Atmosphere–soil carbon transfer as a function of soil depth

Abstract

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 30 centimetres)5,6,7, 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 30–100 centimetres 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 10 centimetres. 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.

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Fig. 1: Observed proportions of new carbon in 112 soil profiles.
Fig. 2: Meta-analysis of carbon age distribution over 55 tropical grassland and forest soil profiles.
Fig. 3: Depth distribution of the carbon that has been transferred from the atmosphere to soil organic matter between 1965 and 2015.

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Acknowledgements

We thank C. Marol, S. Milin and P. Signoret for contributing to additional isotopic analyses, as well as the scientists who provided numerical data from their published studies. We thank the French Agence Nationale de la Recherche for funding through the projects Dedycas (14-CE01-0004) and Equipex Aster-CEREGE (ANR-10-EQPX-24) and for supporting the Institute National de la Recherche Agronomique (INRA) Laboratory UR-1138 through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01). This is Laboratoire des Sciences du Climat et de l’Environnement contribution no. 6464.

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Nature thanks I. Janssens and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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J.B. led the study and drafted the manuscript. All authors contributed equally to data provision and processing, and commented on and provided edits to the original manuscript. J.C. supervised the statistical analysis.

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Correspondence to Jérôme Balesdent.

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Extended data figures and tables

Extended Data Fig. 1 Locations of the study sites.

Source of background image: Visible Earth, NASA.

Extended Data Fig. 2 Kinetics of new-carbon incorporation for the depth layers 0–30 cm and 30–100 cm.

The respective logarithmic regressions y = 0.30 × log10(x) − 0.07 for 0–30 cm and y = 0.26 × log10(x) − 0.23 for 30−100 cm indicate that the duration required to replace one-third of the carbon is on average seven times longer in the subsoil than the topsoil.

Extended Data Table 1 Proportion of new carbon in topsoil: multivariate linear regression
Extended Data Table 2 Proportion of new carbon in subsoil: multivariate linear regression
Extended Data Table 3 Age distribution of carbon over 55 tropical grassland and forest soil profiles
Extended Data Table 4 Depth incorporation of new carbon in subsoil: multivariate linear regression
Extended Data Table 5 Median depth of new carbon: multiple linear regression
Extended Data Table 6 Depth distribution of carbon transferred from atmosphere to SOM in 1965–2015

Supplementary information

Supplementary Information

This file contains references to the study sites.

Reporting Summary

Supplementary Table

This file contains the complete dataset of raw primary data, calculated data and ancillary information analysed and generated during the current meta-analysis.

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Balesdent, J., Basile-Doelsch, I., Chadoeuf, J. et al. Atmosphere–soil carbon transfer as a function of soil depth. Nature 559, 599–602 (2018). https://doi.org/10.1038/s41586-018-0328-3

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