Diversity and Composition of Bacterial Communities During Soil Development and Horizon Formation.

The composition and dynamics of bacterial communities in soil respond inextricably to habitat characteristics. Based on this we sought to understand whether bacterial community composition and diversity changes vertically, between two diagnostic horizons, within a soil profile. We determined the composition of bacterial communities by constructing 16 rRNA clone libraries from A and B horizons along a soil developmental gradient of 5K , 45K, and 77K years since deposition of the sandy aeolian parent materials. The bacterial communities in both horizons were dominated by similar groups: Acidobacteria, Alpha-Proteobacteria, Planctomycetes. All of the diversity indices furthermore, indicated that bacterial diversity and richness (chao 1, Shannon, Simpson indices) were greater in the A than the B horizon. Bacterial communities in the B horizon, however, are not merely a subset of the communities in the A horizon. Yet, in developmentally immature soils (5K), cluster analysis of the 22 dominant taxonomic units revealed relatively similar communities in the A and B horizons, sharing 9 of the 14 (64%) taxa identified (>97% sequence similarity). As the soils developed and aged (45 and 77K), and each horizon developed further into its own unique habitat, the communities of the A and B horizons followed suit and also become more dissimilar, sharing only ~37% of the taxonomic units. This study shows that habitat type, more so than spatial proximity, is an important determinant of community composition in soil.