3.3 Bacterial community structures at phylum and class levels
The bacteria from sixteen soil samples exhibited similar diversity but different abundances. A total of 42 identified phyla were observed, and the 4,790,305 high-quality sequences classified at the phylum level were affiliated to 36 Bacterial phyla and 6 Archaea phyla. Bacteria were numerically dominant relative to Archaea. The dominant phyla across all samples were: Acidobacteria , Actinobacteria ,Bacteroidetes , Chloroflexi , Cyanobacteria ,Firmicutes , Gemmatimonadetes , Ignavibacteriae ,Nitrospirae, Planctomycetes , Proteobacteria ,Thaumarchaeota andVerrucomicrobia (relative abundance more than 0.5%). The dominant taxa in the analyzed soil samples were:Proteobacteria (39.17%), Chloroflexi (15.81%),Acidobacteria (7.19%), Actinobacteria (6.34%),Planctomycetes (5.82%), followed by a second group with a lower but still important percentage of distribution across all the samples, i.e., Bacteroidetes (4.44%), Firmicutes (1.98%),Gemmatimonadetes (1.53%), Ignavibacteriae (1.51%). The taxa with a lower relative distribution were Cyanobacteria(0.99%), Nitrospirae (0.95%), Verrucomicrobia (0.70%) and Thaumarchaeota (0.56%). The relative bacterial community abundance at the phylum level was shown in Fig. 5a. For the CK treatment, Proteobacteria accounted for 36.54% of the total soil bacteria, whereas the percentage increased to 39.29%, 39.42% and 40.78% for NF1, NF2 and NF3 treatments, respectively.Chloroflexi , another predominant phylum, was 17.12% of the total bacteria in the CK treatment soil, which significantly decreased to17.58%, 15.30%, and 13.57% in NF1, NF2 and NF3 treatment soils, respectively. Moreover, significant shifts were also observed forFirmicutes and Nitrospirae across all the treatments.
Figure 5
Figure 5b illustrates the relative bacterial community abundance at the class level. In total, 73 classes were classified across all soil samples, of which 23 classes had relative abundance of more than 0.5%. The relative abundance of unassigned classes was 18.68%, and the dominant classes (relative abundance more than 0.5%) across all samples included Deltaproteobacteria (11.79%), Anaerolineae(10.61%), Alphaproteobacteria (10.27%),Betaproteobacteria (7.94%), Gammaproteobacteria (7.41%),Actinobacteria (6.13%) and Planctomycetia (5.15%). The bacteria classes with lower but important percentage (relative abundance between 1% and 5%) comprised of Acidobacteria_Gp 6 (2.65%),Ignavibacteria (1.49%), Cytophagia (1.43%),Sphingobacteriia (1.33%), Nitrospira (1.32%),Gemmatimonadetes (1.20%), Bacilli (1.19%) andAcidobacteria_Gp 10 (1.05%). The relative bacterial community abundance at the class level was presented in Table 6. Increasing trend was observed for classes of Alphaproteobacteria ,Gammaproteobacteria , Planctomycetia and Nitrospirawith the increment of N fertilization rates. For instance, the relative abundance of Nitrospira increased from 1.09% to 1.13%, 1.35% and 1.65% at 150, 300, and 450 kg N hm-2y-1 rates, respectively. This trend was also observed for Nitrospira at the phylum level. The N fertilization decreased the relative abundance of classes Anaerolineae ,Acidobacteria_Gp 6, Cytophagia , Bacilli andAcidobacteria_Gp 10. The relative abundance of classesDeltaproteobacteria , Betaproteobacteria ,Actinobacteria , Ignavibacteria , Sphingobacteriiaand Gemmatimonadetes was not statistically different among various N fertilization rates, indicating that the effect of N fertilization on the above bacterial classes was not obvious.
Table 6