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