The world’s soils store more carbon than is present in biomass and in the atmosphere1. Little is known, however, about the factors controlling the stability of soil organic carbon stocks2,3,4 and the response of the soil carbon pool to climate change remains uncertain5,6. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m depth) stimulated the microbial mineralization of 2,567 ± 226-year-old carbon. Our results support the previously suggested idea7 that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile1,8,9 could however stimulate the loss of ancient buried carbon.
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We thank S. Révaillot, J. Messaoud, S. Millon, G. Alavoine and O. Delfosse for assistance during the incubation and for chemical and isotopic analysis. We thank J.-F. Soussana, J. Bloor, R. Hakkenberg, P. Loiseau, V. Maire, A. Chabbi and K. Klumpp for critical comments on the manuscript. The Lehrstuhl für Bodenkunde, TU München, is acknowledged for providing the NMR spectrometer. The Hydrasa Laboratory (University of Poitiers) is acknowledged for providing the X-ray diffractometer. This work was financially supported by the INRA Ecology Department, the MED (GESSOL) and the ANR (BIOMOS and C Profond).
Author Contributions S.F. conceived and designed this study; C.R. performed NMR and FTIR analyses; P.B. performed clay mineralogy analyses; B.M. performed 13C analyses; N.B. and S.F. jointly conducted the incubation; S.F. wrote the manuscript; S.B. commented on the manuscript, the modelling and the statistical analyses; and all authors took part in the interpretation of the results.
This file contains Supplementary Figure S1 which presents a picture of the studied soil profile (a) and a schematic of the main finding of the study (b), Supplementary Figure S2 which presents the XRD diffractogram of clay fractions, Supplementary Table S1 which presents the distribution of soil organic carbon content and storage along the studied soil profile, Supplementary Table S2 which compares the percentage of chemical groups obtained by the integration of NMR spectra, Supplementary Data 1 which presents the results of the chemical characterization of soil carbon by FTIR-spectroscopy, Supplementary Data 2 which explains why the mean residence time of particulate organic matter is 6.4 years and not 101 years, Supplementary Method 1 which presents the model used to analyze the role of soil minerals in the stability of deep carbon, and Supplementary Method 2 which provides the details of the model simulation estimating the mean residence time of soil organic carbon and particulate organic matter from their 14C content.
About this article
Global Biogeochemical Cycles (2019)