Metagenomic Insights into Corn Stalk Biochar-Mediated Carbon Cycling and Microbial Community Shifts in Black Soil Soybean Rhizosphere

Despite extensive research on biochar’s effects on soil properties, the molecular mechanisms linking carbon-cycling functional genes to soil organic carbon dynamics under biochar application remain poorly understood. This study evaluated the effects of corn stalk biochar applied at different rates (0, 300, 600, and 900 kg/hm2) on soil physicochemical properties, microbial community structure, carbon cycling genes, and soybean (HeiNong551) growth in Northeast China’s black soil region. Metagenomic analysis and metabolic pathway analysis were also conducted. Biochar application significantly improved soil fertility by increasing pH, moisture content, and various SOC fractions, while modulating the activities of key soil enzymes, including β-glucosidase, alkaline phosphatase, cellulase, and polyphenol oxidase. Metagenomic analysis revealed that biochar substantially enhanced soil microbial richness and diversity, with significant enrichment of beneficial bacterial genera, including Nocardioides and Sphingomicrobium. Functional gene analysis demonstrated that biochar promoted soil carbon cycling by increasing the abundance of Glycosyltransferases, Glycoside Hydrolases, and Auxiliary Activities, while decreasing the abundance of Carbohydrate-Binding Modules, Carbohydrate Esterases, and Polysaccharide Lyases. Analysis of 22 key carbon cycling pathway genes indicated that biochar enhanced carbon fixation via the reductive tricarboxylic acid cycle (korA gene) and promoted the degradation of easily mineralizable SOC components, while suppressing stable carbon degradation pathways.