world, and Iran has been no exception (Zhao et al., 2013). ...
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This study investigated the change in soil organic carbon (SOC), soil labile carbon (C), soil total nitrogen (N), particulate organic carbon (POC), mineralizable soil C (Min-C) in soil aggregate fractions, and soil particles size at 0–30 cm soil depth of the semi-steppe rangelands in Central Iran (Sheida and Khargosh regions). The study used a fully randomized design with four land uses in the rangelands including covered with rainfed wheat (CR0), abandoned rangelands (AR) for 5–15 years (AR15), abandoned rangeland for 16–40 years (AR40), and permanently uncultivated rangelands (UR) as the reference sites that were replicated three times (n = 3) with at most 2 km distance from each other in an area of 25 ha. For each treatment, the soil samples were randomly obtained at 0–30 cm soil depth in three replicated plots of 2 × 2 m using a core size of 7 cm in diameter. The samples were mixed to take a composite soil sample (12 soil samples in each region) in June 2018. The results indicated that soil POC is the most sensitive fraction for the identification of shifts in the total SOC after the cultivation cessation because by increasing the abandoned land and the cessation of cropland activities, the POC has also increased. In conclusion, after the cessation of rainfed cropping and the recovery of natural vegetation, the increases in soil microorganism activity can result in a higher nutrient cycling in the degraded rangeland ecosystems.
... Other works reported that the amounts of SOC associated with the coarse fractions (F > 50 μm) in sandy soils, mostly comprising particulate organic matter (Christensen 2001), are an important part of the total SOC contents and contribute to SOC storage after a change in cropland management (Fujisaki et al. 2018a;Manlay et al. 2002;Zinn et al. 2007). Moreover, in a complementary investigation using a Rock-Eval method to assess the thermal stability of soil organic matter (SOM), in the same territories and management practices as in the present study, Malou et al. (2020) revealed that SOM of these soils was dominated by thermally labile forms that mineralize quickly (Sebag et al. 2016). It is especially important for such Arenosols, as crop production relies on fast cycling organic matter (Wood et al. 2016). ...
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Soil organic carbon (SOC) is essential for the productivity of agroecosystems and for mitigating climate change. Because the SOC contents of sandy soils are usually small, the effects of agricultural management upon SOC stocks in such soils have been insufficiently studied. In West sub-arid Africa, the coarse-textured soils (mostly Arenosols) are diversely managed by smallholders. In this study, we aimed to quantify SOC stocks in cultivated soils of that region, in a context where agricultural practices rely mainly upon organic inputs derived from various integrated crop-livestock systems. SOC stocks were estimated for the 0–30 cm depth in 1,813 plots in Senegal’s groundnut basin. We found that SOC stocks in farmers’ fields varied between 2.3 and 59.8 Mg C ha-1 (mean ± standard deviation, 14.6 ± 0.14 Mg C ha-1). SOC stocks were influenced slightly by soil type, but were only weakly correlated to soils’ clay and silt contents. SOC stocks differed significantly among the three studied village territories due to contrasting livestock-raising systems. Average stocks were significantly higher in plots close to housings (home-fields), which receive larger amounts of organic inputs, than in plots farther from the village (out-fields). Thus, the organic inputs to home-fields improves soil C stocks of these sandy soils in the short term. Innovative agricultural practices in the studied area probably need to target options for managing all fields optimally. Those options will require continuous application of organic products—a measure that will in turn require solutions for improving availability or management of local organic resources.
... Open-system programmed-pyrolysis techniques such as Rock-Eval, have found increased application for geochemical screening of hydrocarbon-plays (Espitalié et al., 1977;Peters and Cassa, 1994;Peters et al., 2005;Hazra et al., 2019a). The Rock-Eval technique, apart from finding its usage in hydrocarbon-plays, has been used for characterizing organic matter in soil and recent sediments (Sanei et al., 2005;Sebag et al., 2016Sebag et al., 2018Malou et al., 2020), soil contamination studies (Poot et al., 2009), etc. The reason for the extensive usage of Rock-Eval by researchers working in different fields is due to its rapid analysis technique and generation of reliable data. ...
Article
The Rock-Eval pyrolysis-stage derived parameters such as free hydrocarbons (S1), heavier pyrolysis-hydrocarbons (S2), pyrolyzable carbon (PC) and pyrolysis Tmax (from S2 curve) have received considerable interest for source-rock screening and thermal maturity assessment. On the other hand, the Rock-Eval oxidation-stage S4CO2 curve, which gives the amount of residual carbon (RC), only recently has received some interest. While the pyrolysis-stage S2 temperature-peak (Tmax) is conventionally used as a maturity proxy, in this work we show that the temperature-peak of S4CO2 curve (S4Tmax) can also be used as a thermal maturity proxy for shales. For overmature and low-TOC shale samples, showing asymmetric S2 shape and concomitantly producing doubtful Tmax, the S4 curves showed symmetric nature and consequently the S4Tmax was observed to be a reliable thermal maturity estimate. While the S4Tmax clearly resolved immature and overmature shales, for the early mature and peak mature shales the S4Tmax showed overlapping values. S4Tmax of pre-pyrolyzed and pyrolyzed masses showed good positive correlation with differential scanning calorimetry temperature-peak (DSCTpeak), and consequently indicated its applicability as a thermal maturity proxy. When early mature pre-pyrolyzed samples were directly analyzed using the Rock-Eval oxidation stage, the S4 curves showed formation of two sub-peaks, and consequently the Tmax was observed to decrease. It is recommended that analysts and interpreters should thoroughly cross-check S2 curves before reporting data, and in case of asymmetric or unreliable S2 curves, the S4Tmax can be used as a maturity proxy.
... Among the selected sites, most of farming activities followed the rainy season, which is uni-modal from June to October, with annual rainfall between 500 to 900 mm. Soils vary, but are mostly sandy with low fertility (Malou et al., 2020). ...
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Pearl millet (Pennisetum glaucum L.) plays a critical role in smallholder food security in sub-Saharan Africa. The production of pearl millet has, however, stagnated or even declined due to several factors. The objective of this study was to assess farmer perceptions on production constraints and varietal preferences in Senegal. A survey was conducted involving 150 randomly selected farmers from 15 villages, in five representative rural communities of Senegal. A semi-structured questionnaire was used, supplemented by focus group discussions. Results revealed that parasitic Striga weed was the most constraining factor to pearl millet production across the rural communes. This was followed by low soil fertility and insect pests in that order. Other constraints included lack of machinery for sowing, plant diseases, drought, seed-eating birds, limited access to land for pearl millet cultivation and limited seed availability. Among the traits for varietal preference, farmers unanimously considered grain yield as the most important trait. Other important traits mentioned were adaptation to drought, adaptation to low soil fertility and earliness. These production constraints and varietal preference should be integrated in the profile of the national pearl millet breeding programmes in order to improve the productivity and adoption of bred-cultivars.
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Changes in agricultural land productivity have become a crucial criterion for assessing the effectiveness of land consolidation (LC). In this study, agricultural land productivity under LC was observed and monitored in terms of two important factors: vegetation productivity and soil productivity, which were represented by net primary productivity (NPP) and soil organic matter (SOM), respectively. Based on spatial and temporal variations before and after the implementation of LC, the response of vegetation and soil productivity to LC was further discussed. The main results are as follows: (1) The effectiveness of LC could be quantitatively confirmed in terms of both vegetation and soil productivity. Compared with vegetation productivity, soil productivity had stronger and more positive response to LC. (2) The effectiveness of LC was characterized by temporal attenuation, which specifically showed that the improvement of vegetation and soil productivity followed a trajectory of "increase–stability" after the implementation of LC. (3) The effectiveness of LC was seasonal, coinciding with the farming period. In other words, the effectiveness of LC became completely prominent with the implementation of agricultural activities. This phenomenon was particularly more pronounced from the aspect of soil productivity.
