Objective To explore the influence of soil salinization on nitrogen transformation, the distribution patterns of nitrogen-fixing and denitrifying bacteria along a natural salinity gradient in coastal soils were analyzed in this study.

Method The qPCR and terminal restriction fragment length polymorphism (T-RFLP) techniques were used to study the distributions of community abundance and composition of nitrogen-fixing (nifH gene) and denitrifying bacteria (nirS, nirK and nosZ genes) along the salinity gradient (0.64% ~ 5.18%) in saline soils of the southern coast of Laizhou Bay and the estuary of the Yellow River. According to the range of soil salinity, the samples were divided into three gradients: low salinity (0.64% ~ 0.76%), medium salinity (1.25% ~ 2.39%), and high salinity (3.49% ~ 5.18%).

Result Nitrogen-fixing bacteria's abundance in the low salinity samples was significantly higher than those in the medium and high salinity (P < 0.05). Spearman’s correlation showed that the copy number of nifH gene was significantly positively correlated with soil NO₃⁻ concentration (P < 0.05), while Shannon index was significantly positively related to soil mean grain size (d0.5) (P < 0.05). The canonical correspondence analysis (CCA) showed that nifH gene community structure was significantly controlled by salinity (P = 0.04). NirK genotype dominated in denitrifying bacteria in the saline soils. Salinity had no significant effect on the copy numbers of nirK, nirS and nosZ genes, but the (nirK + nirS)/nosZ ratio significantly increased with the increasing salinity. Inorganic nitrogen was a key environmental factor affecting the community structure of denitrifying bacteria.

Conclusion In summary, degree of soil salinization in coastal zones significantly affects the abundance and community composition of soil nitrogen-fixing bacteria and the ratios of functional genes in the denitrification, which may affect soil nitrogen transformation processes and N₂O emission, and further influence global climate.