Jérôme Balesdent

Atmosphere–soil carbon transfer as a function of soil depth

The exchange of carbon between soil organic carbon (SOC) and the atmosphere affects the climate1,2 and—because of the importance of organic matter to soil fertility—agricultural productivity3. The dynamics of topsoil carbon has been relatively well quantified4, but half of the soil carbon is located in deeper soil layers (below 30xa0centimetres)5–7, and many questions remain regarding the exchange of this deep carbon with the atmosphere8. This knowledge gap restricts soil carbon management policies and limits global carbon models1,9,10. Here we quantify the recent incorporation of atmosphere-derived carbon atoms into whole-soil profiles, through a meta-analysis of changes in stable carbon isotope signatures at 112 grassland, forest and cropland sites, across different climatic zones, from 1965 to 2015. We find, in agreement with previous work5,6, that soil at a depth of 30–100xa0centimetres beneath the surface (the subsoil) contains on average 47 per cent of the topmost metre’s SOC stocks. However, we show that this subsoil accounts for just 19 per cent of the SOC that has been recently incorporated (within the past 50 years) into the topmost metre. Globally, the median depth of recent carbon incorporation into mineral soil is 10xa0centimetres. Variations in the relative allocation of carbon to deep soil layers are better explained by the aridity index than by mean annual temperature. Land use for crops reduces the incorporation of carbon into the soil surface layer, but not into deeper layers. Our results suggest that SOC dynamics and its responses to climatic control or land use are strongly dependent on soil depth. We propose that using multilayer soil modules in global carbon models, tested with our data, could help to improve our understanding of soil–atmosphere carbon exchange.This study of whole-soil carbon dynamics finds that, of the atmospheric carbon that is incorporated into the topmost metre of soil over 50 years, just 19 per cent reaches the subsoil, in a manner that depends on land use and aridity.

Labelled microbial culture as a calibration medium for C-13-isotope measurement of derivatized compounds: application to tert-butyldimethylsilyl amino acids

RATIONALEnCompound-specific stable carbon isotope analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) is widely used in studies of environmental or biological functioning. In the case of derivatized molecules, a calibration might be required due to added non-analyte carbon and in some cases non-stoichiometric recovery by the mass spectrometer.nnnMETHODSnTwo biological materials of known isotopic composition were produced by microbial cell cultures on either (13) C-labelled glucose or non-labelled glucose as sole source of carbon. Subsequent hydrolyzed amino acids were derivatized as tert-butyldimethylsilyl (tBDMSi) derivatives and analyzed by GC/C/IRMS. The (13) C-enrichment measurements were used as a direct calibration to calculate the original (13) C/(12) C ratios of individual amino acids. We tested this calibration on both known and unknown samples.nnnRESULTSnFor the main proteinogenic amino acids we could determine the number of non-analyte added carbon atoms and assess the non-stoichiometrical recovery of tBDMSi carbon atoms, due to their incomplete oxidation in the combustion step of GC/C/IRMS. The calibration enabled the determination of the natural abundances (δ(13) C values) of amino acids with an average accuracy of ±1.1 ‰. We illustrate the application of the calibration to determine the (13) C/(12) C ratios of amino acids, and the associated uncertainty, in biological and plant materials.nnnCONCLUSIONSnThe analysis of a labelled microbial cell culture offers a straightforward, rapid and reliable estimate of non-analyte carbon contribution to stable isotope composition. We recommend this method as a calibration or a control in artificial or natural (13) C-tracing experiments. Copyright

Quantification of vertical solid matter transfers in soils during pedogenesis by a multi-tracer approach

PurposeVertical transfer of solid matter in soils (bioturbation and translocation) is responsible for changes in soil properties over time through the redistribution of most of the soil constituents with depth. Such transfers are, however, still poorly quantified.Materials and methodsIn this study, we examine matter transfer in four eutric Luvisols through an isotopic approach based on 137Cs, 210Pb(xs), and meteoric 10Be. These isotopes differ with respect to chemical behavior, input histories, and half-lives, which allows us to explore a large time range. Their vertical distributions were modeled by a diffusion-advection equation with depth-dependent parameters. We estimated a set of advection and diffusion coefficients able to simulate all isotope depth distributions and validated the resulting model by comparing the depth distribution of organic carbon (including 12/13C and 14C isotopes) and of the 0–2-μm particles with the data.Results and discussionWe showed that (i) the model satisfactorily reproduces the organic carbon, 13C, and 14C depth distributions, indicating that organic carbon content and age can be explained by transport without invoking depth-dependent decay rates; (ii) translocation partly explains the 0–2-μm particle accumulation in the Bt horizon; and (iii) estimates of diffusion coefficients that quantify the soil mixing rate by bioturbation are significantly higher for the studied plots than those obtained by ecological studies.ConclusionsThis study presents a model capable of satisfactorily reproducing the isotopic profiles of several tracers and simulating the distribution of organic carbon and the translocation of 0–2-μm particles.