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Soil organic matter (OM) is a complex heterogeneous mixture: resulting from decomposition and organo-mineral interactions, it challenges characterization in terms of composition and biogeochemical stability. From this perspective, the Rock-Eval® method is a rapid and efficient thermal analysis, which combines quantitative and qualitative information on soil OM, including several parameters related to thermal stability. This approach has already been used to monitor changes in OM properties at landscape, cropland, and profile scales. This study aims to assess the stability of soil organic matter pools by characterizing grain-size fractions from forest litters and topsoils using Rock-Eval® thermal analyses. Organic and topsoil samples were selected from a beech forest located in Normandy (France), whose management has been documented for the last 200 years. Fractionation by wet sieving was used to separate large debris (>2000 μm), coarse (200–2000 μm), and fine particulate organic matter (50–200 μm) in organic samples, and coarse (200–2000 μm), medium (50–200 μm), and fine (<50 μm) fractions in topsoils. Rock-Eval® was able to provide thermal parameters sensitive enough to study fine-scale soil processes. In organic layers, quantitative and qualitative changes are all explained by progressive decomposition of labile organic compounds from plant debris to the finest organic particles. On the other hand, the grain size fractions of the topsoil display different characteristics: indeed, the coarse organo-mineral fractions show high C contents, but with a different composition and a higher thermal stability and degree of decomposition than the plant debris forming the organic layers. These results are consistent with previous studies concluding that the microbial activity is more effective in this fraction. The finest fractions of topsoil reveal low C contents and the highest thermal stability, but also a low degree of decomposition, which can be explained by stronger interactions with the mineral matrix. Therefore, it is suggested that the dynamics of OM present in the different size fractions be interpreted in the light of a plant-microbes-soil continuum. Finally, three distinct thermal stability C pools are highlighted through the grain-size heterogeneity of soil OM: free-coarse organic matter (large debris, coarse and fine particles), weakly-protected organic matter in (bio)aggregates (coarse fraction of topsoil), and stabilized organic matter in fine fractions of topsoil, the latter resulting from interactions inside organo-mineral complexes. These results allow Rock-Eval® thermal parameters to be used in order to empirically illustrate the conceptual models emphasizing the role of drivers played by the gradual decomposition and protection of the most thermally labile organic constituents.
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Geochemical indicators are emerging as important predictors of soil organic carbon (SOC) dynamics, but evidence concerning the role of calcium (Ca) is scarce. This study investigates the role of Ca prevalence in SOC accumulation by comparing otherwise similar sites with (CaCO3-bearing) or without carbonates (CaCO3-free). We measured the SOC content and indicators of organic matter quality (C stable isotope composition, expressed as δ13C values, and thermal stability) in bulk soil samples. We then used sequential sonication and density fractionation (DF) to separate two occluded pools from free and mineral-associated SOC. The SOC content, mass, and δ13C values were determined in all the fractions. X-ray photoelectron spectroscopy was used to investigate the surface chemistry of selected fractions. Our hypothesis was that occlusion would be more prevalent at the CaCO3-bearing site due to the influence of Ca on aggregation, inhibiting oxidative transformation, and preserving lower δ13C values. Bulk SOC content was twice as high in the CaCO3-bearing profiles, which also had lower bulk δ13C values, and more occluded SOC. Yet, contrary to our hypothesis, occlusion only accounted for a small proportion of total SOC (< 10 %). Instead, it was the heavy fraction (HF), containing mineral-associated organic C, which accounted for the majority of total SOC and for the lower bulk δ13C values. Overall, an increased Ca prevalence was associated with a near-doubling of mineral-associated SOC content. Future investigations should now aim to isolate Ca-mediated complexation processes that increase organo-mineral association and preserve organic matter with lower δ13C values.
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In this work, we assess whether or not ramped thermal oxidation coupled with determination of the radiocarbon content of the evolved CO2 can be used to isolate distinct thermal fractions of soil organic matter (SOM) along with direct information on the turnover rate of each thermal fraction. Using a 30-year time series of soil samples from a well-characterized agronomic trial, we found that the incorporation of the bomb spike in atmospheric 14CO2 into thermal fractions of increasing resistance to thermal decomposition could be successfully modeled. With increasing temperature, which is proportional to activation energy, the mean residence time of the thermal fractions increased from 10 to 400 years. Importantly, the first four of five thermal fractions appeared to be a mixture of fast- and increasingly slower-cycling SOM. To further understand the composition of different thermal fractions, stepped pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS) experiments were performed at five temperatures ranging from 330 to 735 ∘C. The Py-GC/MS data showed a reproducible shift in the chemistry of pyrolysis products across the temperature gradient trending from polysaccharides and lipids at low temperature to lignin- and microbe-derived compounds at middle temperatures to aromatic and unknown compounds at the highest temperatures. Integrating the 14C and Py-GC/MS data suggests the organic compounds, with the exception of aromatic moieties likely derived from wildfire, with centennial residence times are not more complex but may be protected from pyrolysis, and likely also from biological mineralization, by interactions with mineral surfaces.
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Sandy soils cover approximately 900 million ha worldwide particularly in arid and semiarid regions. There are extensive areas of sandy soils under cultivation, but the soil fertility is often low and dependent on the levels of soil organic carbon (SOC). Here, we review SOC levels of sandy soils across the world using pedon databases, data from literature, and three detailed case studies. Pedons were selected from five major databases and the pedons had a minimum of 850g sand kg1in the top 30cm. Sandy soils occur in all climates and several soil orders (mainly Alfisols, Entisols, Inceptisols, Spodosols, Ultisols). Soil organic carbon was highest in sandy soils of the temperate and cold zones (mean 19 g kg-1), and lowest in soils of the arid zone (<5 g kg-1). The SOC concentrations was highest when mean annual precipitation was between 700 and 1300mm, and SOC was lower when annual temperatures were higher. Sandy soils under forest had on average23 g SOC kg-1 whereas the soils under prairie had on average 7 g SOC kg-1. Spodosols had the highest SOC concentrations, and SOC was lowest in Aridisols. The average A-horizon thickness of sandy soils was 17 cm, and it was thickest in Mollisols, and thinnest in Inceptisols. The thickness of the A-horizon is important since most of the SOC of sandy soils is found in the A-horizon. Over 32,000 papers have been published on sandy soils since 1914, and in the past 10 years, some 1000–1800 papers are annually published on sandy soils. We extracted papers since the early 1990s, and grouped them by continent, land cover, soil order, and focus of the research. Three soil-forming processes have been extensively studied in relation to SOC and sandy soils: podzolization, humification, and melanization. Several studies found that SOC increases when sandy soils were fertilized, but sandy soils become more water repellent with an increase in SOC and a decrease in pH. From published literature, it was found that SOC was high in sandy soils under forest (<90 g kg-1) and grassland (<187 g kg-1), and low in sandy soils under agriculture (<38 g kg-1) or under shrub (<36 g kg-1). Lastly, we present case studies from Southern Africa, Western Europe, and North America. Sandy soils cover about 273 million ha in Africa, and SOC rarely exceeds 10 g SOC kg-1 in the top 30cm. The plaggen soils of Western Europe have high SOC (up to 66 g kg-1) due to the long-term additions of sod, litter, manure, and sea sand to increase the soil fertility. In the Wisconsin Central Sand Plains, SOC stocks in soils under agriculture were high (60 Mg ha-1) compared to the soils under forest (15 Mg ha-1) and grassland (25 Mg ha-1). Available water capacity increased with an increase in SOC. From this review, we conclude that sandy soils are found throughout the world, and the sandy soils in the temperate and cold zones have the highest SOC levels. In sandy soils, SOC increased the cation exchange capacity and lowered the bulk density. Soil organic carbon levels can be significantly increased in sandy soils under agriculture when amendments are made in combination with irrigation.
