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

Abstract

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.