Still, the environment selects: disentangling the Effect of Distance Decay on Soil Bacterial Communities

Soil bacterial communities are central to ecosystem functioning, yet the relative importance of dispersal limitation, environmental selection, and biotic interactions in shaping their spatial turnover remains unresolved. Distance–decay relationships (DDRs)—the decline in community similarity with geographic distance—are commonly observed for microbial communities, but their underlying drivers across ecologically relevant spatial scales remain unclear. We analyzed soil bacterial communities in temperate grasslands across two regions in Germany to quantify the contribution of geographic distance, soil physicochemical properties, plant community composition, and plant traits to bacterial β-diversity. Generalized linear modeling, variation partitioning, and commonality analysis revealed a clear DDR, with bacterial similarity declining by ~5% for each doubling of geographic distance. However, soil physicochemical heterogeneity accounted for over 50% of the explained variation in the bacterial DDR and nearly 30% of the total variation in community composition. Plant community composition independently explained additional variation, while plant functional traits had only marginal effects. Notably, fine-scale environmental heterogeneity within sites contributed to high turnover over short distances, indicating strong abiotic filtering even at the plot scale. Then, to further assess microbial distribution patterns, we examined the relationship between taxon abundance and spatial range. We found that rare taxa were both locally and broadly distributed, suggesting that rarity does not necessarily constrain dispersal. Dominant taxa, particularly from Proteobacteria and Firmicutes, were consistently broadly distributed, in accordance with generalist lifestyles. In contrast, we found a small, but taxonomically diverse group of highly abundant taxa which were distributed only over intermediate ranges, suggesting that dispersal limitation does not constrain dominance. Our results demonstrate that soil bacterial DDRs emerge primarily from environmental filtering and plant–soil interactions, with a secondary role for spatial separation. These findings highlight the importance of integrating spatially explicit sampling with soil and vegetation data in microbial biogeography, and shed light on the complex patterns of dispersal limitation in microbial communities.