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The study area of the Chobe Enclave (northern Botswana) is defined as mostly covered by Arenosols in available maps. However, recent explorations of the area showed that soils are more diverse than expected. This is because of complex interactions between current alluvial deposition processes, paleo-environmental effects (ancient alluvial deposition, ancient wind-blown sand deposits) and ongoing hydrological effects and colluvial effects on topographic gradients. An in-depth exploration of both soils and vegetation in the area was conducted with the aim (i) to survey the soil diversity at the Chobe Enclave, (ii) to study soil dynamics and identify the key factors of this diversity, and (iii) to create a soil map based on the analysis of the soil-vegetation relationship. For this purpose, thirty-six soil profiles were extensively described according to the World Reference Base for soil resources. In order to better classify these soils, physicochemical parameters, such as pHH2O, exchangeable cations, and particle size distributions, were measured for a specific set of soils (n = 16), representative of their diversity. To assess Soil Organic Matter (SOM) dynamics, samples were studied using Rock Eval pyrolysis. Results show a high soil diversity and heterogeneity with the presence of (i) Arenosols, as expected, but also of (ii) organic-rich soils, such as Chernozems, Phaeozems, and Kastanozems, (iii) salty/sodic soils, such as Solonchaks and Solonetz, and finally (iv) calcium-rich soils, such as Calcisols. Analyses of the different actors driving the soil diversity emphasized the importance of the surficial geology, composed of different sand deposits (red sands/white sands), carbonate and diatomite beds, as well as ancient salt deposits, in which high proportions of exchangeable Na⁺ were found, associated with high pHH2O (up to 11.3). In addition, as a parameter, the topography creates a complex hydrological system in the Chobe Enclave and therefore, induces a notable soil moisture gradient. Moreover, this study stressed the key role of termites: not only do they modify physicochemical patterns of soils, but they also decay and incorporate large quantities of fresh plant materials into soils. Finally, the analysis of Organic Matter (OM) showed that the Soil Organic Carbon (SOC) is composed essentially by recalcitrant Organic Carbon (OC) substances, such as charcoal, a common carbon type of tropical soils.
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Resilient, productive soils are necessary to sustainably intensify agriculture to increase yields while minimizing environmental harm. To conserve and regenerate productive soils, the need to maintain and build soil organic matter (SOM) has received considerable attention. Although SOM is considered key to soil health, its relationship with yield is contested because of local-scale differences in soils, climate, and farming systems. There is a need to quantify this relationship to set a general framework for how soil management could potentially contribute to the goals of sustainable intensification. We developed a quantitative model exploring how SOM relates to crop yield potential of maize and wheat in light of co-varying factors of management, soil type, and climate. We found that yields of these two crops are on average greater with higher concentrations of SOC (soil organic carbon). However, yield increases level off at ∼2% SOC. Nevertheless, approximately two-thirds of the world's cultivated maize and wheat lands currently have SOC contents of less than 2%. Using this regression relationship developed from published empirical data, we then estimated how an increase in SOC concentrations up to regionally specific targets could potentially help reduce reliance on nitrogen (N) fertilizer and help close global yield gaps. Potential N fertilizer reductions associated with increasing SOC amount to 7% and 5% of global N fertilizer inputs across maize and wheat fields, respectively. Potential yield increases of 10±11% (mean±SD) for maize and 23±37% for wheat amount to 32% of the projected yield gap for maize and 60% of that for wheat. Our analysis provides a global-level prediction for relating SOC to crop yields. Further work employing similar approaches to regional and local data, coupled with experimental work to disentangle causative effects of SOC on yield and vice versa, is needed to provide practical prescriptions to incentivize soil management for sustainable intensification.
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Improved soil management is increasingly pursued to ensure food security for the world's rising global population, with the ancillary benefit of storing carbon in soils to lower the threat of climate change. While all increments to soil organic matter are laudable, we suggest caution in ascribing large, potential climate change mitigation to enhanced soil management. We find that the most promising techniques, including applications of biochar and enhanced silicate weathering, collectively are not likely to balance more than 5% of annual emissions of CO2 from fossil fuel combustion.
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Our understanding of mechanisms governing soil organic matter (OM) stability is evolving. It is gradually becoming accepted that soil OM stability is not primarily regulated by the molecular structure of plant inputs, but instead by the biotic and abiotic properties of the edaphic environment. Moreover, several experimental studies conducted in artificial systems have suggested that mechanisms regulating OM stability may differ with depth in the soil profile. Up to now however, there is very limited field-scale evidence regarding the hierarchy of controls on soil OM dynamics and their changes with soil depth. In this study, we take advantage of the high heterogeneity of ecological conditions occurring in the alpine belt to identify the major determinants of OM dynamics and how their significance varies with depth in the soil profile. Aboveground litter, mineral topsoil, and subsoil samples originating from 46 soil profiles spanning a wide range of soil and vegetation types were analysed. We used Rock-Eval pyrolysis, a technique that investigates the thermal stability of OM, as an indicator of OM dynamics. Our results show a clear divergence in predictors of OM thermal stability in the litter, topsoil, and subsoil layers. The composition of OM correlated with its thermal stability in the litter layer but not in mineral soil horizons, where the supply rate of fresh organic material and the physical and chemical characteristics of the pedogenic environment appeared important instead. This study offers direct confirmation that soil OM dynamics are influenced by different ecosystem properties in each soil layer. This has important implications for our understanding of carbon cycling in soils under a changing climate.
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The 4‰ initiative launched by the French government at COP21 in Paris in December 2015 aspires to increase global soil organic carbon (SOC) stocks at a rate of 0.4% per year. We conducted a systematic literature review on SOC storage under agroforestry and conservation agriculture systems in sub-Saharan Africa, where we reported 66 and 33 cases for both systems respectively. The results showed that SOC storage rates were significantly higher than 4‰ yr−1 in fallows and in multistrata agroforestry systems (P = 0.0001 and 0.0178, respectively), but not in alley cropping and parklands systems. For conservation agriculture, SOC storage rates were only significantly higher than 4‰ yr−1 (P = 0.0438) when all three principles were applied, i.e. no- or minimum tillage combined with crop residue retention and intercropping or rotation. The data showed very large variability in SOC storage rates as the result of various factors, including previous land-use history, experimental set up and approach used to determine SOC storage (diachronic versus synchronic approach), soil type, depth of soil sampling, type of crops and management, and duration of the experiment. SOC storage rates significantly decreased with time in the agroforestry systems (P = 0.0328). However, we were unable to find significant relationships with initial SOC stocks or tree density. Given the limited published data and the high variability in results, no significant relationships between SOC storage rates and site variables were found for conservation agriculture. We argue that there is a potential for SOC storage in agricultural soils of sub-Saharan Africa, as illustrated by SOC gaps observed on smallholder farms. Low SOC levels are, however, to a great extent the result of limited resources of most smallholder farmers. Practices such as agroforestry and conservation agriculture can restore SOC in these soils, but the 4‰ initiative has to be implemented on the grounds of the positive impact on crop productivity rather than on climate change mitigation. The efficiency in doing so will depend on the specific situations and will need economic support to smallholder farmers, including the promotion of good markets for sale of extra produce and for input supply, effective private support and policy, such as credit schemes and subsidies for inputs, and efficient extension services which incentivize farmers to invest in new technologies.
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The 4‰ initiative to sequester carbon in soils has the potential to connect sustainable development goals, enhance food security and mitigate climate change by utilizing waste organic residues.
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A large fraction of soil organic matter (OM) resists decomposition over decades to centuries as indicated by long radiocarbon residence times, but the mechanisms responsible for the long-term (multi-decadal) persistence are debated. The current lack of mechanistic understanding limits our ability to accurately predict soil OM stock evolution under climate and land-use changes. Using a unique set of historic soil samples from five long-term (27–79 years) bare fallow experiments, we demonstrate that despite wide pedo-climatic diversity, persistent OM shows specific energetic signatures, but no uniform chemical composition. From an energetic point of view, thermal analyses revealed that combustion of persistent OM occurred at higher temperature and provided less energy than combustion of more labile OM. In terms of chemical composition, persistent OM was H-depleted compared to OM present at the start of bare fallow, but spectroscopic analyses of OM functional groups did not reflect a consistent chemical composition of OM across sites, nor substantial modifications with bare fallow duration. The low energy content of persistent OM may be attributed to a combination of reduced content of energetic C–H bonds or stronger interactions between OM and the mineral matrix. Soil microorganisms thus appear to preferentially mineralize high-energy OM, leaving behind material with low energy content. This study provides the first direct link between long-term persistence of OM in soil and the energetic barriers experienced by the decomposer community.
