Challenges in Integrating DOM Chemodiversity into Kinetic Models of Soil Respiration for Improved Carbon Cycling Predictions

Chemodiversity of dissolved organic matter (DOM) has been proposed as an ecosystem property controlling the microbial metabolism; thus, the fate of carbon (C) in soils. Recent research suggests that accounting for DOM chemodiversity can improve the accuracy of process-based C cycling models; however, this approach has never been validated at continental U.S. scale. In this study, we used statistical and kinetic modeling approaches to evaluate how DOM chemodiversity affects soil respiration and whether incorporating it in kinetic models improves respiration prediction. We utilized paired high resolution FTICR-MS descriptions of DOM chemistry and soil respiration rate measurements from 63 topsoils across the USA, provided by the Molecular Observation Network. Regression analysis revealed that DOM alpha diversity (defined as the number of detected organic compounds) interacted nonlinearly with dissolved organic C (DOC) and water-extractable total nitrogen (WETN) concentrations. Soils with high DOC and WETN concentrations, showed decreased soil respiration with increasing alpha diversity, while soils with low DOC and WETN concentrations showed increased respiration. Therefore, DOM chemodiversity controlled the plausible tradeoff in microbial metabolism leading to either loss of C through respiration or SOM buildup through increased microbial growth. This finding implies that chemodiversity, as a parameter, has the potential to increase the accuracy of soil C cycling models. To evaluate respiration rate dependence on chemodiversity as a parameter, we tested three kinetic models: (1) as a function of DOC concentration only, (2) with model parameters informed by average DOM chemodiversity and (3) by chemodiversity calculated within chemical classes of DOM. All three models predicted respiration with similar accuracy. This inability suggests that current kinetic formulations do not adequately represent chemodiversity–microbial metabolism interactions. Therefore, we advise future studies to explore the effects of DOM chemodiversity, with consideration of its interactions with tradeoffs in microbial traits and environmental conditions.