Effect of water-saving irrigation on the N2O dynamics and the contribution of exogenous and endogenous nitrogen to N2O production in paddy soil using 15N tracing

Water-saving irrigation techniques can effectively mitigate methane emissions from rice fields but typically enhance nitrous oxide (N2O) emissions due to changes in soil moisture content. However, few studies have examined the effect of different water regimes on N2O production using different species of exogenous nitrogen (N) fertilizer application. Furthermore, the contributions of exogenous N fertilizer and endogenous N of soil for N2O productions have not been elucidated. Using a series of lysimeters, three types of water management were employed in rice cultivation in this study: conventional irrigation management (CIM), alternate wetting and drying (AWD), and dry cultivation management (DCM). Ammonium nitrate (NH4NO3) fertilizer was applied as base, tillering, and heading fertilizer, which was then separated into two groups labeled with N-15 stable isotope (> 99 %) at two different N positions: (NH4NO3)-N-15 and (NH4NO3)-N-15. Higher peaks of N2O flux were observed following drainage of floodwater for AWD and DCM treatments. In addition, (N2O)-N-15 fluxes were clearly separated into two different patterns between (NO3-)-N-15 and (NH4+)-N-15 fertilizer application. Application of (NO3-)-N-15 fertilizer rapidly released (N2O)-N-15 but the (N2O)-N-15 flux for (NH4+)-N-15 fertilizer application slowly increased with a hysteresis effect, suggesting denitrification was the main pathway for (N2O)-N-15 production in paddy soil with (NO3-)-N-15 fertilizer added in a flooded situation. Conversely, (NH4+)-N-15 may need to be converted into (NO3-)-N-15 through nitrification, then denitrified and finally released as (N2O)-N-15. Furthermore, N2O production was likely enhanced by nitrification-denitrification processes due to the water level alternating near (th)e ground surface for AWD treatment. Consequently, the total N2O emission over the rice growing period was the highest from the AWD soils, at 2.4 times that for CIM treatments. The high proportion of N2O-N derived from soil endogenous N sources (81 % of total N2O emission) in the AWD treatment indicates that the alternating wetting-drying of soil stimulated N2O production using endogenous N mineralized from soil organic matter. Furthermore, water management of paddy soil affected N uptake by rice plants. The total N uptake by rice plants was significantly higher for CIM than for AWD and DCM treatments.