Impact of Chamaecrista rotundifolia cover cropping on soil elemental cycling in karst agroecosystems.

Karst desertification causes severe soil erosion, hydrological imbalance, and biodiversity loss, thereby threatening ecosystem resilience and agricultural productivity. Soil carbon (C), nitrogen (N), phosphorus (P), and sulfur (S) cycling underpin ecosystem stability, yet how agricultural management practices regulate multi-element cycling in degraded karst agroecosystems remains poorly understood. Here, we conducted a four-year field trial in a karst mango orchard in southwestern China and integrated metagenomic sequencing with comprehensive soil environmental profiling to investigate how C. rotundifolia cover cropping reshapes soil elemental cycling. Compared with conventional tillage (CK), C. rotundifolia cover cropping (Y) significantly (p < 0.05) altered topsoil microbial functional profiles and nutrient availability. Carbon cycling potentials, including gene associated with aerobic respiration (cox1/3), fermentation, and CO₂ assimilation-were enriched under C. rotundifolia cover cropping. Nitrogen cycling potentials were enhanced through increased representation of genes involved in denitrification (nirK/S, nosZ), nitrogen acquisition, and nitrate reduction (nirA, narB), while phosphorus cycling was promoted through enrichment of the PhoR-PhoB phosphate regulatory system. Sulfur transformation potentials were also altered, with increased representation of genes involved in sulfate reduction, oxidation, and sulfonate utilization. Cover cropping substantially improved soil fertility, increasing SOC (+35.4%), NH4 +-N (+31.8%), TN (+35.1%), and AP (+105.9%), while reducing exchangeable Al3+ concentration by 60.9%. Microbial community restructuring was characterized by decreased Actinobacteriota and Chloroflexota and increased Bacteroidota and Proteobacteria, which exhibited central roles in elemental cycling networks. Mantel analyses identified exchangeable Al3+ as a dominant environmental constraint shaping microbial communities involved in C, N, P, and S cycling. Collectively, these findings demonstrate that C. rotundifolia cover cropping enhances karst soil resilience through a coupled microbial-geochemical pathway, in which improved soil chemical conditions and microbial functional reorganization jointly restore elemental cycling capacity.