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Despite its relevance to long term carbon sequestration in soils, there is to date no successful experimental way to isolate all the stable pool of soil organic carbon (SOC), i.e. SOC that has a residence time of centuries to millennia. Long term bare fallows (LTBF) offer a unique opportunity to study stable SOC, as without carbon inputs and with continuing biodegradation and mineralization, SOC becomes progressively enriched in its most stable components. Here, we took advantage of the 42 plots LTBF experiment of Versailles (France), where C inputs stopped 80 years ago, to test the relevance of chemical oxidation methods to isolate a pool of carbon stable at the centennial timescale. To do so, we studied the effect of two oxidizing reagents -hydrogen peroxide, HO, and sodium hypochlorite, NaOCl on soil total organic carbon (TOC) content after 0 and 79 years of LTBF. If these methods can isolate centennial stable C, then chemical oxidation resistant-C from 0 and 79 years of LTBF should equate, in amount, with TOC after 79 years of LTBF. Results showed that chemical oxidation strongly decreased TOC in both soils. Oxidation-resistant OC accounted for ca.1 mgC g soil, which is five times smaller than TOC in the untreated LTBF soils, after 79 years of biological oxidation. Moreover, the amount of oxidation-resistant OC was significantly lower in LTBF compared to LTBF. We conclude that neither centennial stable carbon nor older stable carbon can be successfully quantified by these chemical oxidation methods. [ABSTRACT FROM AUTHOR]

Clay minerals have a major role in soil aggregation because of their large specific surface area and surface charges, which stimulate interactions with other mineral particles and organic matter. Soils usually contain a mixture of clay minerals with contrasting surface properties. Although these differences should result in different abilities of clay minerals regarding aggregate formation and stabilization, the role of different clay minerals in aggregation has been seldom evaluated. In this study, we took advantage of the intrinsic mineral heterogeneity of a temperate Luvisol to compare the role of clay minerals in aggregation. First, grassland and tilled soil samples were separated in water into aggregate-size classes based on the aggregate hierarchy model. Then, clay mineralogy and organic C in the aggregate-size classes were analysed. Interstratified minerals containing swelling phases accumulated in aggregated fractions compared with free clay fractions under the two land-uses. The accumulation increased with decreasing aggregate size from large macroaggregates (> 500 µm) to microaggregates (50-250 µm). Carbon content and carbon-to-nitrogen ratio followed the opposite trend. This fully supports the aggregate hierarchy model, which postulates an increasing importance of mineral reactivity in smaller aggregates than in larger aggregates in which the cohesion relies mostly on physical enmeshment by fungal hyphae or small roots. Consequently, differences in the proportion of the different 2:1 clay minerals in soils can influence their structure development. Further research on the links between clay mineralogy and aggregation can improve our understanding of mechanisms of soil resistance to erosion and organic matter stabilization. [ABSTRACT FROM AUTHOR]

Soil is the major terrestrial reservoir of carbon and a substantial part of this carbon is stored in deep layers, typically deeper than 50 cm below the surface. Several studies underlined the quantitative importance of this deep soil organic carbon (SOC) pool and models are needed to better understand this stock and its evolution under climate and land-uses changes. In this study, we tested and compared three simple theoretical models of vertical transport for SOC against SOC profiles measurements from a long-term bare fallow experiment carried out by the Central-Chernozem State Natural Biosphere Reserve in the Kursk Region of Russia. The transport schemes tested are diffusion, advection and both diffusion and advection. They are coupled to three different formulations of soil carbon decomposition kinetics. The first formulation is a first order kinetics widely used in global SOC decomposition models; the second one, so-called "priming" model, links SOC decomposition rate to the amount of fresh organic matter, representing the substrate interactions. The last one is also a first order kinetics, but SOC is split into two pools. Field data are from a set of three bare fallow plots where soil received no input during the past 20, 26 and 58 yr, respectively. Parameters of the models were optimised using a Bayesian method. The best results are obtained when SOC decomposition is assumed to be controlled by fresh organic matter (i.e., the priming model). In comparison to the first-order kinetic model, the priming model reduces the overestimation in the deep layers. We also observed that the transport scheme that improved the fit with the data depended on the soil carbon mineralisation formulation chosen. When soil carbon decomposition was modelled to depend on the fresh organic matter amount, the transport mechanism which improved best the fit to the SOC profile data was the model representing both advection and diffusion. Interestingly, the older the bare fallow is, the lesser the need for diffusion is, suggesting that stabilised carbon may not be transported within the profile by the same mechanisms than more labile carbon. [ABSTRACT FROM AUTHOR]

