Disentangling Drivers of Soil Bacterial and Fungal Diversity on Tropical Islands.

Island area and variations in climate (e.g., temperature and precipitation) are widely known to affect the insular-dwelling soil bacterial and fungal communities. Such effects can be context-dependent, and many factors can determine the diversity of soil microbes. For example, island area and climate factors can directly influence soil bacterial and fungal communities, as well as exert an indirect effect by altering plant communities and/or soil properties. However, we lack a comprehensive mechanistic understanding of the relative importance of these potential drivers. To explore the key factors affecting insular soil microbial community dynamics, we selected 20 representative tropical islands in the South China Sea and established two to eight permanently marked plots based on the island area. Then, we investigated the plant community composition and measured growth strategy-related plant traits (i.e., specific leaf area and leaf dry matter content). Concurrently, we analyzed a series of soil properties (i.e., pH, salinity, organic carbon, total nitrogen, total phosphorus, total potassium, and carbon/nitrogen ratio) and the diversity of soil bacterial and fungal communities. For the soil bacterial community, based on structural equation modeling (SEM) analysis, we found an indirect effect of climate factors and island area on bacterial richness through their influence on plant richness, in addition to the direct effect of island area on bacterial richness. In contrast, for the soil fungal community, we found that the influences of temperature and precipitation are primarily indirect via changes in soil pH and the community-weighted mean (CWM) value of plant leaf dry matter content. Overall, this study highlights that island-dwelling soil bacterial and fungal communities are shaped by island-specific plant community and soil pH, which may interplay with macroscopic threats like ongoing sea level rise and biological invasions, thus providing crucial insights into the dynamics of soil microorganisms under global change scenarios.