Study of the Effects of Co-Application of Biochar and Biogas Slurry on Nitrogen Cycling Enzyme Activity and Nitrogen Use Efficiency

This study investigated the effects of combined biochar and biogas slurry application on Acrisols in Hangzhou, Zhejiang Province. Biochar was applied at 0–2% (C0–C3) with or without slurry substitution (B0, B1) to evaluate changes in soil properties, nitrogen-cycle-related enzymes, and functional genes. The joint amendment significantly increased soil pH, reduced bulk density, and enhanced total nitrogen, with the C3B1 treatment showing marked improvements in ammonium, nitrate, and total nitrogen compared with sole chemical fertilization (p < 0.05). Co-application also suppressed ammonia volatilization and N2O emissions, while stimulating nitrate reductase, nitrite reductase, urease activities, and gene abundances. Regression analysis identified nitrate reductase as the primary driver of nitrogen use efficiency, and correlation analysis indicated that ammonium and total nitrogen were strongly associated with enzyme activities. Overall, biochar–slurry integration improved soil fertility and nitrogen utilization while mitigating nitrogen losses.