Microbial cell density and interaction potential in soil are governed by the extent of habitable pore surfaces

Soil microbes drive terrestrial biogeochemical cycles, yet their fine-scale spatial distribution and occupancy of pore space remain poorly understood. Using a multidisciplinary approach, we resolve microbial colonization of soil pores at a micrometer resolution and assess how cell density shapes potential microbial interactions. We derive a general scaling law that converts X‑ray microtomography outputs into micrometer-scale estimates of habitable surface area. Across diverse soils, we found that microbial abundance scaled positively with habitable surface area, and that bacteria occupied a substantially larger fraction of pore surfaces than previously recognized. In addition, the diameter of soil pores was found to modulate the potential for bacterial interactions, which likely span phylogenetic lineages. These findings revise prevailing views of microbial colonization of soil pores and have direct implications for conceptualizing microbial interactions including modelling of microbially mediated processes.