Reducing Surface Wetting Proportion of Soils Irrigated by Subsurface Drip Irrigation Can Mitigate Soil N₂O Emission

To reveal the impact of soil moisture distributions on nitrous oxide (N₂O) emissions from wet soils irrigated by sub-surface drip irrigation (SDI) with different surface soil wetting proportions, pot experiments were conducted, with surface irrigation (SI) as a control. Results indicated that irrigation triggered N₂O pulsing effect in all SDI treatments, yet N₂O values reduced with the decrease of surface soil wetting proportions of SDI irrigated soils, and the occurrence times were lagged. The peak N₂O fluxes and the corresponding soil water filled pore space (WFPS), as well as the coefficients of determination (R²) of the exponential function between N₂O fluxes and soil WFPS, decreased with the reduction of surface soil wetting proportions with SDI treatment, and from the central sub-region to the periphery sub-region. The pulse period contributed most to the reduction of N₂O emissions in SDI compared to SI treatments and should be a key period for N₂O emission mitigation. Over the whole experimental period, the area-weighted average cumulative N₂O fluxes from SDI treatments were 82.3⁻157.3 mg N₂O m^-2 lower than those from SI treatment, with periphery sub-regions of R3 and R4 (radius of 19⁻27 cm and 28⁻36 cm from the emitter horizontally) contributing to more than 75.8% of the total N₂O emission mitigation. These results suggest that reducing surface soil wetting proportions or the increments of topsoil WFPS for SDI irrigated soils is a promising strategy for N₂O emission reduction.