Microbial Heterogeneity Regulates C, N, and P Cycling Responses to Precipitation in Casuarina equisetifolia Forests

While many environmental variables (e.g., temperature) exhibit only moderate variation, precipitation is a primary driver of soil carbon–nitrogen–phosphorus cycling in forest ecosystems, with soil microorganisms mediating these processes. Within the unique micro-ecosystems of Casuarina equisetifolia coastal shelterbelts, it remains unclear how microbial communities regulate carbon–nitrogen–phosphorus cycling under precipitation gradients. This study aimed to investigate the responses of microbial communities and key functional genes related to carbon–nitrogen–phosphorus cycling in C. equisetifolia forests on Hainan Island under varying precipitation regimes. Microbial diversity across soil layers exhibited a unimodal relationship with mean annual precipitation, with upper-layer communities more compositionally diverse than lower-layer communities. Actinobacteria, Proteobacteria, and Acidobacteria dominated the soil bacterial community and were the primary contributors to key functional genes related to soil carbon–nitrogen–phosphorus cycling, indicating that these phyla are the main functional taxa driving these elemental cycles. Microbial communities responded to precipitation changes through assemblages of precipitation-sensitive functional genes that modulated specific metabolic pathways underlying carbon, nitrogen, and phosphorus cycling. Furthermore, available phosphorus primarily drove microbial metabolic responses to precipitation changes. In conclusion, precipitation is the predominant driver of soil carbon, nitrogen, and phosphorus cycling in C. equisetifolia forests by directly regulating soil water content and nutrient availability. Precipitation also indirectly drives the spatial differentiation of microbial communities and selective enrichment of precipitation-sensitive functional genes across soil layers. The spatial heterogeneity of soil microorganisms, their key functional genes along the precipitation gradient and between soil layers, including Actinobacteria, Proteobacteria, and Acidobacteria, and their precipitation-sensitive key functional genes are critical regulatory machinery that orchestrate carbon, nitrogen, and phosphorus cycling across precipitation regimes and soil depths.