Dual role of biochar in reducing nitrogen leaching and reshaping 0–200 cm Soil nitrogen migration in AWD-managed alluvial soil paddies

Alternate wetting and drying irrigation (AWD) is widely adopted to save water in crop production, yet it may enhance nitrogen (N) leaching and deep migration in alluvial soil with high water percolation, posing risks to groundwater quality and N use efficiency. A 3-yr field experiment was conducted to elucidate the dual role of biochar in reducing N leaching and reshaping 0–200 cm soil N migration in AWD-managed alluvial soil paddies with three treatments in total, including continuous flooding irrigation (CF) without biochar, AWD without biochar, and AWD with 20 t ha −1 maize straw biochar. The shift from CF to AWD significantly reduced water percolation by 29 % but increased NO 3 - -N leaching and inorganic N deep migration (100–200 cm) in most cases. AWD combined with biochar lowered water percolation by 36 % over CF alone and reduced inorganic N leaching by 24 % over AWD alone. In addition, AWD combined with biochar increased topsoil (0–40 cm) inorganic N and total N accumulation by 151–280 % while reducing their deep migration (150–2350 %) compared to AWD alone. The reduction in inorganic N deep migration stemmed from the decreased soil solution NO₃⁻-N at 120–200 cm, and the enhanced surface soil NH 4 + -N and inorganic N at 40–100 cm. This study highlights the potential of biochar as a valuable soil amendment that effectively decouples the trade-off between water-saving irrigation and environmental degradation, offering a sustainable strategy for N management in high-percolation cropping systems. • Alternate wetting and drying irrigation (AWD) increased cumulative NO 3 - -N leaching. • AWD generally increased inorganic N deep migration (100–200 cm). • Biochar mitigated the risk of soil solution NO 3 - -N downward migration. • Biochar decreased soil inorganic N and total N migration along 100–200 cm profile.