Nitrification and coupled nitrification-denitrification at shallow depths are responsible for early season N2O emissions under alternate wetting and drying management in an Italian rice paddy system

Abstract There is increasing pressure to reduce water use in irrigated rice production to save water, reduce methane emissions and reduce grain arsenic uptake arising from anaerobic conditions. However, under such practices emissions of nitrous oxide (N 2 O) often increase. Rice systems generally exhibit strong stratification of environmental conditions that drive the balance between N 2 O production and consumption, and ultimately the emissions of N 2 O. We investigated how the introduction of alternate wetting and drying (AWD) relative to conventional flood (FLD) irrigation modifies the depth distribution of environmental conditions and nutrients (NO 3 − , NH 4 + , dissolved organic carbon, soil redox (Eh) and water filled pore space, (WFPS)). We then examined how these variables related to N 2 O production and consumption via the measurement of δ 15 N-N 2 O emitted/poreair , δ 15 N-NO 3 − , N 2 O turnover and subsurface N 2 O fluxes at five depths (5, 12.5, 25, 50 and 80 cm). These measurements, together with N 2 O surface emissions were taken on six days surrounding a broadcast urea fertilizer application and for six days surrounding the onset of final drainage. The highest emissions were observed in the AWD treatment at the onset of measurements. These emissions were driven by high NH 4 + availability and could mainly be attributed to nitrification directly or indirectly via coupled nitrification-denitrification in the upper depths. In both irrigation treatments, an increase in NO 3 − and dissolved N 2 O concentrations and a drop in δ 15 N-NO 3 − values indicated rapid and ephemeral nitrification following the fertilization, but without significant effects on N 2 O surface emissions. At 50 and 80 cm, δ 15 N-N 2 O poreair was enriched relative to upper depths, pointing to N 2 O reduction at these depths in both treatments. We conclude that the increased N 2 O emissions under AWD compared to FLD management were associated with enhanced nitrification in the upper soil layers during plant establishment and thus related to basal N fertilization and mineralization of native soil N rather than in-season fertilization.