Abstract: The diazotrophs assemblages associated with the roots of the mangrove species Kandelia obovata exhibit remarkable species richness, which is integral to nitrogen cycling within mangrove ecosystems. However, the ecological adaptation mechanisms of these diazotrophs within mangrove ecosystems remain inadequately elucidated. In this study, we collected 75 samples each of root - endosphere, rhizosphere, and bulk soil from mangrove ecosystems in Leizhou, South China. Using high - throughput sequencing and molecular ecological approaches, this study analyzed the composition, diversity, complexity, stability of diazotrophs and their main driving factors in Kandelia obovata's root endosphere, rhizosphere, and non - rhizosphere areas. The results reveal that Alphaproteobacteria, Deltaproteobacteria, and Gammaproteobacteria dominate the diazotrophic communities, with Alphaproteobacteria exhibit the highest relative abundance. Specifically, the relative abundances of Alphaproteobacteria in the endorhizosphere, rhizosphere, and non-rhizosphere soils were 54.9%, 37.5%, and 29.1%, respectively. The α-diversity of diazotrophic communities in the rhizosphere of Kandelia obovata is minimal, whereas their stability is maximal. Notably, complexity and stability were negatively correlated in the rhizosphere and bulk soil but positively correlated within the endorhizosphere. In both the rhizosphere and non-rhizosphere regions of Kandelia obovata, a significant negative correlation exists between complexity and stability. Spearman correlation analysis indicated that salinity and total nitrogen are principal determinants of diazotrophic community complexity across distinct regions of mangrove root-associated zones. In the non-rhizosphere and endosphere, diazotrophic community complexity exhibits significant positive correlations with both salinity and total nitrogen. Conversely, in the rhizosphere, diazotrophic community complexity displays significant negative correlations with salinity and total nitrogen. The primary factors influencing the stability of diazotrophic communities in mangrove root-associated zones differ among regions: in the non-rhizosphere, stability is significantly negatively correlated with nitrate nitrogen, whereas in the rhizosphere, it is significantly positively correlated with salinity. The present study augments the comprehension of the mechanisms governing the sustenance of complexity and community stability among diazotrophic communities within mangrove ecosystems. These insights hold substantial significance for the restoration and preservation of mangrove vegetation in saline-alkali regions.

Key words: diazotrophic communities, root-associated zones, mangrove ecosystem, co - occurrence network, complexity, stability