3.4 Differences in bacterial classes among different N rates
Principal coordinates analysis (PCoA) was performed to determine the
extent of treatment differentiation with regard to the N fertilization
rates. The PCoA demonstrated clear separations in the bacterial
communities at class level under different N fertilization rates (Fig.
6). The first principal component (PC1, 40.7% of contribution rate),
which explains the majority of variations in the data, represented
presence or absence of N fertilization. The second principal components
(PC2), representing N fertilization rates, explained 17.9% of the data
variance. In total, 58.6% of variance of species was explained by the
two principal components. The two components separated the community
composition by differences in the N fertilization rates which was the
only difference among the treatments. Obviously, one soil sample with
300 kg N hm-2 y-1 was clustered into
the group of 150 kg N hm-2 y-1.
Despite this, the PCoA results suggested that soil bacterial classes
were well separated from different N application rates, and the change
in N rates brought about changes in the bacterial community structure.
Figure 6
The similarity and differences of the sixteen dominant bacterial classes
was further presented in the bacterial community heatmap (Figure 7). The
cluster structure showed four main groups of class which shared a
peculiar composition and abundance among the samples. The control soil
samples were discriminated from other samples treated with N
fertilization, suggesting clear distinction of bacterial community
structure between the treatments with and without N fertilization.
Samples with 450 kg N hm-2 y-1 rate
were also discerned easily, such as Alphaproteobacteria ,Anaerolineae and Actinobacteria , whereas the distinction
between treatments of 150 kg N hm-2y-1 and 300 kg N hm-2y-1 rates was not obvious. Actually, most groups of
classes, although showing varying abundance, appeared grouped together
and uniformly distributed across all the samples with the increase of N
rates. For 150 kg N hm-2 y-1 and 300
kg N hm-2 y-1 treatments, all the
classes had almost the same abundance degree, resulting in the
uncertainty in cluster discrimination. Overall, this indicated that the
capacity of N fertilization in shaping bacterial communities was not as
important as that of soil salinity. This was also witnessed by the
statistics of relative abundance in Table 6.
Figure 7