High salinity suppresses nitrogen cycle genes and shifts nitrifier communities in the black mangrove rhizosphere

High salinity inhibits the nitrogen cycle, which is crucial to biogeochemical changes in coastal mangrove ecosystems. We examined Avicennia germinans rhizosphere soil over a salinity gradient (electrical conductivities of 5.27 mS cm⁻¹–38.64 mS cm⁻¹) to see how high salinity affects the bacterial community and metabolic nitrogen activities. Amplicon sequencing of the 16S rRNA gene examined the bacterial population profile, whereas full shotgun metagenome sequencing assessed functional genetic potential. Bacillus, Desulfuromonas, Methyloceanibacter, and Nitrospira dominated the genera, whereas Proteobacteria, Actinobacteria, and Bacteroidetes dominated the phyla. Nitrospirae dominated at high salinity. High soil salinity suppressed nitrogen cycle gene abundances: nifH, nxrAB, nirS, nirK, norB, nirB, and nirA. Ammonia-oxidizing bacteria like Nitrosococcus and Nitrosomonas decreased with salinity in the nitrifier population discovered by amplicon sequencing. Nitrite-oxidizing bacteria like Nitrospira and Nitrospina rose at high salinity, whereas Nitrococcus and Nitrolancea declined. Salinity reduces nitrogen gene abundances in most nitrifier community members, inhibiting the nitrogen cycle.