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The stability of soil organic matter (SOM) as it relates to resistance to microbial degradation has important implications for nutrient cycling, emission of greenhouse gases, and C sequestration. Hence, there is interest in developing new ways to quantify and characterise the labile and stable forms of SOM. Our objective in this study was to evaluate SOM under widely contrasting management regimes to determine whether the variation in chemical composition and resistance to pyrolysis observed for various constituent C fractions could be related to their resistance to decomposition. Samples from the same soil under permanent pasture, an arable cropping rotation, and chemical fallow were physically fractionated (sand: 2000-50 μm; silt: 50-5 μm, and clay: <5 μm). Biodegradability of the SOM in size fractions and whole soils was assessed in a laboratory mineralization study. Thermal stability was determined by analytical pyrolysis using a Rock-Eval pyrolyser, and chemical composition was characterized by X-ray absorption near-edge structure (XANES) spectroscopy at the C and N K-edges. Relative to the pasture soil, SOM in the arable and fallow soils declined by 30% and 40%, respectively. The mineralization bioassay showed that SOM in whole soil and soil fractions under fallow was less susceptible to biodegradation than that in other management practices. The SOM in the sand fraction was significantly more biodegradable than that in the silt or clay fractions. Analysis by XANES showed a proportional increase in carboxylates and a reduction in amides (protein) and aromatics in the fallow whole soil compared to the pasture and arable soils. Moreover, protein depletion was greatest in the sand fraction of the fallow soil. Sand fractions in fallow and arable soils were, however, relatively enriched in plant-derived phenols, aromatics, and carboxylates compared to the sand fraction of pasture soils. Analytical pyrolysis showed distinct differences in the thermal stability of SOM among the whole soil and their size fractions; it also showed that the loss of SOM generally involved preferential degradation of H-rich compounds. The temperature at which half of the C was pyrolyzed was strongly correlated with mineralizable C, providing good evidence for a link between the biological and thermal stability of SOM.
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Ce numéro d'Innovations Agronomiques comprend les articles correspondant aux présentations du colloque « Atténuation des Gaz à Effet de Serre par l’agriculture » qui s'est tenu à Versailles le 4 juin 2014.
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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.
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This paper reviews our current knowledge and understanding on carbon storage in soils under the effect of climate change and land management. For the French metropolitan territory, the carbon storage potential and possible effects of climate change on carbon decrease are of the same order of magnitude. It might be more important to preserve existing large stocks (for example in peat) than to try to create new ones. Whatever the changes are, they will be very difficult to prove within carbon accounting over short periods.
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Institut de recherche pour le développement (l.R.D. • ex-Orstom), laboratoire d'agronomie de "Institut national de recherche agronomique (Tnra). 78850 Thiverval-Grignon (France). 140 Anneke DE Rouw RéSUMÉ D'AUTEUR Le Sahel subit des risques de sécheresse et ne dispose que de sols pauvres. Pour lutter contre la diminution de la fertilité des champs, les paysans ont recours à deux techniques: la mise en jachère et la fumure organique (essentiellement sous forme de restitution par le parcage). Dans la zone d'étude, coexistent des systè-mes de cultures sur jachères, peu ou assez dégradées, et des systèmes de culture avec fumure organique. Du fait des dépÔts de poussières pendant la période de jachère, la surface du sol s'enrichit en éléments fins, puis le sol devient relativement riche en matière or-ganique. Lors d'une période de culture sans intrants, le stock en matière organi-que diminue et les éléments fms sont sujets aux érosions éolienne et hydrique par suite des sarclages et du défrichement. Une succession des cycles culture-jachère renforce ces pertes. Le parcage extensif arrive à atténuer les pertes en matière organique mais pas celles en éléments fins. L'analyse de l'horizon superficiel (matière organique et texture) permet de distinguer quatre systèmes de culture: sans intrants sur jachère -soit dix ans de culture et plus de quinze ans de ja-chère, soit quatre à cinq ans de culture et trois à quatre ans de jachère; culture permanente avec parcage -soit après plus de quinze ans de jachère, soit après trois à quatre ans de jachère.
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Carbon stored in soils worldwide exceeds the amount of carbon stored in phytomass and the atmosphere. Despite the large quantity of carbon stored as soil organic carbon (SOC), consensus is lacking on the size of global SOC stocks, their spatial distribution, and the carbon emissions from soils due to changes in land use and land cover. This article summarizes published estimates of global SOC stocks through time and provides an overview of the likely impacts of management options on SOC stocks. We then discuss the implications of existing knowledge of SOC stocks, their geographical distribution and the emissions due to management regimes on policy decisions, and the need for better soil carbon science to mitigate losses and enhance soil carbon stocks.
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Heterogeneity in soil fertility in these smallholder systems is caused by both inherent soil-landscape and human-induced variability across farms differing in resources and practices. Interventions to address the problem of poor soil fertility in Africa must be designed to target such diversity and spatially heterogeneity. Data on soil management and soil fertility from six districts in Kenya and Uganda were gathered to understand the determinants of soil heterogeneity within farms. Analysis of the variance of soil fertility indicators across 250 randomly selected farms (i.e., 2607 fields), using a mixed model that considered site, sampling frame, farm type, and field as random terms, revealed that the variation in soil organic C (6.5-27.7 g kg(-1)), total N (0.6-3.0 g kg(-1)), and available P (0.9-27 mg kg(-1)) was mostly related to differences in the inherent properties of the soils across sites (50 to 60% of total variance). Exchangeable K+ (0.1-1.1 cmol((+)) kg(-1)), Ca2+ (1.5-14.5 cmol((+)) kg(-1)), Mg2+ (0.6-3.7 cmol((+)) kg(-1)), and pH (5.1-6.9) exhibited larger residual variability associated with field-to-field differences within farms (30 to 50%). Soil fertility indicators decreased significantly with increasing distance from the homesteads. When this variable was included in the model, the unexplained residual variances-associated with soil heterogeneity within farms-were 38% for soil C; 32% for total N; 49% for available P; 56, 49, and 38% for exchangeable K+, Ca2+ and Mg2+, respectively; and 49% for the pH. In allocating nutrient resources, farmers prioritized fields they perceived as most fertile, reinforcing soil heterogeneity. Categorization of fields within a farm with respect to distance from the homestead, and soil fertility classes as perceived by farmers, were identified as entry points to target soil fertility recommendations to easily recognizable, distinct entities.
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Organic matter (OM) is a key component of soils but information on its chemistry and behavior in soils is still incomplete. Numerous methods are commonly used to characterize and monitor OM dynamics, but only a few include the qualities required to become routine techniques i.e. simple, rapid, accurate and at low cost. Rock–Eval pyrolysis (RE pyrolysis) is a good candidate, as it provides an overview of OM properties by monitoring four com- ponents related to the main major classes of organic constituents (from A1 for the labile biological constit- uents to A4 for the mature refractory fraction). However, a question is still pending: do these four major classes used in the literature reflect a pertinent compositional chemical counterpart? 13C Nuclear Magnetic Resonance Spectroscopy in the solid state (13C CPMAS NMR) has been used to answer this question by collecting information on structural and conformational characteristics of OM. Moreover, in order to avoid the blurring effect of pedogenesis on OM dynamics, a ‘‘less complex OM’’ source, i.e. compost samples, has been used. Results showed significant and high determination coefficients between classes, indi- ces (of transformation of plant biopolymers, humifi- cation...) from RE pyrolysis, and the main classes of OM characterized by 13C NMR, e.g. A1 & A2 with labile/easily degradable components (alkyl C et O-alkyl C), A3 & A4 with humified OM (with aromatic C and phenolic C). The R index (contribution of bio- macromolecules) is correlated with phenolic and aromatic C, whereas the I index (related to immature OM) refers to labile––easily degradable components (alkyl C et O-alkyl C). The results confirm the pertinence of RE pyrolysis to monitor OM dynamics.
