Abstract: Casuarina equisetifolia is a typical coastal shelterbelt tree species. The characteristics of soil diazotrophic microbial communities among different varieties of C. equisetifolia remain poorly understood. In this study, we investigated the physicochemical properties, structure of soil diazotrophic communities and their key driving factors across six varieties (Xingwang, Fada, Qiantou, Ping’an, Ruyifeng, and Jixianglong) from the National C. equisetifolia Germplasm Repository in Hui’an, Fujian Province. The results showed that: 1) There were significant differences in soil physicochemical properties among different varieties, with soil moisture ranging from 7.7% to 15.6%, organic matter content from 6.3% to 9.8%, and clay proportion from 0.1% to 0.3%. All variables, as well as the activities of N-acetyl-glucosaminidase (25.77-92.12 nmol·g^-1·h^-1) and leucine aminopeptidase activity (12.18-59.04 nmol·g^-1·h^-1) were highest in ‘Ruyifeng’. 2) The abundance of nifH gene ranged from 4.5×10⁵ to 1.6×10⁶ copies·g^-1 soil, with the highest value being observed in ‘Jixianglong’. The α-diversity indices of diazotrophic microbial communities (based on the nifH gene) were significantly higher in Ping’an than in the other varieties. Principal coordinate analysis based on Bray-Curtis distance revealed relatively high diazotrophic microbial community similarity between Jixianglong and Xingwang, as well as between Qiantou and Fada, whereas the Ping’an exhibited the most distinct community composition. 3) The dominant diazotrophs across all six varieties were Bradyrhizobium (relative abundance>40%), and there were significant differentiations in community composition. The community composition of Ping’an variety differed most markedly from others. 4) Soil moisture, pH, and organic matter cotent were the main factors driving diazotrophic abundance, with organic matter content having the most significant effect. Organic matter content and clay fraction were identified as key drivers of community diversity. Structural equation modeling showed that soil pH and moisture had significant negative effects on diazotrophic abundance but positive effects on diversity. In conclusion, different varieties of C. equisetifolia significantly influence the abundance and diversity of diazotrophic communities by modulating rhizosphere soil conditions, thereby playing a key role in the construction of coastal protection forests and soil microbial succession. Among the varieties, Jixianglong was suitable for nitrogen-deficient coastal soils to enhance nitrogen supply, Ping’an could better adapt to environments with high variability or where rhizosphere stability was required, and Ruyifeng exhibited higher enzyme acti-vities in soils with relatively high organic matter content.

Key words: Casuarina equisetifolia variety, diazotrophic bacteria, community structure, coastal protection forest