4.2. Relative abundances of the bacterial phyla and classes under saline environment and different N fertilization rates
Relative abundance of phyla Proteobacteria , Chloroflexi ,Acidobacteria , Actinobacteria , Planctomycetes ,Bacteroidetes and Firmicutes accounted for more than 80% of total bacterial richness. Correlation analysis on relative abundance of dominant bacterial phyla and N fertilization rates showed that N fertilization rates were positively correlated withProteobacteria (r =0.562) and Planctomycetes(r =0.492), and negatively correlated with Chloroflexi(r =-0.624) and Firmicutes (r =-0.688). Most of the bacterial phyla found in this area were also reported by Canfora et al. (2014) in a semiarid Mediterranean environment, Gao et al. (2015) in coastal salt-affected alluvial area, Ahmed et al. (2018) in mine salt stressed soil, Ren et al. (2018) in a salt-affected desert ecosystem, and Rath et al. (2019) along a salt-affected lake ecosystem. Based upon meta-analysis and retrieved sequences from the two databases GenBank and RDP, Ma and Gong (2013) summarized that 90% of the bacterial sequences were categorized into six phyla, including Proteobacteria ,Actinobacteria , Firmicutes , Acidobacteria ,Bacteroidetes , and Chloroflexi . Similarly, Delgado-Baquerizo et al. (2018) concluded that five dominant bacterial phyla in mudflats and paddy soil were Proteobacteria ,Bacteroidetes , Chloroflexi , Acidobacteria andPlanctomycetes . These bacterial phyla were also discovered in our study, although the absolute and relative richness varied among these phyla. This was not unexpected considering that the average ECe of soil samples decreased from 9.3 dS m-1 on late May 2015 to 2.78 dS m-1on early June 2018. This meant that soil salinity decreased from moderately saline level (ECe, 8-16 dS m-1) to very slightly saline level (ECe, 2-4 dS m-1) according to soil salinization classification proposed by Soil Survey Division Staff (1993). In addition to the above soil salinity related bacterial phyla, some phyla widely distributed in nonsaline soil were also found in our study. This group consisted of the following phyla:Gemmatimonadetes , Ignavibacteriae ,Cyanobacteria/Chloroplast , Nitrospirae ,Verrucomicrobia , Thaumarchaeota , Armatimonadetesand Euryarchaeota , although some phyla were classified as “salinity related” by some authors (Naz et al., 2010; Chambers et al., 2016). Correlation analysis also showed that N fertilization rates had significantly positive influence on Gemmatimonadetes(r =0.819) and Nitrospirae (r =0.895), and negative influence on Armatimonadetes (r =-0.529) andEuryarchaeota (r =-0.589). On the other hand, some bacterial phyla, showing an equal distribution across all the sites, were not correlated with and influenced by the N fertilization rates. Generally, the capacity of N fertilization in shaping bacterial communities was not as important as that of soil salinity, and this conclusion could be drawn from the comparison among most of previous literatures.
At the class level, more than 60% of total bacterial richness belonged to seven classes: Deltaproteobacteria , Anaerolineae ,Alphaproteobacteria , Betaproteobacteria ,Gammaproteobacteria , Actinobacteria andPlanctomycetia . As the division of phylum Proteobacteria ,Deltaproteobacteria , Alphaproteobacteria ,Betaproteobacteria and Gammaproteobacteria were predominant bacterial classes. Decrease trend was found for the relative abundance of Deltaproteobacteria when amendments were made to the salt-affected soils (Bello-López et al., 2014). Wu et al. (2006) found that the relative abundance of Alphaproteobacteria and theGammaproteobacteria increased with soil salinity, whereas theBetaproteobacteria decreased with salinity. The findings of Yousuf et al. (2014) also showed that Alphaproteobacteria was the dominant group in the high-salinity soil or water in terms of abundance, whereas Betaproteobacteria was more important in nonsaline environment and Gammaproteobacteria was a minor component. This was in line with Kirchman et al. (2004) who concluded that marine systems were typically dominated by Alphaproteobacteria andCytophaga -like bacteria, whereas Betaproteobacteriaappeared to be the dominant group in freshwater systems. However, in another study by Valenzuela-Encinas et al. (2008), who found that, for soils from the former lake Texcoco, the dominant class ofProteobacteria in both high and low saline soils wasGammaproteobacteria , whereas Alphaproteobacteria was dominant in medium saline soils. In this study, the relative abundance of bacterial Alphaproteobacteria (r =0.520) andGammaproteobacteria (r =0.612) showed significantly positive correlation with N fertilization rates, and positive correlation was also observed between N fertilization rates andDeltaproteobacteria , Actinobacteria andPlanctomycetia , whereas Anaerolineae exhibited negative correlation with N fertilization rates. When considering the bacteria classes with lower but important percentage, N fertilization rates was found to be negatively correlated with relative abundance ofCytophagia (r =-0.652) and Bacilli(r =-0.511), and positively correlated with relative abundance ofNitrospira (r =0.806). This indicated that, under soil saline environment, N fertilization increased the relative abundance ofAlphaproteobacteria , Gammaproteobacteria andNitrospira , whereas decreased the relative abundance ofCytophagia and Bacilli (Fig. 8). In the semiarid irrigated area, Guo et al. (2018) found that no significant difference in bacterial composition occurred under slightly saline water irrigation with varying levels of N fertilization, and long-term slightly saline water irrigation (EC= 1.7 dS m-1) increased the abundances of Actinobacteria and Nitrospira in soils. This was different from the findings of our study. The possible reason was that bacterial community was more sensitive to soil moisture than N fertilization rates for the semi-arid irrigated soil, whereas the level of N fertilization prevailed under the soil saline environment when soil moisture was not a constraining factor (Sun et al., 2018).