Variation in Soil Bacterial and Fungal Diversity, Community Composition, and Co-Occurrence Patterns Across Nearshore Islands in the Northern South China Sea

Soil microbial biogeography on nearshore islands remains less well understood than that on remote archipelagos, particularly when bacterial and fungal diversity, community composition, and co-occurrence associations are evaluated together. We characterized bacterial 16S rRNA gene and fungal ITS communities in 36 topsoil samples collected from five nearshore islands in the northern South China Sea and examined their associations with plant diversity, soil physicochemical properties, and climatic and geographic conditions. Bacterial Shannon diversity, ACE richness, and Simpson diversity differed significantly among islands, as did fungal ACE richness, whereas fungal Shannon and Simpson diversity showed no significant differences. Bray–Curtis-based analyses revealed island-associated differentiation in bacterial community composition and fungal community composition, although multivariate dispersion also differed among islands. Sample-level subnetworks derived from an integrated co-occurrence network showed that the number of bacteria–bacteria edges differed significantly among islands, whereas the numbers of bacteria–fungi and fungi–fungi edges did not. Exploratory Spearman correlation analyses and variation partitioning indicated that microbial properties were more broadly associated with soil physicochemical properties and climatic–geographic conditions than with plant diversity. By evaluating both microbial kingdoms within the same field campaign, this study identifies partly distinct bacterial and fungal biogeographic and co-occurrence patterns and provides a baseline for future microbial biodiversity monitoring on nearshore islands.