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

Abstract 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.