Influence of microbial diversity on soil organic carbon dynamics highlighted by a 13C-labelling technique

EA GenoSol EcolDur CT3International audienceSoil organic matter (SOM) represents the main pool of carbon within the biosphere, estimated at roughly twice that in atmospheric CO2. In agrosystems, organic amendments are common to maintain soil C stocks. However, recent studies have revealed that these practices can lead to a priming effect, corresponding to enhanced release of CO2 into the atmosphere, due to over-mineralisation of soil organic carbon. Therefore, appropriate decisions regarding organic input management require better understanding of the biogeochemical cycles related to SOM dynamics. As soil microorganisms are major actors in SOM turnover, their diversity is likely to influence SOM dynamics. In this context, the relationship between microbial diversity and SOM turnover should be characterised more precisely. Three levels of soil microbial diversity (D1>D2>D3) were obtained by serial dilutions of a cropland soil suspension. Gamma-ray sterilised soil microcosms were subsequently inoculated with the three levels of microbial diversity to investigate the consequences of the erosion of microbial diversity. Wheat residues labelled at more than 95% with 13C were incorporated into microcosms. Decomposition of these labelled residues and of indigenous organic matter was assessed during a 60 day-incubation experiment. The enrichment in 13C was measured in CO2 fluxes, pressurised hot water-extractable organic carbon (PHWEOC) and soil organic matter (SOM) to obtain information about the mineralised, labile and stable organic matter pools, respectively. During this study, we observed no difference in quality and quantity of PHWEOC for the three levels of microbial diversity in the control experiment, when fresh organic matter (FOM) was not added into soils. In contrast, the input of 13C-labelled FOM resulted in changes in the quality and amount of PHWEOC in the amended microcosms, revealing the influence of microbial diversity on the labile organic matter pool. Results also showed that the intensity of soil respiration as well as of the primingeffect induced by the addition of plant residues was strongly linked to microbial diversity, with the highest CO2 production measured when microbial diversity was greatest. These results illustrate the importance of considering microbial diversity as a predictive variable for organic carbon storage/release in soils. The 13C-labelling technique is suggested as a suitable technique for monitoring both the incorporation of FOM into labile and stable soil organic matter compartments, and the amount of carbon mineralised from FOM (13CO2) and SOM (12CO2). Lastly, this study clearly shows that the 13Clabelling technique is highly relevant for priming effect prevision and quantification