Soil is the major terrestrial reservoirs of carbon, and a substantial part of this carbon is stored in deep layers, typically deeper than 50 cm below the surface. Several studies underlined the quantitative importance of this deep Soil Organic Carbon (SOC) pool and models are needed to better understand this stock and its evolution under climate and land-uses changes. In this study, we test and compare 3 simple theoretical models of vertical transport for SOC against SOC profiles measurements from a long-term bare fallow experiment carried out by the Central-Chernozem State Natural Biosphere Reserve named after V.V. Alekhin, in the Kursk Region of Russia. The transport schemes tested are diffusion, advection or both diffusion and advection. They are coupled to two different formulations of soil carbon decomposition kinetics. The first formulation is a first order kinetics widely used in global SOC decomposition models; the second one links SOC decomposition rate to the amount of fresh organic matter, representing a "priming effect". Field data are from a set of three bare fallow plots where soil received no input during the past 20, 26 and 58 yr respectively. Parameters of the models were optimized using a Bayesian method. The best results are obtained when SOC decomposition is assumed to be controlled by fresh organic matter. In comparison to the first-order kinetic model, the "priming" model reduces the underestimation of SOC decomposition in the top layers and the over estimation in the deep layers. We also observe that the transport scheme that improved the fit with the data depends on the soil carbon mineralization formulation chosen. When soil carbon decomposition is modelled to depend on the fresh organic matter amount, the transport mechanisms which improves best the fit to the SOC profile data is the model representing both advection and diffusion. Interestingly, the older the bare fallow is, the lesser the need for diffusion is. This suggests that stabilized carbon may not be transported within the profile by the same mechanisms than more labile carbon. [ABSTRACT FROM AUTHOR]

Soil moisture is of primary importance for predicting the evolution of soil carbon stocks and fluxes, both because it strongly controls organic matter decomposition and because it is predicted to change at global scales in the following decades. However, the soil functions used to model the heterotrophic respiration response to moisture have limited empirical support and introduce an uncertainty of at least 4% in global soil carbon stock predictions by 2100. The necessity of improving the representation of this relationship in models has been highlighted in recent studies. Here we present a data-driven analysis of soil moisture-respiration relations based on 90 soils. With the use of linear models we show how the relationship between soil heterotrophic respiration and different measures of soil moisture is consistently affected by soil properties. The empirical models derived include main effects and moisture interaction effects of soil texture, organic carbon content and bulk density. When compared to other functions currently used in different soil biogeochemical models, we observe that our results can correct biases and reconcile differences within and between such functions. Ultimately, accurate predictions of the response of soil carbon to future climate scenarios will require the integration of soil-dependent moisture-respiration functions coupled with realistic representations of soil water dynamics. [ABSTRACT FROM AUTHOR]

Soil moisture is of primary importance for predicting the evolution of soil carbon stocks and fluxes, both because it strongly controls organic matter decomposition and because it is predicted to change at global scales in the following decades. However, the soil functions used to model the heterotrophic respiration response to moisture have limited empirical support and introduce an uncertainty of at least 4% in global soil carbon stock predictions by 2100. The necessity of improving the representation of this relationship in models has been highlighted in recent studies. Here we present a data-driven analysis of soil moisture-respiration relations based on 90 soils. With the use of linear models we show how the relationship between soil heterotrophic respiration and different measures of soil moisture is consistently affected by soil properties. The empirical models derived include main and moisture interaction effects of soil texture, organic carbon content and bulk density. When compared to other functions currently used in different soil biogeochemical models, we observe that our results can correct biases and reconcile differences within and between such functions. Ultimately, accurate predictions of the response of soil carbon to future climate scenarios will require the integration of soil-dependent moisture-respiration functions coupled with realistic representations of soil water dynamics. [ABSTRACT FROM AUTHOR]

In this paper, we attempt to analyse the respective influences of land-use and climate changes on the global and regional balances of soil organic carbon (SOC) stocks. Two time periods are analysed: the historical period 1901-2000 and the period 2000-2100. The historical period is analysed using a synthesis of published data as well as new global and regional model simulations, and the future is analysed using models only. Historical land cover changes have resulted globally in SOC release into the atmosphere. This human induced SOC decrease was nearly balanced by the net SOC increase due to higher CO and rainfall. Mechanization of agriculture after the 1950s has accelerated SOC losses in croplands, whereas development of carbon-sequestering practices over the past decades may have limited SOC loss from arable soils. In some regions (Europe, China and USA), croplands are currently estimated to be either a small C sink or a small source, but not a large source of CO to the atmosphere. In the future, according to terrestrial biosphere and climate models projections, both climate and land cover changes might cause a net SOC loss, particularly in tropical regions. The timing, magnitude, and regional distribution of future SOC changes are all highly uncertain. Reducing this uncertainty requires improving future anthropogenic CO emissions and land-use scenarios and better understanding of biogeochemical processes that control SOC turnover, for both managed and un-managed ecosystems. [ABSTRACT FROM AUTHOR]

