Objectives The effect and mechanism of biochar and nitrification inhibitor on reducing the emission of main active nitrogen gas in paddy was studied to evaluate the mitigation effects of single or combined application of biochar and nitrification inhibitor.

Methods A typical paddy soil was collected in Zhejiang Province for a potted experiment inside the airtight boxes. Five treatments were set up: no nitrogen fertilizer (N0), urea (Urea), urea + nitrification inhibitor (DMPP), urea + biochar (BC), urea + nitrification inhibitor + biochar (BCDM). Gas chromatography and Drage-Tube method were used to monitor soil N₂O and NH₃ emissions during crop growth. After basal, tillering and panicle fertilization, the surface and leakage water were sampled for measuring inorganic N concentrations. After rice harvest, fresh soil samples were collected to extract soil microbial DNA for determing the number of key functional microorganisms (AOB, AOA and nosZ), using qPCR technology.

Results After base fertilization, all the treatments did not increase NH₄⁺-N concentration in surface water significantly. After tillering and panicle fertilization, BC and DMPP treatments significantly increased NH₄⁺-N concentration in surface water up to 35.4 mg/L, while BCDM and DMPP treatments reduced NO₃⁻-N concentration in surface water by 80.0% and 56.9%. Compared with Urea treatment, BC and DMPP alone or combined application significantly reduced the cumulative N loss in surface water, with the highest reduction 95.6% in BCDM. The cumulative N leakage loss at 20 cm and 40 cm soil depth was 3.67 kg/hm² in Urea, which were reduced by 16.5%, 27.9% and 37.4% in BC, BCDM and DMPP treatments, respectively. BC, DMPP and BCDM treatment also significantly reduced the cumulative N₂O emission by 31.5%, 64.0% and 57.6%, respectively. DMPP significantly increased ammonia cumulative emission by 10.5%, while BC combined with DMPP or BC alone decreased ammonia cumulative emission by 25.2% and 21.6%. During the growth period of rice, the indirect N₂O emission caused by NH₃ emission accounted for 83.9%, 90.2%, 90.5%, 52.9% and 82.0% of the total N₂O emission in BC, BCDM, DMPP, N0 and Urea treatments, respectively. qPCR results showed that compared with Urea treatment, BC and DMPP alone or combined application significantly increased the amoA gene copy number of AOA by 48.0%−73.4%, and decreased the amoA gene copy number of AOB by 62.7%−195.6%; DMPP alone or combined with BC increased the copy number of nosZ gene by 3.7%−14.8%, thus promoting the N₂O reduction ability, and reducing N₂O emission.

Conclusions During rice growing period, the combined application of biochar and DMPP can significantly reduce the copy number of amoA-AOB gene in soil, decrease the nitrification process, and reduce NO₃⁻-N concentration, thus reduce the inorganic nitrogen loss in surface and leakage water. At the same time, the combined application of biochar and DMPP can increase the copy number of N₂O reductase nosZ gene, which is a key function of active nitrogen gas production process, thus decline the total emission of N₂O (N₂O direct emission and NH₃ indirect emission) by 32.2%, suggesting that the combination is an effective way on the reduction of N₂O emission and NH₃ volatilization.