Spatial substrate heterogeneity limits microbial growth as revealed by the joint experimental quantification and modeling of carbon and heat fluxes

Spatial heterogeneity is a pervasive feature of soils, affecting the distribution of carbon sources as well as their microbial consumers. Heterogeneous addition of substrates typically results in delayed microbial growth compared to homogeneous addition, and this effect has frequently been attributed to spatial separation of microorganisms from their food. We investigated the importance of two other potential causes of this effect, the availability of nutrients and oxygen, by measuring heat and CO2 release along with O2 consumption from soil samples after homogeneous or heterogeneous addition of glucose as well as with or without further addition of a nutrient solution. We then employed a microbial-explicit model to quantitatively interpret our observations. The results revealed that delayed growth after spatially heterogeneous substrate addition was primarily caused by nutrient limitation. While sufficient co-location of all entities - substrate, microorganisms, and nutrients - is required for optimal growth, spatial separation of glucose and microorganisms only played a minor role in our experiment. Model simulations captured the dynamics based on aerobic growth and maintenance, utilizing a simple formulation of nutrient limitation coupled with dynamic transition of microbes between activity and dormancy. The model predicted an overall lower microbial activity over the course of the incubation in treatments with heterogeneous substrate addition. Despite reduced rates, neither the experimental carbon and energy balances nor modeling showed an effect of heterogeneity on the growth efficiency after 50 h of incubation. In all treatments, energy use efficiency exceeded carbon use efficiency by 9-21%. We found no evidence of anaerobiosis. The application of a bioenergetic framework facilitated the interpretation of complex experimental data and quantitatively captured the mechanisms underlying the effects of spatial heterogeneity.