Objectives Amendment of biochar significantly affected soil nitrous oxide (N₂O) emission. It is still unclear about the underlying microbial mechanism for N₂O emission, particularly in the acidic vegetable soil. By integrating the field experiments with indoor incubations and using real-time quantitative PCR technology, the microbial mechanism for effects of biochar on N₂O emissions from an intensive vegetable field was investigated.

Methods Soil samples were collected from a three-years’ field treatment with 2² factorial design at N rates of 0 and 1.25 × 10³ kg/hm² and biochar rates of 0 and 40 t/hm², namely control (CK), N fertilizer (N), biochar (Bc) and N fertilizer with biochar (N + Bc). The functional genes of ammonia oxidizing archaea (amoA-AOA), ammonia oxidizing bacteria (amoA-AOB), nitrite reductase (nirK, nirS) and nitrous oxide reductase (nosZ), soil pH, inorganic N concentration, in conjunction with soil N₂O emissions were measured periodically.

Results Compared with CK, biochar application significantly increased the concentration of SOC by 27.1% and TN by 8.2%, significantly decreased the copy numbers of amoA-AOB and increased the nosZ genes by 11.0% and 21.2%, and had no significant effects on N₂O emissions. Applying nitrogen fertilizer significantly reduced soil pH, increased N₂O cumulative emissions, increased the content of inorganic nitrogen and nirK, nirS, nosZ gene copy numbers (P < 05), compared with CK treatment. Compared with N treatment, biochar and nitrogen fertilizer combined application (N + Bc) significantly increased the abundance of amoA-AOA, amoA-AOB, nirK, nirS and nosZ gene copy numbers by 68.1%, 39.3%, 21.1%, 19.8% and 48.4%, respectively (P < 0.05), decreased the (nirK + nirS)/nosZ ratio, and reduced N₂O emissions by 33.3%. The N₂O emission peak appeard at 1–5 d during the indoor incubation, and the emission rate reached N 1.70 × 10³ and 1.76 × 10³ ng/(kg·h) for the N and N + Bc treatment, respectively. Correlation analysis indicated that N₂O emission showed significant positive correlation with the nosZ gene copy numbers (P < 0.05) and the content of NH₄⁺-N (P < 0.01), while significant negative correlation with pH (P < 0.01) during the incubation.

Conclusions The amendment of biochar could decrease N₂O emissions from the acidic vegetable soil, which is mainly due to the accelerated N₂O reduction via decreasing the ratio of (nirK + nirS)/nosZ.