5. Conclusions

This study revealed that consecutive N fertilization accelerated the improvement of soil chemical and microbial properties for coastal salt-affected Fluvo-aquic soil under paddy rice- winter wheat rotation. Results of 16S rRNA gene sequencing showed that soil bacterial community structures well responded to the N fertilization rates and community richness indexes increased with the N fertilization rates. Phyla ofProteobacteria , Chloroflexi , Acidobacteria ,Actinobacteria and Planctomycetes were the dominant phylum across all soil samples, whereas Deltaproteobacteria ,Anaerolineae , Alphaproteobacteria ,Betaproteobacteria , Gammaproteobacteria ,Actinobacteria and Planctomycetia were predominant bacterial classes at the class level. Although the impact of N fertilization on shaping bacterial communities was nowhere near as significant as that of soil salinity, N fertilization resulted in the increase in the relative abundance of classesAlphaproteobacteria , Gammaproteobacteria ,Planctomycetia and Nitrospira , and the decrease inAnaerolineae , Acidobacteria_Gp 6, Cytophagia ,Bacilli and Acidobacteria_Gp 10. Four bacterial classes were well separated from different N fertilization rates. However, community heatmap showed that most groups of classes appeared grouped together and uniformly distributed for different N fertilization rates. Redundancy analysis (RDA) indicated that the community structure ofAlphaproteobacteria , Planctomycetia and Nitrospirawas significantly influenced by PNR, and the community structure ofActinobacteria was significantly influenced by CMR. Overall, N fertilization improved soil nutrients and metabolic activities to more suitable microhabitats for bacteria, and bacterial community evolved by replacing less adapted species with better adapted ones for coastal salt-affected soil under paddy rice-winter wheat rotation.