3.3 Soil bacteria community compositions
According to 16S rRNA gene sequences, the number of sequences per sample ranged from 31458 to 172704 at a 97% sequence identity threshold. Overall, a total of 8232 OTUs were identified. Actinobacteria (14.5%–32.6% relative abundance), Proteobacteria (16.5%–28.7% relative abundance), Acidobacteria (15.5%–37.1% relative abundance), Chloroflexi (10.5%–21.6% relative abundance), and Gemmatimonadetes (4.0%–6.9% relative abundance) were considered the dominant phyla associated with residue decomposition (Fig. 4a–b). These five phyla accounted for 96.4% of all sequence reads (Fig. 4).
N application, tillage × soil depth, and N × tillage interaction significantly influenced the bacterial (16S) community compositions (Table S4). For the dominant phyla, the relative abundances of Acidobacteria, Planctomycetes, and Firmicutes increased with soil depth, while the relative abundances of Proteobacteria, Actinobacteria, Bacteroidetes, and Verrucomicrobia declined with soil depth (Fig. 4). Compared with CT, NT increased the relative abundances of Proteobacteria and Bacteroidetes in the 0–25 cm layer. The relative abundances of Bacteroidetes increased with an increasing in N application under NT, while N application had no effect on them under CT. Tillage management also had no influence on the relative abundances of Chloroflexi. Furthermore, N2 increased the relative abundances of Chloroflexi compared with N1 and N3 under CT, whereas the relative abundances of Chloroflexi of N1 were higher than N2 and N3 under NT in both layers.