Enhancing soil fertility and organic carbon stability with high-nitrogen biogas slurry: Benefits and environmental...

Large-scale livestock farming has increased the amount of excrement, leading to ecological and environmental issues. To address this, sustainable solutions like biogas slurry (BS) are needed to enhance soil fertility and soil organic carbon (SOC). This study investigated the effects of conventional nitrogen fertilizer (NPK), standard BS (BS1), and high-nitrogen input BS treatments (BS5, BS7, and BS11) on soil quality, nutrient dynamics, SOC fractions, and environmental risks across soil depths (0-20, 20-40, and 40-60 cm). Field experiments revealed that BS application significantly enhanced the soil quality index (SQI) by 32.5 %-61.6 % in the 0-20 cm soil layer and ecosystem multifunctionality (EMF) by 42.3 %-169.0 % across all layers compared to NPK. Mineral-associated organic carbon (MOC) dominated SOC stabilization (57.2 %-91.3 % of total SOC), with BS11 increasing MOC content by 76.3 % in the 40-60 cm soil layer. High-nitrogen BS treatments increased microbial biomass carbon (MBC) by 318.1 % (BS11) and β-glucosidase activity by 144 % (BS7) in 0-20 cm soil, indicating enhanced soil enzyme activities. However, excessive nitrogen inputs induced nitrate accumulation in the 40-60 cm soil layer, where NO₃^--N concentrations surged to 97.2 %-200.6 % above NPK levels. Vertical nutrient migration also triggered subsoil acidification, with the pH decreasing by 5.0 % under the BS11 treatment. These results underscore the dual role of BS: while it enhances SOC stability (via MOC dominance) and multifunctionality, nitrogen inputs must be capped below 300 kg N ha^-1 yr^-1 to mitigate leaching risks. The findings provide actionable insights for optimizing BS applications in sustainable agriculture, balancing soil health improvements with environmental safeguards.