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 δ13C measurements for benchmarking SOC turnover in global models. We used 13C 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 13C enrichment with soil depth as follows: 1) the Suess effect and CO2 fertilization, 2) the relative 13C enrichment of roots compared to leaves, and 3) 13C discrimination associated with microbial activity. We tested if the upgraded soil module was able to reproduce the vertical profile of δ13C 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 δ13C, 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 δ13C. This new soil module is an emerging tool for the diagnosis and improvement of global SOC models.
Highlights
Global soil organic carbon (SOC) simulated by Earth system models (ESMs) may be consistent with global observations, but the uncertainty surrounding models output remains collectively high (Huntzinger et al, 2017; Todd‐Brown et al, 2013)
Our new module represents some of the processes that have been hypothesized to lead to a 13C enrichment with soil depth as follows: 1) the Suess effect and CO2 fertilization, 2) the relative 13C enrichment of roots compared to leaves, and 3) 13C discrimination associated with microbial activity
ORCHIDEE‐SOM represents the adsorption of dissolved organic carbon (DOC) to soil minerals using an equilibrium‐partition coefficient (Nodvin et al, 1986), which implies that the DOC is split into two pools as follows: free DOC in soil solution and DOC adsorbed on soil minerals
Summary
Global soil organic carbon (SOC) simulated by Earth system models (ESMs) may be consistent with global observations, but the uncertainty surrounding models output remains collectively high (Huntzinger et al, 2017; Todd‐Brown et al, 2013). Using observational isotopic data to constrain soil turnover times is not yet possible for ESMs, because global models usually do not represent either stable carbon or radiocarbon isotopes even though some attempts have been carried out with ORCHIDEE for radiocarbon (Tifafi et al, 2018). Radiocarbon has been used for decades to constrain SOC turnover times in a 1‐D model (Coleman et al, 1997) and has been used to indirectly evaluate ESMs (He et al, 2016).
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