Modeling population-level controls on soil microbial turnover

Soil is organizationally complex and spatially heterogeneous with exceptional microbial diversity that varies in time and space. Hotspots of microbial activity are prevalent, yet they are patchy and periodic and it remains intractable to represent this level of detail in macro-scale soil microbial models. Most macro-scale microbial models have, therefore, been focused on exploring theory and capturing select processes in a simplified way. However, effective equations that account for population- and community-level controls may be needed to suitably capture emergent feedbacks at macro-scales. In this study, we explore the effective relationships that emerge between spatially aggregated carbon pools in a micro-scale soil model with competition and space constraints. Specifically, we use an individual-based, spatially explicit model to simulate the response of soil microbes to a range of scenarios with increasing carbon inputs, including spatially-uniform (homogeneous) and spatially-clumped (heterogeneous) increases, where the input flux integrated over the total area is the same in both scenarios. The latter is meant to mimic hotspots of carbon inputs, for example, in the rhizosphere or near preferential flow paths. We also explore threshold effects on microbial activity when varying the degree of spatial clumping. We find that competition between microbes and the probability of invasion from neighboring microsites plays a critical role in emergent density-dependent dynamics of microbial growth and turnover. Our study elucidates the role of population-level controls on microbial turnover at macro-scales, and motivates careful consideration of scale-dependent model representations.