Biochar-mediated regulation of carbon-nitrogen dynamics in aerobic composting of biogas residue

To clarify the migration and transformation of carbon (C) and nitrogen (N) during biochar-assisted biogas residue aerobic composting and improve compost quality, this study established five treatment groups with biochar addition ratios of 0 % (CK), 2.5 % (T1), 5.0 % (T2), 7.5 % (T3), and 10.0 % (T4). The findings demonstrated that the addition of biochar decreased the cumulative emission of CO2, CH4, NH3, and N2O by 12.14–30.60 %, 29.48–42.65 %, 21.49–44.83 %, and 17.41–34.15 %, respectively, in comparison to CK. The addition of biochar can decrease the amount of nitrogen lost during the composting process and increase the amount of nitrogen present in the form of ammonium salt, which will decrease the mineralization of organic nitrogen and facilitate the conversion of NH4+-N to NO3−-N. The addition of biochar can successfully lower the nitrogen loss by 23.61–41.08 % and the carbon loss by 15.06–30.57 %. Among all treatments, the 10 % biochar addition group exhibited the most significant effects, achieving the highest efficiency in C and N retention and the strongest mitigation of greenhouse gas emissions. These findings demonstrated that biochar addition is an effective strategy to optimize biogas residue composting by reducing nutrient loss and gaseous pollution.