Horizon-specific bacterial community composition of German grassland soils as revealed by pyrosequencing-based analysis of 16S rRNA genes.

The diversity of bacteria in soil is enormous and soil bacterial communities can vary greatly in structure. Here, we employed pyrosequencing-based analysis of the V2-V3 16S rRNA gene region to characterize the overall and horizon-specific (A and B horizon) bacterial community composition in 9 grassland soils, which covered three different land use types. The entire data set comprised 752,838 sequences of which 600,544 could be classified below the domain level. The average number of sequences per horizon was 41,824. The dominant taxonomic groups present in all samples and horizons were Acidobacteria, Betaproteobacteria, Actinobacteria, Gammaproteobacteria, Alphaproteobacteria, Deltaproteobacteria, Chloroflexi, Firmicutes, and Bacteroidetes. Despite these overarching dominant taxa, the abundance, diversity, and composition of bacterial communities were horizon-specific. The estimated bacterial diversity (H') was higher in the A horizons than in the corresponding B horizons and was positively correlated with the organic carbon content, the total nitrogen content, and the C:N ratio, which decreased with soil depth. In addition, it appeared that lower land use intensity results in higher bacterial diversity. The majority of sequences affiliated to the Actinobacteria, Bacteroidetes, Cyanobacteria, Fibrobacteres, Firmicutes, Spirochetes, Verrucomicrobia, Alpha-, Beta-, and Gammaproteobacteria were derived from A horizons, whereas the majority of the sequences related to the Acidobacteria, Chloroflexi, Gemmatimonadetes, Nitrospira, TM7, and WS3 originated from B horizons. The distribution of bacterial phylogenetic groups and subgroups in the different horizons correlated with soil properties such as organic carbon content, total nitrogen content, and microbial biomass. For example, members of Acidobacteria were predominant across all samples, but the horizon-specific distribution of the different acidobacterial subgroups showed correlations to the latter factors.