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The “4 per Thousand” and “Adapting African Agriculture” are bold and innovative initiatives adopted at COP21 in Paris and COP22 in Marrakesh, respectively. These initiatives are soil-centric and based on adoption of soil-restorative and improved agricultural practices. The objective of this article is to discuss the merits and challenges of South–South Cooperation (SSC) in promoting the adoption of best management practices (BMPs) such as conservation agriculture (CA) and sustainable intensification (SI). Basic principles of CA are: retention of crop residue mulch, incorporation of cover crops and complex rotations, integrated nutrient management and elimination of soil disturbance. The strategy of SI is to produce more from less by enhancing the eco-efficiency, reducing waste, and restoring soil health. Whereas CA has been successfully adopted in Brazil, Argentina, Chile and other regions of South America, its potential of harnessing agronomic and ecologic benefits has not been realized in Sub-Saharan Africa, South Asia, and elsewhere in The Global South. The strategy of SSC is pertinent because of the ten basic principles or tenets: lack of hierarchy, equal participation in all decision-making processes along with transparency, trust, mutual respect, and accountability. However, several concerns have been raised regarding issues such as land grab, and access to resources etc. Based on the scientific concepts of SI, producing more from less, even a triangular cooperation (TAC) or South-South-North (SSNC) cooperation can be developed to achieve adaptation and mitigation of climate change, advance food security, improve degraded soils and restore soil health through soil organic carbon (SOC) sequestration, and advance Sustainable Development Goals (SDGs) of the U.N. A widespread adoption of CA and SI through SSC, TAC or SSNC can advance SDGs including #1 (end poverty), #2 (eliminate hunger), #6 (clean water), #13 (climate action), and #15 (life on land). Of the global cropland area under CA estimated at ∼180 million hectare (Mha) in 2015–16, land area under CA is only 2.7 Mha in Africa and 13.2 Mha in Asia. SSC, TAC and SSNC can build upon the existing and on-going initiatives by national and international organizations.
Thesis
Les systèmes agro-sylvo-pastoraux (SASP) d’Afrique de l’Ouest sont des agro-écosystèmes limités en biomasses et en nutriments. Le recyclage des nutriments et les transferts de fertilité sont traditionnellement rythmés par la mobilité des troupeaux de ruminants conduits en extensif. Les agro-éleveurs pratiquent le parcage nocturne de leurs troupeaux pour concentrer la matière organique, dans les champs à proximité des habitations afin de sécuriser une production vivrière suffisante à leurs besoins. Dans un contexte de croissance démographique et de réduction des parcours naturels au profit des zones cultivées, le système d’élevage « traditionnel », basé sur une forte mobilité intra-terroir villageois, est remis en cause. Les stratégies adoptées par les agro-éleveurs sont, soit (i) l’éloignement des troupeaux du terroir villageois pendant des périodes plus ou moins longues par des pratiques de transhumance saisonnière vers des régions moins peuplées et disposant de davantage de ressources fourragères ; soit (ii) plus récemment, des pratiques d’intensification avec des animaux gardés à l’étable au sein du terroir villageois et nourris avec des aliments concentrés, achetés sur le marché local. Ces changements de systèmes d’élevage ont possiblement des conséquences importantes sur les flux de biomasses et les cycles des nutriments au niveau du ménage et du territoire. Il convenait de les évaluer en termes d’impacts sur le fonctionnement et la durabilité des SASP.A cet effet, le modèle multi-agents TERROIR a été développé et implémenté sur la plateforme de modélisation GAMA. Il simule l’effet de changements dans l’organisation du paysage et des systèmes d’élevage sur les flux de biomasse et d’azote aux différents niveaux d’organisation du territoire : la parcelle, le troupeau, le ménage et le terroir villageois. Le modèle simule les échanges de biomasses entre une centaine de ménages comportant des stratégies et des pratiques différentes. Cela inclut les transferts spatiaux de biomasses orchestrés par plusieurs centaines de troupeaux se déplaçant de façon indépendante sur un millier de parcelles. Le modèle synthétise ces flux par un ensemble d’indicateurs issus de deux méthodes d’analyse (« Ecological Network Analysis » et « System Gate Balance ») pour décrire la structure, le fonctionnement et la durabilité de l’agroécosystème, en termes de productivité, d’efficience, d’autonomie, de recyclage, de transferts spatiaux et de bilan de nutriments. Le modèle a été conçu et paramétré à partir des données disponibles sur les agroécosystèmes de savane en Afrique de l’Ouest et il a été évalué à partir des données observées dans deux terroirs villageois du bassin Arachidier au Sénégal où les pratiques des agro-éleveurs sont particulièrement contrastées.Le modèle TERROIR a été utilisé pour explorer les impacts des dynamiques territoriales observées sur la période 1920-2015 dans le bassin Arachidier au Sénégal, une zone agricole à transition agraire rapide et avancée. Les résultats soulignent une réorganisation du cycle de l’azote et une tendance générale à l’intensification des flux et à l’augmentation de la dépendance des agroécosystèmes vis-à-vis de sources extérieures de nutriments. Cependant, le recyclage et les transferts spatiaux de nutriments internes aux agrosystèmes restent à des niveaux élevés. L’intégration sol-plantes-animaux-hommes et l’hétérogénéité spatiale de la répartition des ressources fertilisantes apparaissent comme deux propriétés persistantes des agro-écosystèmes étudiés. Consolider cette intégration et cette organisation spatiale seraient ainsi un gage pour la durabilité des futurs systèmes agricoles qui émergeront dans un contexte de poursuite de la forte croissance démographique et de changement climatique.
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Stabilization of organic matter (OM) against decomposition and its de-stabilization leading to mineralization are important processes controlling greenhouse gas emissions and carbon sequestration in soils. Soil organisms, particularly earthworms, may greatly influence these processes through their potential impact on the interaction of OM with clay minerals, a mechanism which may protect OM at medium (years) to long (decades, centuries) time scales. The aim of this study was to use earthworms and pure minerals in order to enhance carbon sequestration through the formation of aggregates containing particulate OM and organo-mineral associations during a composting experiment. To this end we compared OM transformations in composting treatments with and without (i) clay minerals (montmorillonite) and (ii) epigeic earthworms (Eisenia andrei and Eisenia foetida). We used density fractionation of the end-products to quantify the amount of aggregates including organo-mineral associations formed during 196 days. Their chemical composition and stability were analyzed by elemental analyses, Rock-Eval 6 thermal analysis and solid-state ¹³C nuclear magnetic resonance spectrometry. Our results indicated that the addition of minerals led to the formation of heavy fractions most probably containing aggregates and organo-mineral associations in treatments with and without earthworms. While OM showed higher oxidative transformation in all treatments, addition of earthworms changed organic carbon (OC) and nitrogen (N) concentrations in specific density fractions and increased the thermal stability of OM in heavy density fractions. These fractions contained higher proportions of aromatic and proteinaceous material. We conclude that earthworms under composting conditions may be able to effectively stabilize OM, though the formation of aggregates and/or organo-mineral associations containing greater proportions of microbial-derived material.
