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 (O-2) supply and demand strongly influence soil redox. Migration of O-2 into the soil primarily occurs via gas diffusion, expressed as relative gas diffusivity (D-p/D-o), 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 D-p/D-o but detailed studies examining the relationship between D-p/D-o 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 D-p/D-o 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 N-15 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, N-2, inorganic-N and soluble carbon, while D-p/D-o 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 D-p/D-o 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 D-p/D-o and the soil aeration status, the draining of soils, especially soils high in organic matter, may enhance O-2 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 O-2, are needed.