To mimic microbes with models: Development and application of the microbially explicit soil model MIMICS+ in cold climates

Abstract

A major uncertainty in current climate projections is whether soils will act as a net sink or source of atmospheric carbon. Through decomposition and respiration, soil microbes like bacteria and fungi largely control the carbon exchange between soils and the atmosphere. These exchanges are dependent on factors like temperature, moisture, and nutrient availability. Yet, the traditional soil sub-models used in Earth System Models lack a proper representation of these processes. This work presents and applies a new soil model, MIMICS+. It builds on an existing model (MIMICS) which represents the decomposition of dead plant material and soil organic matter by two microbial groups. With MIMICS+, we extend this framework by introducing symbiotic fungi (mycorrhiza), a nitrogen cycle, and vertical layers. The model performs better or on par with a traditional model when compared to forested Norwegian sites, and broadly captures climatic and litter quality controls on decomposition rates. We also show that the presence of mycorrhizal fungi can affect carbon storage and respiration through nutrient competition with microbial decomposers. In the future MIMICS+ will represent an improved soil sub-model in the Norwegian Earth System Model, thus contributing to a deeper understanding of how soil and microorganisms respond to climate change.