Article
Much effort has been spent on formulating guidelines for inorganic fertilizer use in millet crops in Sub-sahelian farms. However, these guidelines do not take into account the diversity of manuring practices. In this study we analyzed over two years (2016–2017) the use efficiency of an inorganic NPK fertilizer as affected by the two most contrasted categories of organic manure strategies (OMS) found in millet fields of central Senegal. 19 farmers’ fields were selected in a village typical of that region, 11 and 8 of which respectively corresponding to categories OMS1 and OMS2 as follows: OMS1, locally referred to as Homefields, were fields continuously cropped with millet over the last 15 years, having received organic manure regularly in the past, and manured again at the onset of the 2016 rainy season. OMS2 fields locally referred to as Outfields, were not manured in 2016 and were rarely manured in the past. Four of them were continuously cropped with millet and the others had followed a triennial millet-peanut-fallow rotation. In 2017, no manure was applied in any of the OMS1 or OMS2 fields. A pairwise treatments with and without the same inorganic fertilizer dose was applied in each field in both 2016 and 2017 cropping seasons. In 2016, the higher the manure application, the higher the use efficiency of the inorganic fertilizer applied. The use efficiency of the inorganic N was most closely related to soil bulk density and P availability. In 2017, with no new manure amendment, millet yield in OMS1 was about three times higher than in 2016. It was close to the water-limited yield, suggesting that the residual effect of the manure applied in 2016 was high. The use efficiency of the inorganic N was generally low under these conditions. In OMS2, millet yield and use efficiency of inorganic fertilizer remained low in both years. The crop rotation with peanuts did not enrich the soil compared to the millet returning every year, but it reduced Striga hermontica infestation and increased the millet 1000-grain weight. The methodological approach developed here may help in formulating guidelines to deal with the diversity of farming practices in Sub-sahelian villages.
Article
Soil structure formation in alluvial soils is a fundamental process in near-natural floodplains. A stable soil structure is essential for many ecosystem services and helps to prevent river bank erosion. Plants and earthworms are successful soil engineering organisms that improve the soil structural stability through the incorporation of mineral and organic matter into soil aggregates. However, the heterogeneous succession of different textured mineral and buried organic matter layers could impede the development of a stable soil structure. Our study aims at improving the current understanding of soil structure formation and organic matter dynamics in near natural alluvial soils. We investigate the effects of soil engineering organisms, the composition, and the superimposition of different alluvial deposits on the structuration patterns, the aggregate stability, and organic matter dynamics in in vitro soil columns, representing sediment deposition processes in alluvial soils. Two successions of three different deposits, silt–buried litter–sand, and the inverse, were set up in mesocosms and allocated to four different treatments, i.e. plants, earthworms, plants + earthworms, and a control. X-ray computed tomography was used to identify structuration patterns generated by ecosystem engineers, i.e. plant root galleries and earthworm tunnels. Organic matter dynamics in macro-aggregates were investigated by Rock-Eval pyrolysis. Plant roots only extended in the top layers, whereas earthworms preferentially selected the buried litter and the silt layers. Soil structural stability measured via water stable aggregates (%WSA) increased in the presence of plants and in aggregates recovered from the buried litter layer. Organic matter dynamics were controlled by a complex interplay between the type of engineer, the composition (silt, sand, buried litter) and the succession of the deposits in the mesocosm. Our results indicate that the progress and efficiency of soil structure formation in alluvial soils strongly depends on the textural sequences of alluvial deposits.
Article
Refining our understanding of how soil structure develops is important because soil structure has a major influence on plant growth. Recent studies show positive correlations between soil organic matter and soil structure. However, the question remains: how are the relationships between soil organic matter (SOM) parameters and soil structure characteristics affected by the long-term fertilization of a sandy soil? In contrast to most other studies on SOM vs. soil structure, the present study is composed of long-term field experiments (a total of 3 experiments) with durations of 25, 41 and 94 years. In this paper, the impact of the long-term application of mineral fertilizers and manure on the SOM and soil structure of a sandy soil is quantified, and the relationships between the SOM and soil structure of a sandy soil with a dependence on the length of fertilizer application are determined. Soil samples were collected from all three long-term field experiments in central Poland, which were located at Skierniewice experimental station including a 94-year-old experiment with mineral fertilization, a 41-year-old experiment with mineral fertilization and a 25-year-old experiment with mineral fertilization + farmyard manure (FYM) in a 4-year cycle. In the spring of 2017, soil samples were collected (Co – no fertilizers, NPK – NPK fertilizers, CaNPK – CaNPK fertilizers). In the 94-year-old experiment, the content of soil organic carbon (SOC) in Co, NPK and CaNPK was 4.07, 5.89 and 5.99 g kg ⁻¹ , respectively. An increase in the SOC content under fertilization was also found in the other two experiments. In the 25-year-old experiment, the SOC contents in FYM, FYM + NPK and FYM + CaNPK were 6.07, 8.36 and 7.63 g kg ⁻¹ , respectively, and in the 41-year-old experiment, the SOC contents in Co, NPK and CaNPK were 6.38, 10.1 and 7.80 g kg ⁻¹ , respectively. The content of labile carbon (C L ) increased significantly in the fertilized treatments only in the 94-year-old experiment. The contents of humic substances significantly increased in the soil of fertilized treatments in the 25- and 41-year-old experiments. In all fertilized treatments, the humus quality significantly decreased. After 94 years of mineral fertilization, the content of water-stable macroaggregates (WSA ma ) in size fractions > 5 and 5–2 mm was significantly higher in the NPK than in the Co and CaNPK treatments. After 94 years of mineral fertilization, the mean weight diameter of aggregates for dry sieving (MWD d ) differed between Co (0.41) and both treatments of mineral fertilization (NPK: 0.88; CaNPK: 1.70). The application of FYM with CaNPK resulted in a statistically significant decrease in aggregate stability (Sw) in the 25-year-old experiment. For Co and NPK treatments in the 41-year-old experiment, the contents of WSA ma > 5 mm were 41% and 51% lower, respectively, whereas the content of WSA ma 5–3 mm was lower by 40% and 50%, respectively, which was higher than that for the CaNPK treatment. The aggregate stability significantly decreased due to NPK application in the soil of the 41-year-old experiment. The number of correlations between SOM parameters and the soil structure decreased in the following order: 94-year-old experiment with mineral fertilization > 41-year-old experiment with mineral fertilization > 25-year-old experiment with mineral fertilization + FYM.
Book
Soils are neither good nor bad, but some have inherent or acquired characteristics that may or may not suit our intended use. Unsuitable characteristics are considered to be soil problems, soil constraints or soil limitations. Only twelve percent of global land is right for agricultural production without much limitation. Some soils have severe limitations for crop production. These soils are so called ‘problem soils’. Many of them do not have enough fertility to be productive; some are arid and saline; some are very sandy and dry; and some are wet and waterlogged for most of the growing season. The global demand for food, wood, fuel, fiber, medicine and other plant products for the 7.2 billion current world population has created such an immense pressure on global soil resources that even the most fertile soils are losing their productive capacity. We are being compelled to bring more and more unsuitable or marginally suitable soils under cultivation. Unless innovative and integrated soil, crop and environmental management practices are adopted for their improvement and sustainable use, further degradation is inevitable. This book, Management of Soil Problems, identifies the problems and discusses management options in a smooth and reader-friendly style. It will be useful for students and professionals of soil science, agriculture, forestry, geography and environmental sciences.
Article
The “4 per Thousand” initiative was launched at the 21st Conference of Parties (COP21) in December 2015 to address global climate change through the aspirational goal of increasing soil organic carbon (SOC) stock of the world to 40-cm depth by an average annual rate of 4%. Small landholders (SLHs), often faced with difficult bio-physical and socio-economic conditions, are the principal managers of soil in India. There are 117 million SLHs representing 85% of the total operational holdings, cultivating over 72 million ha of land, and meeting 50–60% of India's food requirement. The agricultural soils of SLHs are strongly depleted of SOC and nutrient reserves. Therefore, the challenge of feeding 1.7 billion people in India by 2050 will depend on increasing the current productivity levels by restoring the depleted soils of SLHs. According to our estimates, soils of SLHs currently contain 1370–1770 Tg C and, which can be increased to 2460–2650 Tg C by 2050 through large-scale adoption of best management practices (BMPs) including balanced application of nutrients, compost, agroforestry, and conservation agriculture. A wide spread adoption of these practices can enhance C sequestration by 70–130 Tg CO2e per annum and produce 410–440 million Mg of food grains accounting for 80–85% of the total requirement by 2050. In this paper we propose strategies for achieving the dual objectives of advancing food security, the “4 per Thousand” target and mitigating climate change in India.
