Nitric acid-modified co-pyrolyzed biochar from gentamicin mycelial residue and biomass regulates cattle manure composting: Reducing carbon loss and enhancing humification.

Aerobic composting is an effective technology for organic waste recycling; however, it is often hindered by substantial carbon loss, mainly in the form of CO2 and CH4, and slow humification. To address these challenges, this study evaluated the effects of nitric acid (HNO3)-modified co-pyrolyzed biochar derived from gentamicin mycelial residue, food waste, and wheat straw on the composting of cattle manure. The results showed that, compared with the control, unmodified co-pyrolyzed biochar group (GBC) increased cumulative CO2 emissions by 24.85%, but reduced cumulative CH4 emissions by 45.90%. In contrast, the HNO3-modified co-pyrolyzed biochar group (NGBC) demonstrated a comprehensive mitigation effect, reducing cumulative CO2 and CH4 emissions by 2.38% and 60.46%, respectively. Furthermore, NGBC achieved the highest increase in humic acid content (22.72%). The enhanced performance is attributed to the improved specific surface area and surface functionality of the modified biochar, which optimized the composting microenvironment by prolonging the thermophilic period and enriching key bacterial taxa (e.g., Actinobacteria and Firmicutes) crucial for lignocellulose degradation. Metabolic prediction further revealed that HNO3 modification redirected microbial carbon metabolism from mineralization towards humification, facilitating the conversion of organic carbon into stable humus. This study provides a technological approach for the efficient treatment of organic solid waste.