The stability of soil carbon is a major source of uncertainty for the prediction of atmospheric CO2 concentration during the 21st century. Isolating experimentally the stable soil carbon from other, more vulnerable, pools is of prime importance for calibrating soil C models, and gaining insights on the mechanisms leading to soil organic carbon (SOC) stability. Long-term bare fallow experiments, in which the decay of SOC is monitored for decades after inputs from plant material have stopped, represent a unique opportunity to assess the stable organic carbon. We synthesized data from 6 bare fallow experiments of long-duration, covering a range of soil types and climate conditions, at Askov (Denmark), Grignon and Versailles (France), Kursk (Russia), Rothamsted (UK), and Ultuna (Sweden). The conceptual model of SOC being divided into three pools with increasing turnover times, a labile pool (∼years), an intermediate pool (∼decades) and a stable pool (∼several centuries or more) fits well with the long term SOC decays observed in bare fallow soils. The modeled stable pool estimates ranged from 2.7 gCkg-1 at Rothamsted to 6.8 gCkg-1 at Grignon. The uncertainty over the identification of the stable pool is large due to the short length of the fallow records relative to the time scales involved in the decay of soil C. At Versailles, where there is least uncertainty associated with the determination of a stable pool, the soil contains predominantly stable C after 80 years of continuous bare fallow. Such a site represents a unique research platform for future experimentation addressing the characteristics of stable SOC and its vulnerability to global change. [ABSTRACT FROM AUTHOR]

The extension of viticulture on hill slopes in the Mediterranean basin might be responsible for a loss of organic carbon from the soil, as a result of increasing runoff and erosion. The loss of C makes soil aggregates less stable, which in turn increases erosion risk. We should use indicators of these risks and so be able to identify the most threatened areas. We first tested four widely used methods for determining aggregate stability for their ability to predict runoff and erosion; all include slaking and give indices such as rates of stable (> 200 μm) macro-aggregates ( MA200) or mean weight diameter ( MWD). We selected three Mediterranean vineyards characterized by brown calcareous soils and on which we had previously studied the effects of simulated rainfall. The tests were made on the soils from five farming situations, corresponding to 14 simulated rainfalls with an intensity of 60 mm hour−1. The two indices that best predict erosion are those resulting from the method of Le Bissonnais ( MWD in its logarithmic form and MA200). Then, we analysed the relations between aggregate stability and soil properties on 68 soil samples from various locations within the three selected sites. A very significant correlation between the two indices of Le Bissonnais’s method and soil organic carbon content was found and we propose a pedotransfer function linking these indices to soil characteristics. The results stress the need for farming practices that preserve the organic C in Mediterranean vineyard soils in order to reduce erosion risks. We believe that our findings might well apply to similar soils elsewhere in the world. Résumé Le développement de la viticulture sur les sols en pente du bassin Méditerranéen pourrait entraîner une diminution de la teneur en C de ces sols et une aggravation du ruissellement et de l’érosion. La perte de C rend les agrégats moins stables, ce qui augmente le risque d’érosion. Des indicateurs de ces risques sont nécessaires afin d’identifier les surfaces les plus menacées. Nous avons dans un premier temps comparé quatre méthodes différentes, couramment utilisées pour mesurer la stabilité structurale des sols afin d’évaluer leur capacitéà prédire le ruissellement et l’érosion; toutes mettent en jeu de l’éclatement des agrégats et produisent des indices tels que la proportion de macroagrégats stables > 200 μm ( MA200) ou le diamètre moyen pondéré des agrégats stables ( MWD). Nous avons choisi trois vignobles méditerranéens, caractérisés par des sols bruns calcaires. Les tests de stabilité structurale ont été réalisés sur les sols de cinq sites cultivés, correspondant à 14 simulations de pluie avec une intensité de 60 mm heure−1. Nous avons trouvé des relations significatives entre la stabilité structurale et le comportement lors d’une pluie simulée, les deux meilleurs prédicteurs de l’érosion étant les indices MWD logarithmique et MA200 issus du test de Le Bissonnais. Dans un deuxième temps, nous avons analysé les relations entre ces indices et les caractéristiques de 68 échantillons de sols dans les 3 sites étudiés. Nous avons trouvé une relation hautement significative entre les deux indices et la teneur en C des sols et nous proposons une fonction de pédotransfert reliant ces indices aux caractéristiques des sols. Nos résultats montrent le besoin de pratiques culturales préservant la teneur en C des sols de vignobles Méditerranéens afin de limiter les risques d’érosion. [ABSTRACT FROM AUTHOR]

