Acid-modified biochar increases grain yield and reduces reactive gaseous N losses and N-related global warming potential in alternate wetting and drying paddy production system

Biochar amended soils reduce fertilizer N losses and suppress greenhouse gas emissions. However, biochar can increase NH 3 volatilization. H 2 SO 4 -modified biochar has been studied as a means to achieve the advantages of biochar while reducing volatilization, especially under alternate wetting and drying irrigation ( I AWD ). In contrast to continuously flooded irrigation ( I CF ), I AWD is a water-saving technology that repeatedly dries and re-floods fields. A 3-year field experiment was conducted with two irrigation regimes ( I CF and I AWD ) as main plots and 0 (control), 20 t ha −1 biochar, and 20 t ha −1 H 2 SO 4 -modified biochar as subplots. I AWD produced 7.6–14.8% more reactive gaseous N losses (NH 3 and N 2 O) and emitted 2.02 times N-related global warming potential (GWP N ) of I CF . Biochar increased NH 3 volatilization by 35.6% in the first year and decreased it by 22.4% and 24.8% in the second and third years, respectively, while H 2 SO 4 -modified biochar decreased NH 3 volatilization each year. The increased NH 3 volatilization was caused by the higher NH 4 + concentration and pH in the floodwater and surface soil due to increasing N inputs and alkalinity from biochar. The decrease in the following two years was attributed to pH returning to the pre-treatment level and continued biochar absorption of NH 4 + from the floodwater. Both biochar and H 2 SO 4 -modified biochar significantly reduced seasonal N 2 O emissions. H 2 SO 4 -modified biochar coupled with I AWD mitigated the initial increases in NH 3 volatilization and reactive gaseous N losses in the first year, and increased grain yield, decreased reactive gaseous N losses, and GWP N compared with the I AWD without biochar throughout the three years. The use of acid-modified biochar could produce higher grain yield with lower reactive gaseous N losses and GWP N for application in the I AWD paddy systems, which benefits sustainable agricultural production. • AWD produces more reactive gas N losses and N-related global warming potential (GWP N ). • Biochar ( B 20 ) suppresses NH 3 loss after soil pH returns to the pre-treatment level. • Acid-modified biochar ( B 20-AM ) mitigates initial increase in NH 3 loss and reduces reactive gaseous N losses. • Both B 20 and B 20-AM play a significant role in mitigating N 2 O emissions. • B 20-AM coupled with AWD produces higher yield with lower reactive gas N losses and GWP N .