13C-14C relations reveal that soil 13C-depth gradient is linked to historical changes in vegetation 13C

Aims The understanding of the dynamics of subsoil (>30 cm) soil organic matter (SOM) is critical to predict the future evolution of the carbon cycle. Stable carbon isotopes ratios (C-13/C-12) are helpful to study the dynamics of SOM, but their variations with depth are still speculative. Methods Several studies indicated that the C-13/C-12 ratio of C-3 vegetation decreased over time more than that of atmospheric CO2 did. From these studies, we modelled the average variation of delta C-13 values of vegetation from 20,000 years Before Present (BP) to today. Then, we conducted a meta-analysis of the delta C-13 vs increment C-14 values relations in forty-five soil profiles sampled all around the world. Results We first found evidence of the change in SOM delta C-13 values with the sampling year of the profile. Then, by converting increment C-14 values into mean calendar age of SOM, we showed that 40% of the change in SOM delta C-13 values was explained by the historical change in plant delta C-13 values. Conclusion We conclude that the average increase of SOM delta C-13 values with depth was mostly linked to the change in vegetation delta C-13 values over the last 20,000 years. The variance around the trend was attributed to the contribution of root derived carbon and to soil processes such as interaction of SOM with minerals or to microbial processes.