Wetting and drainage cycles in two New Zealand soil types: Effects on relative gas diffusivity and N2O emissions

Nitrous oxide (N₂O) is a potent greenhouse gas generated in agricultural soils by microbial processes that vary according to soil redox. Soil oxygen (O₂) supply and demand strongly influence soil redox. Migration of O₂ into the soil primarily occurs via gas diffusion, expressed as relative gas diffusivity (Dₚ/Dₒ), and is influenced by soil structure (air-filled porosity and tortuosity of pores) and soil water content. Soil N₂O emissions have been shown to increase at low values of Dₚ/Dₒ but detailed studies examining the relationship between Dₚ/Dₒ and soil N₂O emissions remain limited, with relatively few soil types examined, and no studies of repeated wetting-drainage cycles. Thus, the objectives of this study were to examine how successive wetting-drainage cycles affected both Dₚ/Dₒ dynamics and associated N₂O emissions in two New Zealand soils; a pallic silt loam and an allophanic loam, with the latter also having a higher organic matter content. Soil cores, repacked to varying density, were wetted up with ¹⁵N enriched NO₃⁻ solution and placed on tension tables where they underwent two consecutive 12-day wetting-drainage cycles from saturation to field capacity (0 to −10 kPa). Over time measurements were made of N₂O, N₂, inorganic-N and soluble carbon, while Dₚ/Dₒ was modelled using soil physical characteristics. For both soils each wetting-drainage cycle induced N₂O fluxes but with 5-fold lower fluxes in the allophanic soil. Greater aggregation and sand content in the allophanic soil generated higher porosity and Dₚ/Dₒ values that were almost always greater than recognized anaerobic limits. Thus, wetting-induced N₂O fluxes observed in the allophanic soil during early drainage were concluded to result from anaerobic or hypoxic pathways of N₂O production potentially within the intra-aggregate zone. While wetting-drainage events induce N₂O emissions by altering Dₚ/Dₒ and the soil aeration status, the draining of soils, especially soils high in organic matter, may enhance O₂ demand generating anaerobic zones conducive to denitrification. Further detailed studies examining the interaction between soil structure and soil organic matter content and their effect on N₂O emissions under wetting-drainage events, with measures of soil O₂, are needed.