Article
Plants and earthworms, as soil ecosystem engineers, play a crucial role during stabilisation of organic matter in soil through its incorporation into soil aggregates. It is therefore essential to better understand the mechanisms and interactions of soil engineering organisms regarding soil organic matter stabilisation. Several methods have already been successfully applied to differentiate soil aggregates by their origin, but they cannot specify the degree of organic matter stability within soil aggregates. Rock-Eval pyrolysis has already been proved to be pertinent for analyses of soil organic matter bulk chemistry and thermal stability, but it has not yet been directly applied to identify biogenic organic matter signatures within soil aggregates. In this study, Rock-Eval pyrolysis was used for the identification of the soil aggregate origin as well as for the determination of the soil organic matter bulk chemistry and thermal stability in a controlled experiment. Mesocosms were set up, containing treatments with a plant, an earthworm species, or both. Water stable soil macro-aggregates>250 μm were sampled and tested with Rock-Eval pyrolysis after a two-month incubation period. Rock-Eval pyrolysis was able to differentiate soil macro-aggregates by their origin, and to identify a specific signature for each treatment. Macro-aggregates from the plant and earthworm treatment were characterized by a mixed signature incoming from the two soil engineers, indicating that both engineers contribute concomitantly to soil aggregate formation. Organic matter thermal stability was not positively affected by earthworms and even tends to decrease for the plant treatment, emphasising that organic matter was mainly physically protected during the incubation period, but not stabilised. However, future research is required to test if signatures for the tested organisms are speciesspecific or generally assignable to other plant and earthworm species.
Article
To improve C sequestration in soils and mitigate climate change, it is essential to understand how nutrient management strategies impact on soil organic carbon (SOC) stocks and labile fractions. This study was designed to explore changes in soil bulk density (BD), SOC concentrations, SOC stocks and soil labile organic C fractions (mineralizable C (Cmin), microbial biomass C (MBC), dissolved organic C (DOC), particulate organic C (POC), light fraction organic C (LFOC) and permanganate oxidizable C (KMnO4-C)) under 26-year fertilization regimes in a wheat-maize rotation system in the North China Plain. Soil from the following six treatments was analyzed: (1) Control with no amendment addition (CK); (2) Standard rate of mineral fertilizer treatment (SMF) reflecting local farmers' practice; (3) Standard rate of organic manure treatment (SMA) with total N input equal to SMF; (4) Half the standard rate of mineral fertilizer plus half the standard rate of organic manure treatment (1/2 SMF + 1/2 SMA); (5) Double standard rate of mineral fertilizer treatment (DMF); (6) Double standard rate of organic manure treatment (DMA). Results showed that all long-term fertilization regimes significantly decreased BD in topsoil compared to CK except for SMF, with treatments that included organic manure resulting in the lowest BDs. Treatments that included organic manure had significantly higher SOC concentrations and stocks than mineral or unfertilized treatments. The organic manure treatments also had higher concentrations of non-labile C but at the same time a higher proportion of labile C than the mineral or unfertilized treatments. This was confirmed by the carbon management index (CMI) which was significantly increased by organic manure addition. Control and mineral fertilized treatments had higher efficiencies of C retention (RE) from added inputs (crop residues only). Differences in Cmin, POC and KMnO4-C were affected by differences in MA-C, however, changes in rhizodeposition-C, stubble-C and root-C significantly affected DOC, MBC and LFOC. This study demonstrates that fertilization strategies that include organic manure can increase the pool of stable C in the surface soil layer, while at the same time increasing concentrations and proportions of labile C. Organic manure use can therefore contribute to improved nutrient cycling services and higher soil quality in the North China Plain.
Article
At the 21st session of the United Nations Framework Convention on Climate Change (UNFCCC, COP21), a voluntary action plan, the '4 per 1000 Initiative: Soils for Food Security and Climate' was proposed under the Agenda for Action. The Initiative underlines the role of soil organic matter (SOM) in addressing the three-fold challenge of food and nutritional security, adaptation to climate change and mitigation of human-induced greenhouse gases (GHGs) emissions. It sets an ambitious aspirational target of a 4 per 1000 (i.e. 0.4%) rate of annual increase in global soil organic carbon (SOC) stocks, with a focus on agricultural lands where farmers would ensure the carbon stewardship of soils, like they manage day-to-day multipurpose production systems in a changing environment. In this paper, the opportunities and challenges for the 4 per 1000 initiative are discussed. We show that the 4 per 1000 target, calculated relative to global top soil SOC stocks, is consistent with literature estimates of the technical potential for SOC sequestration, though the achievable potential is likely to be substantially lower given socio-economic constraints. We calculate that land-based negative emissions from additional SOC sequestration could significantly contribute to reducing the anthropogenic CO2 equivalent emission gap identified from Nationally Determined Contributions pledged by countries to stabilize global warming levels below 2 °C or even 1.5 °C under the Paris agreement on climate. The 4 per 1000 target could be implemented by taking into account differentiated SOC stock baselines, reversing the current trend of huge soil CO2 losses, e.g. from agriculture encroaching peatland soils. We further discuss the potential benefits of SOC stewardship for both degraded and healthy soils along contrasting spatial scales (field, farm, landscape and country) and temporal (year to century) horizons. Last, we present some of the implications relative to non-CO2 GHGs emissions, water and nutrients use as well as co-benefits for crop yields and climate change adaptation. We underline the considerable challenges associated with the non-permanence of SOC stocks and show how the rates of adoption and the duration of improved soil management practices could alter the global impacts of practices under the 4 per 1000 initiative. We conclude that the 4 per 1000 initiative has potential to support multiple sustainable development goals (SDGs) of the 2030 Agenda. It can be regarded as no-regret since increasing SOC in agricultural soils will contribute to food security benefits that will enhance resilience to climate change. However, social, economic and environmental safeguards will be needed to ensure an equitable and sustainable implementation of the 4 per 1000 target.
Article
Organic carbon saturation in soils refers to the theoretical maximum soil organic carbon (SOC) that can be associated with and stabilized on fine silt plus clay particles (F < 20 μm). We reviewed the literature dealing with SOC distribution between soil fractions to evaluate carbon saturation for tropical soils and estimate the C storage potential of cropland. We collected 258 data points on SOC distribution between soil fractions in tropical soils from 84 sites in 27 countries. We used boundary line analysis to estimate the maximum stabilized SOC depending on soil group, clay type and land use. SOC storage potential was calculated as the SOC saturation deficit, the difference between the actual SOC content and the maximum stabilized SOC content. We found that the maximum SOC in the fine fraction of tropical soils (53 g C kg− 1 fine fraction) was lower than previous assessments of global SOC storage scale based mainly on temperate soils. The F < 20 μm fractions were closer to SOC saturation in forest soils than in croplands. The cropland had a higher soil C storage potential, but changing agricultural management practices did not fill the deficit that is calculated using the whole dataset. The deficit was much lower when it was estimated with grassland or cropland data only: this provides a more realistic estimate for SOC storage potential for croplands. The SOC content in the coarser fraction (F > 50 μm) did not depend on soil texture and significantly contributed to the total SOC, especially in sandy soils (41.3%). This is affected by changes in agricultural management practices. We concluded that, although the aim of increasing SOC stabilization originally arose from climate change mitigation strategies, it must now be more viewed as being more relevant to food security and local adaptation to climate change.
