General Rules for Size and Spatial Distribution of Soil Bacterial Communities

Soil water dynamics within a highly fragmented soil physical environment constrain soil bacterial dispersion ranges, modulate diffusion and access to patchy resources. We have used a mechanistic modeling framework that integrates soil hydration status with organic carbon inputs to estimate community size distributions and interaction distances of modeled soil bacterial populations. The resulting spatial patterns of bacterial communities is critical for interpreting soil micro-ecological functioning. Experimental data supported by model results show that soil bacterial cluster sizes often follow an exponentially truncated power law with key parameters that vary with mean soil water content and total carbon inputs across biomes. Similar to human settlement size distributions, tree sizes and other spatially fixed systems in which growth rates are defined by their environment independent of object size (city or a tree), bacterial community size distribution is expected to obey the so-called Gibrat’s law (derived analytically for growth rates independent of community size). Results support generalization in soil using positively skewed distributions of soil bacterial community sizes (e.g., log normal). We show that soil bacteria reside in numerous small communities (with over 90% of soil bacterial communities containing less than 100 cells), supported by theoretical predictions of log-normal distribution for non-interacting soil bacterial community sizes with scaling parameters that vary with biome characteristics.