Nitrous oxide emission dynamics and greenhouse gas intensity under mild alternate wetting and drying with nitrogen management in single-rice paddies

Irrigation regimes and nitrogen (N) management are key levers for reducing water use and nitrous oxide (N₂O) emissions from irrigated rice. Mild alternate wetting and drying (AWD) is widely promoted as a water-saving practice, but its effects on short-lived N₂O pulse events and yield-scaled N₂O-based greenhouse gas intensity (N₂O-based GHGI, calculated from N₂O emissions only) remain uncertain in single-rice systems. Based on a two-year field experiment in Jingzhou, Hubei Province, China, we evaluated four water–nitrogen combinations: CM (continuous flooding, CF, with conventional N), WO (mild AWD with conventional N), NO (CF with ~20% reduced N), and WNO (mild AWD with ~20% reduced N). N₂O fluxes were monitored at high frequency, together with soil moisture, mineral N, dissolved organic carbon (DOC), and denitrifier functional genes. At a given N rate, mild AWD increased seasonal cumulative N₂O emissions by about 40% relative to CF, whereas the reduced-N strategy generally lowered peak fluxes and seasonal totals but only partially offset the AWD-induced increase, yielding WNO ≈ WO > CM ≥ NO. Most N₂O peaks occurred 5–7 days after fertilization, coinciding with increases in soil moisture, NH₄⁺–N, DOC, and denitrifier gene abundances. Under mild AWD, a weaker nosZ response elevated the nirK/nosZ ratio, indicating N₂O production dominance during this high-risk pulse window. Non-rice season emissions were small. Relative to AWD with conventional N input, the directly tested WNO treatment (mild AWD + integrated reduced-N strategy) improved N₂O-based GHGI by approximately 10% but did not reduce seasonal N₂O emissions below the CM baseline. The 5–7 d post-fertilization pulse pattern further suggests that separating N application from major drying–rewetting events warrants future testing.