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