Microbial Strategies Mediated by Diverse Agricultural Practices Stabilize Soil Organic Carbon Under Ultraviolet Radiation Stress

Ultraviolet A (UVA) poses underexplored risks to organic carbon stability, particularly relevant to the fate of mineral-associated organic carbon (MAOC) and particulate organic carbon (POC) under diverse agricultural practices. To determine how UVA stress alters soil carbon stability, we conducted a 32 day microcosm experiment exposing agricultural soil to 0, 384.6, or 798.2 μW/cm 2 UVA, with or without fulvic acid (FA, 0.4%, w / w ), mulching, or their combination. Our study demonstrates that UVA acts as a genotoxic stressor, forcing microorganisms to undergo a metabolic trade-off. Specifically, UVA selectively stimulates decomposer taxa enriched in carbohydrate-active enzymes, such as Bacteroidota and Acidobacteriota, thereby supporting cellular repair. This “repair-driven hypermetabolism” accelerates the decomposition of recalcitrant substrates, significantly reducing stable MAOC content from 8.25 to 7.96 g/kg, whereas POC did not change significantly under UVA alone. Agricultural practices, such as fulvic acid (FA) preferentially promoted MAOC-related microbial communities and functions, whereas mulching enhanced extracellular polymeric substance production and supported POC stabilization. The combined Mulch-FA treatment redirected microbial strategies toward biosynthesis, increased soil organic carbon by 18.77% relative to the control, and reduced the relative abundances of genes involved in cellulose, mannan, xylan, and lactose degradation by 70.58%, 90.75%, 87.27%, and 81.40%, respectively. This study lays a scientific foundation for achieving sustainable agricultural development under accelerating UVA stress.