Lignin has long been suspected to be a major source of stable carbon in soils, notably because of the recalcitrant nature of its polyphenolic structure relative to other families of plant molecules. However, lignin turnover studies have produced conflicting results, most of them suggesting that large proportions of plant-residue lignin decompose within a year of incorporation into soils. Here, we propose a two-reservoir model where lignin in undecomposed plant residue ( Lp) can either reach soil fractions where it is somewhat protected from further decomposition ( Ls) or is transformed to non-lignin products. Model calibration data were obtained through compound-specific 13C isotopic analyses conducted in a zero- to 9-year chronosequence of maize monoculture after wheat in a temperate loam soil of the Paris basin. Lignin was quantified by CuO oxidation as VSC-lignin, i.e. the sum of vanillil- (V), syringyl- (S) and coumaryl-type (C) phenols. Model calibrations indicate that Lp has a turnover rate faster than 1 year and that 92% is mineralized as CO2 or transformed into other non-lignin products, while only 8% reaches the Ls fraction. Estimated turnover rate of the Ls fraction was 0.05 years−1. The model also suggested that about half of Lp was not measured because it had been excluded from the samples in the process of sieving at 5 mm. In conclusion, the model indicates that chemical recalcitrance alone is not sufficient to explain VSC-lignin turnover in soils, and that, functionally, the most relevant mechanism appears to be the transfer of VSC-lignin molecules and fragments from decomposing plant tissues to soil-protected fractions. [ABSTRACT FROM AUTHOR]

Liming is a common agricultural practice worldwide, used for increasing productivity in acid agricultural soils. Liming reduces Al saturation and toxicity and/or increases pH up to values where the availability of nutrients is higher. The effect of this practice on soil properties has been extensively studied, with focus of most studies upon pH, exchangeable cations and productivity. In turn, the effects of liming on soil organic C (SOC) stocks still remain poorly known. The net effect on SOC can be the result of several factors: first, liming increases the soil biological activity, thus favoring the mineralization of organic matter, which should result in CO 2 losses and a decrease of the SOC stocks. Second, liming ameliorates soil structure, increasing the stability of clay assemblages and clay-organic matter bonds, which should bring an increase in SOC physical and physicochemical protection. Finally, as liming ameliorates soil conditions to plant growth, plant productivity increases and also the return of C inputs to soil, thus potentially increasing SOC concentrations. The net effect of these processes is not well understood yet. Still, some overall trends can be deduced from data currently available in the literature. Liming does modify SOC stocks, increasing them in most cases, what seems to be caused by higher C inputs to limed soils due to increased productivity. Reductions in SOC have also been reported, probably in connexion with increased mineralization, whereas the role of improved soil structure remains unclear. Overall, these insights are deduced from published data which are still scarce, so we encourage the scientific community to synthesize unpublished SOC data from existing in situ experiments, in order to enlarge the span of experimental conditions and gain knowledge about the role of such a widespread agricultural practice on SOC stocks. [ABSTRACT FROM AUTHOR]

Abstract: Many studies showed that Fe and Al oxides stabilise organic carbon (OC) in soil. However, none of them examined the influence of spatial scale on the relations linking OC to metal oxides in soil. For this purpose we chose a near neutral soil, where these interactions may not be the dominant mechanism leading to OC storage. The aim of the study was to compare OC–metal relationships at two spatial scales: the field scale and pedological pit scale. The conceptual approach included sampling of soil horizons from 27 pits to a depth of 2m in a 22ha area. At pit scale, 146 samples were collected from a grid over the surface of a selected profile to obtain a high spatial resolution. Samples were analyzed for OC, Fe, Al, and Si originating from amorphous and crystalline oxides. For both sampling scales, relations between OC and metal oxides were statistically investigated per profile and per single horizon. We observed positive relationships involving amorphous Fe and Al oxides for horizons at pit scale suggesting that at this scale amorphous metal oxides may be involved in processes leading to OC storage in near neutral soils. However, such relationships were no longer observable at field scale. These results show that at larger scales, where the abundance of controls for OC storage is increasing, the empirical relationships established at pit scale are not powerful enough to predict the behaviour of a whole landscape. Mechanistic knowledge established at pit scale may thus not easily be scaled up in soils where interaction with metal oxides is not the only control on OC storage. [ABSTRACT FROM AUTHOR]