Article
The decrease of organic matter content in agricultural soils is a problem of great concern to farmers around the world. Indeed, it lowers soil fertility that directly impairs agricultural crop production and affects a number of other soil properties like water retention capacity, aggregation and structure formation, soil mechanical strength or compactibility. Scarcity in plant available water poses a risk to agriculture, especially in drought-prone areas. However, the increase of organic waste recycling in agriculture may also lead to an increase in soil organic matter contents and to changes in related soil properties. Here, we review 17 long-term field experiments (≥9 years) that investigated the effects of organic amendments on organic carbon and water availability in topsoils. We paid particular attention to the effects of added organic matter on soil bulk density or porosity and consequently on plant available water. Our main findings are that (1) plant available water generally improves after organic waste addition (relative changes from −10 to +30 vol%; p = 0.052), (2) organic matter quality affects changes in organic carbon (p < 0.05), (3) it is more suitable for plant available water quantification to use volumetric rather than gravimetric water contents, (4) the value of the matric potential defining field capacity is an issue, (5) pedotransfer functions developed for American soils adequately predicted most water contents at field capacity and wilting point, and (6) prevailing climate and initial organic carbon content may affect plant available water. This review confirms that organic amendments generally induce beneficial effects on plant available water and other soil properties. It also highlights the influence of organic matter quality on soil organic carbon. Compared with a previous review, this study reinforces reported trends of increasing plant available water with organic waste additions. This may be due to a more restrictive selection of recently published data and the use of volumetric water contents. Our findings are significant for sustainable agriculture regarding the sustainable use of organic wastes and water.
Article
Soil organic matter is critical to sustainable agriculture because it provides nutrients to crops as it decomposes and increases nutrient- and water-holding capacity when built up. Fast- and slow-cycling fractions of soil organic matter can have different impacts on crop production because fast-cycling fractions rapidly release nutrients for short-term plant growth and slow-cycling fractions bind nutrients that mineralize slowly and build up water-holding capacity. We explored the controls on these fractions in a tropical agroecosystem and their relationship to crop yields. We performed physical fractionation of soil organic matter from 48 farms and plots in western Kenya. We found that fast-cycling, particulate organic matter was positively related to crop yields, but did not have a strong effect, while slower-cycling, mineral-associated organic matter was negatively related to yields. Our finding that slower-cycling organic matter was negatively related to yield points to a need to revise the view that stabilization of organic matter positively impacts food security. Our results support a new paradigm that different soil organic matter fractions are controlled by different mechanisms, potentially leading to different relationships with management outcomes, like crop yield. Effectively managing soils for sustainable agriculture requires quantifying the effects of specific organic matter fractions on these outcomes.
Article
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.
Article
Depuis les vingt dernières années, des travaux de plus en plus nombreux mettent en oeuvre des fractionnements granulométriques et/ou densimétriques pour caractériser différents compartiments organiques des sols. Toutefois, à la différence des approches de type chimique ou biologique de la matière organique (MO) des sols, il ne semble pas exister d'étude historique exhaustive sur ce sujet. Pourtant, dès 1874, Schloesing publie, dans de bonnes conditions expérimentales et avec esprit critique, un fractionnement granulométrique de la MO des sols en vue de préciser son rôle dans la stabilité de l'agrégation des sols arables. Nous relatons ici ce travail. Schloesing s'interroge sur la répartition de la MO du sol entre le sable et l'argile. Il distingue cinq fractions qu'il nomme : "gros sable", "sable fin", "écaille" (fraction légère à la surface des "sables fins" et se détachant en "écailles" après séchage), et "deux dépôts d'argile". Les résultats sont présentés en trois tableaux avec les bilans pondéraux (99,9%), les teneurs en carbone et azote par gramme de fraction, et par gramme du sol, et la composition minérale de trois des fractions. Une des conclusions de Schloesing est que : "l'argile contient 6,9% de matière organique ; c'est assez pour qu'elle soit réellement modifiée dans sa manière d'agir comme ciment". Ce travail pourrait être publié de nos jours. (Résumé d'auteur)
Article
The application of organic amendments on soils poor in organic matter (OM) can improve long-term soil fertility, but may also enhance the mineralization of native soil organic matter. Three organic amendments, compost, sewage sludge and horse manure, used by urban market gardeners in Dakar, Senegal were analyzed for their OM maturity. Their fate was evaluated in a 45-d agronomic trial in a sandy Arenosol with lettuce. In each case, water-extractable organic matter (WEOM) and humic-like substances (HLS) were isolated from raw amendments and amended soils, and characterized using ultraviolet-visible (UV/Vis) spectroscopy. Results highlighted the general more aromatic character of HLS and WEOM fractions extracted from compost compared to the other two amendments. When applied to soils, however, these differences were not clearly observed. The aromaticity and humification degree of the labile fraction (WEOM) increased with depth in the first 30 cm for all amendments. This indicated the high lixiviation rates that fresh OM underwent in the studied sandy soil. Finally, a statistical analysis of the results was able to discriminate between surface and deeper horizons and between amended- and non-amended soil samples. Spectroscopic indices showed indeed strong increase/decrease with depth linked with the mineralization/humification processes that the fresh OM from amendments underwent during the 45 d of the agronomic trial. This study highlights the potential of spectroscopic techniques to study agricultural amendment organic matter fractions and their fate in soils.
Article
The impact of animal manure application on soil organic carbon (SOC) stock changes is of interest for both agronomic and environmental purposes. There is a specific need to quantify SOC change for use in national greenhouse gas (GHG) emission inventories. We quantified the response of SOC stocks to manure application from a large worldwide pool of individual studies and determined the impact of explanatory factors such as climate, soil properties, land use and manure characteristics. Our study is based on a meta-analysis of 42 research articles totaling 49 sites and 130 observations in the world. A dominant effect of cumulative manure-C input on SOC response was observed as this factor explained at least 53% of the variability in SOC stock differences compared to mineral fertilized or unfertilized reference treatments. However, the effects of other determining factors were not evident from our dataset. From the linear regression relating cumulative C inputs and SOC stock difference, a global manure-C retention coefficient of 12% ± 4 (95% Confidence Interval, CI) could be estimated for an average study duration of 18 years. Following an approach comparable to the Intergovernmental Panel on Climate Change, we estimated a relative SOC change factor of 1.26 ± 0.14 (95% CI) which was also related to cumulative manure-C input. Our results offer some scope for the refinement of manure retention coefficients used in crop management guidelines and for the improvement of SOC change factors for national GHG inventories by taking into account manure-C input. Finally, this study emphasizes the need to further document the long-term impact of manure characteristics such as animal species, especially pig and poultry, and manure management systems, in particular liquid vs. solid storage. This article is protected by copyright. All rights reserved.
Article
Rock–Eval 6 analysis, a well established screening tool for petroleum geochemistry, is being increasingly used to characterise the varying species of organic matter (OM) in the bulk samples of recent aquatic sediments. This is particularly important due to recent scientific attention on the role of OM in biogeochemical distribution of environmentally hazardous compounds (e.g., trace metals) in recent sediment archives. Rock–Eval’s automated use, low sample volume requirements and its high analytical accuracy and precision makes it an ideal tool for relatively rapid screening of OM in sediment cores. However, to date, there has been no broad scale standardisation to determine what may be contributing to each signal (e.g., S1, S2, S3, RC). We have selected a wide variety of representative, pure biochemicals (proteins, lipids, carbohydrates and lignins) and biological standards (phytoplankton, copepods, tree bark and conifer needles) to better understand the Rock–Eval 6’s measured organic matter parameters in the unconventional environmental samples. These data have been corroborated with organic petrographical and elemental (CHNS/O) data. Our results show that small organic molecules (<500 Da) are largely responsible for the S1 hydrocarbon peak while lipids and aquatic biological standards are contributing most in the S2 signal, and in particular the more labile “S2a” signal. Furthermore, carbohydrates, lignins and terrigenous plant standards are most responsible for the S3 signal. We also note that the S3 signals (CO/CO2 ratios: OICO, OICO2 and OIRE6) are the best discriminants for the source of OM. Finally, step wise pyrolysis of biological standards coupled with elemental analysis (CHNS/O) suggests that S2 and, to a lesser extent, S3 (S3CO and/or S3CO2), would be most responsible for metal-binding elements such as S and N, with implications for element biogeochemical cycles.