Mechanisms of mitigating nitrous oxide emission during composting by biochar and calcium carbonate addition

To investigate the mechanisms underlying effects of biochar and calcium carbonate (CaCO3) addition on nitrous oxide (N2O) emissions during composting, this paper conducted a systematic study on mineral nitrogen (N), dissolved organic carbon (C) and N, sources of N2O, and functional genes. Biochar and CaCO3 addition decreased N2O emissions by 26.5-47.8% (9.5-96.9 mg N kg-1 dw) and 13.9-37.4% (12.0-121.0 mg N kg-1 dw) compared to the control (14.3-179.7 mg N kg-1 dw), respectively. The mitigation of N2O emission was caused by decreased contribution of ammonia-oxidizing bacteria (AOB) and fungi to N2O production due to diminished AOB amoA, fungal nirK and P450 gene abundances, or by stimulated N2O reduction to N2 owing to increased abundances of nosZⅠ and nosZⅠⅠ genes under biochar and CaCO3 addition. The findings suggest that the addition of biochar or CaCO3 is effective in mitigating N2O emission during composting.