Combined application of biochar with urease and nitrification inhibitors have synergistic effects on mitigating CH₄ emissions in rice field: A three-year study

Biochar and inhibitors applications have been proposed for mitigating soil greenhouse gas emissions. However, how biochar, inhibitors and the combination of biochar and inhibitors affect CH₄ emissions remains unclear in paddy soils. The objective of this study was to explore the effects of biochar application alone, and in combination with urease (hydroquinone) and nitrification inhibitors (dicyandiamide) on CH₄ emissions and yield-scaled CH₄ emissions during three rice growing seasons in the Taihu Lake region (Suzhou and Jurong), China. In Suzhou, N fertilization rates of 120-280 kg N ha^-1 increased CH₄ emissions compared to no N fertilization (Control) (P < 0.05), and the highest emission was observed at 240 kg N ha^-1, possibly due to the increase in rice-derived organic carbon (C) substrates for methanogens. Biochar amendment combined with N fertilization reduced CH₄ emissions by 13.2-27.1% compared with optimal N (ON, Suzhou) and conventional N application (CN-J, Jurong) (P < 0.05). This was related to the reduction in soil dissolved organic C and the increase in soil redox potential. Addition of urease and nitrification inhibitor (ONI) decreased CH₄ emissions by 15.7% compared with ON treatment. Combined application of biochar plus urease, nitrification and double inhibitors further decreased CH₄ emissions by 22.2-51.0% compared with ON and CN-J treatment. ON resulted in the highest yield-scaled CH₄ emissions, while combined application of biochar alone and in combination with the inhibitors decreased yield-scaled CH₄ emissions by 12.7-54.9% compared with ON and CN-J treatment (P < 0.05). The lowest yield-scaled CH₄ emissions were observed under combined application of 7.5 t ha^-1 biochar with both urease and nitrification inhibitors. These findings suggest that combined application of biochar and inhibitors could mitigate total CH₄ and yield-scaled CH₄ emissions in paddy fields in this region.