Dry-season precipitation reshapes functional gene composition and modulates ecosystem functioning in the tropical...

Alternating drought and precipitation events are increasing in frequency, profoundly reshaping the structure and function of freshwater ecosystems. Microbial communities, as key drivers of C, N, and P cycling, exhibit complex and context-dependent responses to such pulsed hydrological disturbances, yet their regulatory mechanisms remain poorly understood. This study systematically compared microbial functional dynamics before and after precipitation during the dry season in the tropical eutrophic Mirongo River. Results showed that precipitation markedly reduced the Shannon diversity and spatial similarity of C and N cycling functional genes (p < 0.05), and reshaped the overall functional gene composition. Within the dry season, precipitation shifted carbon metabolism toward heterotrophic pathways and reorganized phosphorus metabolism into a more energy-efficient organic-phosphorus hydrolysis route. N cycling exhibited stage-specific responses, with enhanced mid-stage denitrification but constraints on terminal reduction steps. Co-occurrence network analysis revealed a decline in positive associations from 61.26 % before precipitation to 56.46 % after precipitation, accompanied by a shift in keystone nodes from narH to sucC, accC, accB, narG, and fadB. PLS-PM results indicated that precipitation strengthened the direct regulatory effects of environmental factors on microbial functional genes, and that precipitation-induced nutrient changes reversed the influence of microbial diversity on P cycling from positive to negative. The relationships between functional genes and microbial diversity further suggested that C-N-P cycling functions relied predominantly on a few dominant metabolic taxa after precipitation. These findings deepen our understanding of the stability of microbially mediated ecosystem functions under hydrological disturbances and provide important theoretical and practical insights for the management and resilience assessment of freshwater ecosystems in the context of climate change.