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

[Display omitted] • Isotope mapping, qPCR, and inhibition methods to explore N 2 O emission mechanisms. • Biochar and CaCO 3 reduced contribution of AOB and fungi to N 2 O production. • Biochar and CaCO 3 stimulated N 2 O reduction to N 2. • Biochar and CaCO 3 decreased AOB amoA , fungal nirK and P450 gene abundances. • Biochar and CaCO 3 increased nosZ Ⅰ and nosZ ⅠⅠ gene abundances. To investigate the mechanisms underlying effects of biochar and calcium carbonate (CaCO 3) addition on nitrous oxide (N 2 O) emissions during composting, this paper conducted a systematic study on mineral nitrogen (N), dissolved organic carbon (C) and N, sources of N 2 O, and functional genes. Biochar and CaCO 3 addition decreased N 2 O 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 N 2 O emission was caused by decreased contribution of ammonia-oxidizing bacteria (AOB) and fungi to N 2 O production due to diminished AOB amoA , fungal nirK and P450 gene abundances, or by stimulated N 2 O reduction to N 2 owing to increased abundances of nosZ Ⅰ and nosZ ⅠⅠ genes under biochar and CaCO 3 addition. The findings suggest that the addition of biochar or CaCO 3 is effective in mitigating N 2 O emission during composting. [ABSTRACT FROM AUTHOR]