Including Stable Carbon Isotopes to Evaluate the Dynamics of Soil Carbon in the Land‐Surface Model ORCHIDEE

Soil organic carbon (SOC) is a crucial component of the terrestrial carbon cycle and its turnover time in models is a key source of uncertainty. Studies have highlighted the utility of δ(13)C measurements for benchmarking SOC turnover in global models. We used (13)C as a tracer within a vertically discretized soil module of a land‐surface model, Organising Carbon and Hydrology In Dynamic Ecosystems‐ Soil Organic Matter (ORCHIDEE‐SOM). Our new module represents some of the processes that have been hypothesized to lead to a (13)C enrichment with soil depth as follows: 1) the Suess effect and CO(2) fertilization, 2) the relative (13)C enrichment of roots compared to leaves, and 3) (13)C discrimination associated with microbial activity. We tested if the upgraded soil module was able to reproduce the vertical profile of δ(13)C within the soil column at two temperate sites and the short‐term change in the isotopic signal of soil after a shift in C3/C4 vegetation. We ran the model over Europe to test its performance at larger scale. The model was able to simulate a shift in the isotopic signal due to short‐term changes in vegetation cover from C3 to C4; however, it was not able to reproduce the overall vertical profile in soil δ(13)C, which arises as a combination of short and long‐term processes. At the European scale, the model ably reproduced soil CO(2) fluxes and total SOC stock. These findings stress the importance of the long‐term history of land cover for simulating vertical profiles of δ(13)C. This new soil module is an emerging tool for the diagnosis and improvement of global SOC models.