Emergent SOM Dynamics Considering Interactions Between Microbial Physiology, Microbial Competition, Mineral Interactions, Vertical Transport, and Temperature

Abstract:

The range of processes hypothesized to affect Soil Organic Matter (SOM) dynamics is large. Although mechanistic understanding of some of these processes is good (e.g., mineral interactions), it is less well developed for others (e.g., aggregation, microbial community dynamics). Further, interactions between these processes in the complex soil environment are poorly represented in the new generation of mechanistic SOM models. An important component of the modeling problem lies in the under-appreciated numerical modeling issues associated with coupling processes that are tightly inter-connected and span many orders of magnitude in temporal scales (seconds to millennia). Here, I will synthesize results from recent modeling work that integrates (1) explicit representations of microbial physiology; (2) the Equilibrium Chemistry Approximation (ECA) for competitive interactions; (3) process-specific thermodynamically-based temperature sensitivities; (4) enzyme kinetics; (5) and abiotic interactions with mineral surfaces. In several sites (Russian grassland, U.S. midwestern grasslands, Hawaii, and northern California), our modeling results suggest that observed depth profiles of SOM content and radiocarbon signatures are broadly consistent with SOM inputs near the surface, fungal and bacterial activity, advective transport, and mineral interactions and stabilization. Further, when explicitly representing microbial physiology, we demonstrate that the often-observed large temporal variability in temperature sensitivity of SOM decomposition and carbon use efficiency can substantially impact predicted SOM profiles, compared to models that do not include these dependencies. Finally, I will describe integration of these concepts into the global land model CLM4.5 and implications for predicted SOM dynamics under a changing climate.