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International audience; 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.

Increasing soil organic matter (SOM) is of primary importance for maintaining soil fertility and mitigating climate change. Leaving crop residues on top of soil is not always an efficient means of increasing SOM because (i) of the high mineralization of the crop residues, (ii) crop residues may increase the mineralization of existing SOM (priming effect) and (iii) wetting-drying cycles may increase mineralization of SOM and crop residues. Little research has been carried out into these mechanisms under Sudano-Sahelian conditions where the rainfall is mostly irregular with wetting-drying cycles during the transition between the wet and the dry season. To evaluate the effect of wetting-drying cycles on the mineralization of SOM and crop residues and the priming effect, an agricultural soil from the North Region of Cameroon with or without (controls) 13 C-labeled rice straw amendment as crop residues was either subjected to five wetting-drying cycles or maintained at constant water potential after a single rewetting event. Soil samples were incubated for 70 days at 28 °C and the CO 2 and 13 CO 2 emissions and mineral N were monitored. Adding straw (+ 833 μg C g − 1 soil) increased the cumulative CO 2 emissions from the soil (+ 921 μg C-CO 2 g − 1 soil). A positive priming effect was observed (+ 92 μg C-CO 2 g − 1 soil). Only the first wetting cycle created a mineralization flush of the SOM and the straw. This flush did not recur probably because of a lack of labile SOM. However an extra addition of straw after 28 days increased CO 2 emissions but did not result in further mineralization flushes after re-wetting while SOM mineralization was not limited by N availability. We conclude that SOM depletion under Sudano-Sahelian conditions was not explained by SOM mineralization enhancement due to multiple rewetting events or to priming effect following crop residues addition. Indeed, SOM depletion could rather be explained by a high level of mineralization of both, SOM and crop residues, when the soil reached its water retention capacity.

The assessment of the impacts of land-use and management on soil organic carbon (SOC) dynamics is a major environmental concern, as the soil carbon cycle underpins key ecosystem services. However, assessments based on short-term SOC dynamics face methodological and experimental difficulties. Hurisso et al. (2016) proposed a method to assess SOC dynamics by coupling two methods: Permanganate Oxidizable Carbon (POXC) and Basal Soil Respiration (BSR). This method has been used in laboratory on dried and re-wetted soil samples from temperate regions mainly. In our study, we adapted this method to the field and proposed a cost-effective in-field indicator combining the POXC and in situ Basal Soil Respiration (SituResp (R) method). We tested the indicator at four study sites (n = 169 points) within various tropical land-use and management contexts based on rubber, soybean and oil palm cropping systems respectively in Thailand, Cambodia and Indonesia. The results demonstrated the relevance, sensitivity and robustness of the POXC-SituResp (R) indicator to characterize the impact of a gradient of disturbance on SOC dynamics. The results also highlighted the potential of conservation agriculture (no-tillage and crops residues) and compost amendments to accumulate SOC. Rock-Eval (R) analysis showed that POXC-SituResp (R) indicator is negatively linked to excess of potentially mineralizable labile carbon. Carbon pools targeted by the POXC were specified by Rock-Eval (R) pyrolysis measurements to be a rather thermal resistant pool of SOC. Our study confirms that the integrated indicator based on POXC and BSR assess a relative carbon stabilization of SOC pools. This indicator can be measured in the field by a rapid and cost-effective method.

Despite the outstanding potential of Andosols for soil organic carbon (SOC) sequestration, the importance of the different mechanisms involved in protecting SOC against mineralization is still unclear. Previous studies have described allophane particles as having a fractal structure. One hypothesis is that SOC is adsorbed or trapped in this fractal structure and could be less available to microbes and enzymes. This paper aims to determine the fractal structure of allophanic soils, especially in mesopores (2–50 nm), and to establish a correlation between the SOC bioavailability and the fractal features of the allophane aggregates. The present study confirms the fractal structure of allophane aggregates. The structural features of the pores, measured using N 2 adsorption–desorption curves and SAXS for the nano-scale range, showed correlation with SOC bioavailability. SOC bioavailability decreased with the extent of the fractal structure.

Genetically modified crops, which produce Bacillus thuringiensis toxins, release the toxins into soils through root exudates and upon decomposition of residues. Several studies showed that Bacillus thuringiensis toxins adsorbed on pure clay minerals and model clay-OM or oxides associations as well as on soils. We determined the adsorption of the toxin from Bacillus thuringiensis subsp. tenebrionis (Btt), active against Coleoptera, on tropical soils and their constituents. Particle-size fractions were separated from a vertisol, an alfisol and an oxisol and the role of constituents of the clay-size fractions was analysed using clay-size fractions from which specific components had been extracted. The adsorbed amounts and affinities of the soil were in the order, oxisol < alfisol < vertisol. Clay-size fractions adsorbed more, with higher affinities and less reversibility, than the silt or particulate organic matter fractions or bulk soils. In the clay-size fractions, organic matter increased adsorption whereas Fe oxides decreased it, and the mineralogy had a prominent role. Smectitic clay fractions adsorbed much more of the toxin from Btt than kaolinitic ones, in relation with the surface area and charge of these minerals. The features of adsorption of the toxin from Btt on tropical soils and their constituents show that the toxin is likely to be retained by soil after cropping with transgenic plants.

The effects of long-term (19 years) tropical pasture (GP) and market-gardening (MG) on the distribution of soil organic matter (SOM) and carbohydrates across primary organomineral separates were investigated on a Vertisol (clayey, smectitic, Leptic Hapludert) of Martinique (French West Indies). Surface samples were collected in GP and MG plots, separated by wet-sieving into five particle-size fractions (200–2000, 50–200, 20–50, 5–20, 0–5 μm), and analyzed for carbon (C), nitrogen (N), and monosaccharides (MS). Long-term cultivation resulted in significant declines in total SOM present in the soil and caused changes in SOM distribution when compared to the grazed pasture (GP). A major difference between GP and MG was that the proportions of the soil organic C present in the primary organomineral separates were considerably greater in GP than in MG. Losses of C upon cultivation were higher in the coarse sand-sized fraction 200–2000 μm compared with the 0- to 5-μm fraction although the majority of SOM was associated with this one. Concomitantly, cultivation affected significantly the proportion of soil C present as monosaccharides. The finer separate F 0–5 μm accounted for most of the soil monosaccharides and displayed a microbial character. The conditions of hydrolyses used for extracting the monosaccharides revealed the dominance of hemicellulosic sugars in the bulk soils and their organomineral separates