Soil Habitat and Horizon Properties Impact Bacterial Diversity and Composition

Little is known about how bacterial communities differ between soil horizons and in response to horizon development. Soil horizons are distinct habitats that continue to develop and change with age. It was hypothesized that as soil horizons aged and differentiated, that microbial community composition and structure between the horizons would become more different. Clone libraries of the 16S rRNA (ribosomal ribonucleic acid) gene were constructed for soils from the A and B horizons along a soil-age gradient of 5,000, 45,000, and 77,000 yr. Results showed that bacterial communities (16S rRNA genes) in both horizons and across the soil development gradient were dominated by similar groups: Acidobacteria, Alpha-Proteobacteria, and Planctomycetes. Bacterial richness (Simpson's 1/D) was greater in the A (154-337) than the B (29-139) horizon and tended to increase with horizon development. The effect of horizon on the fatty acid-based microbial community structure and physiology indicated a clear horizon effect. Despite the much greater richness in the A than the B horizon, cluster analysis of the 16S rRNA genes of the dominant bacterial taxa indicated that communities in developmentally immature soils (5,000) had similar communities in the A and B horizons, sharing similar to 75% of the sequences (> 97% sequence similarity). As the soils (45,000 and 77,000 yr) developed further, the communities between the A and B horizons diverged, sharing 50% or less of the sequences in the most abundant operational taxonomic units (OTUs). Horizon development during soil genesis seems to be an important determinant of bacterial community composition and structure.