Radiocarbon analysis reveals underestimation of soil organic carbon persistence in new-generation soil models

Abstract

Reflecting recent advances in our understanding of soil organic carbon (SOC) turnover and persistence, a new generation of models increasingly makes the distinction between the more labile soil particulate organic matter (POM) and the more persistent mineral-associated organic matter (MAOM). Unlike the typically poorly defined conceptual pools of traditional SOC models, the POM and MAOM soil fractions can be directly measured for their carbon content and isotopic composition, allowing for fraction-specific data assimilation. However, the new-generation model predictions of POM and MAOM dynamics have not yet been validated with fraction-specific carbon and 14C observations. In this study, we evaluate five influential and actively developed new-generation models (CORPSE, MEND, Millennial, MIMICS, SOMic) with fraction-specific and bulk soil 14C measurements of 77 mineral topsoil profiles in the International Soil Radiocarbon Database (ISRaD). We find that all five models consistently overestimate the 14C content (Δ14C) of POM by 69 ‰ on average, and two out of the five models also strongly overestimate the Δ14C of MAOM by more than 80 ‰ on average, indicating that the models generally overestimate the turnover rates of SOC and do not adequately represent the long-term stabilization of carbon in soils. These results call for more widespread usage of fraction-specific carbon and 14C measurements for parameter calibration and may even suggest that some new-generation models might need to restructure or further subdivide their simulated carbon pools in order to accurately reproduce SOC dynamics.