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 delta C-13 measurements for benchmarking SOC turnover in global models. We used C-13 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 C-13 enrichment with soil depth as follows: 1) the Suess effect and CO2 fertilization, 2) the relative C-13 enrichment of roots compared to leaves, and 3) C-13 discrimination associated with microbial activity. We tested if the upgraded soil module was able to reproduce the vertical profile of delta C-13 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 delta C-13, which arises as a combination of short and long-term processes. At the European scale, the model ably reproduced soil CO2 fluxes and total SOC stock. These findings stress the importance of the long-term history of land cover for simulating vertical profiles of delta C-13. This new soil module is an emerging tool for the diagnosis and improvement of global SOC models.