A microbially-driven and depth-explicit soil organic carbon model constrained by carbon isotopes to reduce equifinality

Abstract. Over the past years, microbially-driven models have been developed to improve simulations of soil organic carbon (SOC), and have been put forward as an improvement to assess of the fate of SOC stocks under environmental change. While these models do include a better mechanistic representation of SOC cycling in comparison to cascading reservoir-based approaches, the complexity of these models implies that data on SOC stocks are insufficient to constrain the additional model parameters. In this study, we constructed a novel depth-explicit SOC model (SOILcarb) that incorporates multiple processes influencing the δ¹³C and Δ¹⁴C values of SOC and assessed if including data on the δ¹³C and Δ¹⁴C value of SOC during parameter reduces model equifinality, the phenomenon that multiple parameter combinations lead to a similar model output. To do so, we used SOILcarb to simulate depth profiles of total SOC and its δ¹³C and Δ¹⁴C values. The results show that when the model is calibrated based on only SOC stock data , the residence time of subsoil organic carbon (OC) is not simulated correctly, thus effectively making the model of limited use to predict SOC stocks driven by, for example, environmental changes. Including data on δ¹³C in the calibration process reduced model equifinality only marginally. In contrast, including data on Δ¹⁴C in the calibration process resulted in simulations of the residence time of subsoil OC consistent with measurements, while reducing equifinality only for model parameters related to the residence time of OC associated with soil minerals. Multiple model parameters could not be constrained even when data on both δ¹³C and Δ¹⁴C were included. Our results show that equifinality is an important phenomenon to consider when developing novel SOC models, or when applying established ones. Reducing uncertainty caused by this mechanism is necessary to increase confidence in predictions of the soil carbon – climate feedback in a world subject to environmental change.