Combined biochar and double inhibitor application offsets NH3 and N2O emissions and mitigates N leaching in paddy fields

The effects of combined biochar and double inhibitor application on gaseous nitrogen (N; nitrous oxide [N₂O] and ammonia [NH₃]) emissions and N leaching in paddy soils remain unclear. We investigated the effects of biochar application at different rates and double inhibitor application (hydroquinone [HQ] and dicyandiamide [DCD]) on NH₃ and N₂O emissions, N leaching, as well as rice yield in a paddy field, with eight treatments, including conventional urea N application at 280 kg N ha⁻¹ (CN); reduced N application at 240 kg N ha⁻¹ (RN); RN + 7.5 t ha⁻¹ biochar (RNB1); RN + 15 t ha⁻¹ biochar (RNB2); RN + HQ + DCD (RNI); RNB1 + HQ + DCD (RNIB1); RNB2 + HQ + DCD (RNIB2); and a control without N fertilizer. When compared with N leaching under RN, biochar application reduced total N leaching by 26.9–34.8% but stimulated NH₃ emissions by 13.2–27.1%, mainly because of enhanced floodwater and soil NH₄⁺-N concentrations and pH, and increased N₂O emission by 7.7–21.2%, potentially due to increased soil NO₃⁻-N concentrations. Urease and nitrification inhibitor addition decreased NH₃ and N₂O emissions, and total N leaching by 20.1%, 21.5%, and 22.1%, respectively. Compared with RN, combined biochar (7.5 t ha⁻¹) and double inhibitor application decreased NH₃ and N₂O emissions, with reductions of 24.3% and 14.6%, respectively, and reduced total N leaching by up to 45.4%. Biochar application alone or combined with double inhibitors enhanced N use efficiency from 26.2% (RN) to 44.7% (RNIB2). Conversely, double inhibitor application alone or combined with biochar enhanced rice yield and reduced yield-scaled N₂O emissions. Our results suggest that double inhibitor application alone or combined with 7.5 t ha⁻¹ biochar is an effective practice to mitigate NH₃ and N₂O emission and N leaching in paddy fields.