Response of organic carbon stability and denitrification potential after long-term biogas slurry instead of chemical fertilizer application in purple soil: a field investigation

Abstract Biogas slurry (BS) is a liquid fertilizer with a low C/N ratio, a high ammonia nitrogen concentration, and a high moisture content. However, repeated BS application may alter soil organic carbon (SOC) stability and nitrogen transformation processes, thereby influencing soil fertility and environmentally relevant N cycling. To evaluate the effects of long-term BS application on SOC stability and soil nitrification and denitrification potentials, topsoil samples (0–20 cm) were collected from fields that had received BS for 10–18 years, along with nearby chemical fertilizer (CF) control fields around livestock and poultry farms. SOC stability indicators, potential nitrification rate (PNR), and denitrification enzyme activity (DEA) were then determined. Compared with CF, the long-term application of BS significantly increased the SOC content by 3.87 g kg −1 and the proportion by 31.29%. The soil particulate matter (POC) content and the proportion of POC were increased by 6.62 g kg −1 and 12.45%, respectively, and the recalcitrant pool (RP) content of SOC and the proportion of RP were increased by 4.56 g kg −1 and 3.63%, respectively. The average PNR and DEA under long-term BS application were high with 4.41 and 0.62 mg kg −1 h −1 , respectively, suggesting that repeated BS application was associated with elevated soil nitrification and denitrification potentials. Soil mineral N, moisture, SOC content and stability jointly explain variation in nitrification/denitrification potential under long-term BS application (R 2 = 0.71). Importance analysis indicated that the levels of soil moisture, nitrate, organic C, and microbial biomass C were the important predictors of DEA. Overall, these findings highlight the need to optimize biogas slurry management to balance soil fertility improvement with potential environmental risks.