Effects of biochar application methods on greenhouse gas emission and nitrogen use efficiency in paddy fields

Biochar application in rice production reduces nitrogen loss and greenhouse gases. We conducted in situ experiments for 3 years, with N₂₁₀B₀ (210 kg N ha^-1) as the control. Two biochar application methods (B₁:15 t ha^-1 biochar applied once and B₂: biochar applied three times at 5 t ha^-1 yr^-1) combined with two nitrogen levels (N₂₁₀: 210 kg N ha^-1 and N₁₆₈: 168 kg N ha^-1) were used. Soil physicochemical properties, CH₄ and N₂O emissions, functional gene abundance, rice yield, and nitrogen use efficiency were analyzed. Both methods improved the physicochemical properties of the soil, however, B₁ was less effective than B₂ in increasing soil pH, bulk density, organic carbon, total nitrogen, and microbial biomass nitrogen in year 3. B₁ had a higher CH₄ emission mitigation effect than B₂ in 3 consecutive years, mainly due to the higher pmoA gene abundance. B₁ showed a higher reduction effect of N₂O emissions compared to B₂ in year 1, but the opposite was observed in years 2 and 3. B₂ had a higher abundance of AOB, nirK, and nosZ genes compared to B₁ in year 3. Compared with N₂₁₀B₀, rice yields were increased by 9.1 %, 9.6 %, and 3.6 % with N₂₁₀B₁, N₂₁₀B_2, and N₁₆₈B₂, respectively, over 3 years, while N₁₆₈B₁ improved yields in the previous 2 years. Biochar improved nitrogen use efficiency over 3 consecutive years directly due to increased use efficiency of panicle fertilizer; the effect of B₁ was greater than that of B₂ during years 1 and 2, while the opposite was observed in year 3. Both Biochar applied once and three times appeared to be promising practices to increase yield and mitigate GHGs. From the GHGI perspective, the biochar applied once combined with 168 kg N ha^-1 can further improve nitrogen use efficiency, and reduce GHGs without hindering improvements in rice yield.