Early season N 2 O emissions under variable water management in rice systems: source-partitioning emissions using isotope ratios along a depth profile

Abstract. Soil moisture strongly affects the balance between nitrification, denitrificationand N 2 O reduction and therefore the nitrogen (N) efficiency and N losses in agricultural systems. In rice systems, there is a need to improve alternative water management practices, which are designed to save water and reduce methane emissionsbut may increase N 2 O and decrease nitrogen use efficiency. In a field experiment with three water management treatments, we measured N 2 O isotope ratios of emitted and pore air N 2 O ( δ 15 N , δ 18 O and site preference, SP) over the course of 6 weeks in the early rice growing season. Isotope ratio measurements were coupled with simultaneous measurements of pore water NO 3 - , NH 4 + , dissolved organic carbon (DOC), water-filled pore space (WFPS) and soil redox potential (Eh) at three soil depths. We then used the relationship between SP × δ 18 O - N 2 O and SP × δ 15 N - N 2 O in simple two end-member mixing models to evaluate the contribution of nitrification, denitrificationand fungal denitrificationto total N 2 O emissions and to estimate N 2 O reduction rates. N 2 O emissions were higher in a dry-seeded + alternate wetting and drying (DS-AWD) treatment relative to water-seeded + alternate wetting and drying (WS-AWD) and water-seeded + conventional flooding (WS-FLD) treatments. In the DS-AWD treatment the highest emissions were associated with a high contribution from denitrificationand a decrease in N 2 O reduction, while in the WS treatments, the highest emissions occurred when contributions from denitrification/nitrifier denitrificationand nitrification/fungal denitrificationwere more equal. Modeled denitrificationrates appeared to be tightly linked to nitrificationand NO 3 - availability in all treatments; thus, water management affected the rate of denitrificationand N 2 O reduction by controlling the substrate availability for each process ( NO 3 - and N 2 O ), likely through changes in mineralization and nitrificationrates. Our model estimates of mean N 2 O reduction rates match well those observed in 15 N fertilizer labeling studies in rice systems and show promise for the use of dual isotope ratio mixing models to estimate N 2 losses.