Introduction of exotic species Sonneratia apetala alters diazotrophic community and stimulates nitrogen fixation in mangrove sediments

Mangrove restoration by exotic mangrove species Sonneratia apetala has been practiced as a strategy to reconstruct mangrove wetlands. Diazotrophs in mangrove sediment are nitrogen suppliers for mangrove ecosystems. However, the response of sediment diazotrophs, especially their nitrogen fixation rates, to the introduction of exotic S. apetala remains unclear. In the present study, we compared the variations in sediment physicochemical properties, N₂ fixation rate, and the composition of total bacterial and diazotrophic communities in mangrove sediments between exotic species (S. apetala) and three native species (i.e., Aegiceras corniculatum, Lumnitzera racemosa, and Rhizophora apiculata). Our results indicated that the introduction of exotic S. apetala significantly (P < 0.05) increased nutrient levels (including TC, TN, and TS) and improved the N₂ fixation rate in sediments. Though S. apetala introduction had not caused significant changes in bacterial and diazotrophic alpha diversity, their community structure and composition were altered. The dominant diazotrophic groups were sulfur-oxidizing (SOB) and sulfate-reducing (SRB) bacteria. The relative abundance of Chromatiaceae (affiliated with SOB) significantly increased (P < 0.05), while the relative abundance of Desulfuromonadaceae (affiliated with SRB) decreased in S. apetala sediments. In addition, several SOB and SRB groups were identified as putative keystone taxa by network analysis, suggesting that SOB/SRB groups potentially played a central role in the biogeochemical cycles involving nitrogen (N) and sulfur (S) of the mangrove ecosystem. Redundancy analysis revealed that C/N and NH₄⁺-N were the key factors (P < 0.05) driving sediment diazotrophic communities. This study provides new insights into the ecological impact of exotic S. apetala introduction, which has important implications for the sustainable management and restoration of mangrove ecosystems.