Soil gas diffusivity controls N₂O and N₂ emissions and their ratio

Knowledge of soil biological and physical interactions with respect to N₂O and N₂ fluxes is essential to ensure that agricultural land management is environmentally and economically sustainable. This study determined how varying soil relative gas diffusivity (Dp/Do) affected cumulative N₂O and N₂ fluxes under simulated ruminant urinary-N deposition. Using repacked soil cores, the effects of varying soil bulk density (rb; from 1.1 to 1.5 Mg m−3) and soil matric potential (y; −10 to −0.2 kPa) on Dp/Do were examined in a Templeton silt loam soil (Udic Haplustept) following the application of simulated ruminant urine (700 kg N ha−1). Fluxes of N₂O and N₂, soil inorganic N, pH, and dissolved organic C (DOC) dynamics were monitored over 35 d. Soil Dp/Do declined as soil bulk density and soil moisture increased. Soil N₂O emissions increased exponentially as Dp/Do decreased until Dp/Do equaled 0.005, where upon N₂O fluxes decreased rapidly due to complete denitrification, such that N₂ fluxes reached a maximum of 60% of N applied at a Dp/Do of <0.005. Regression analysis showed that Dp/Do was better able to explain the variation in N₂O and N₂ fluxes than water-filled pore space (WFPS) because it accounted for the interaction of soil rb and y. This study demonstrates that soil Dp/Do can explain cumulative N₂O and N₂ emissions from agricultural soils. Under grazed pasture systems, potential exists to reduce the emissions of the greenhouse gas N₂O and significant economic losses of N as N₂ if soil management and irrigation can be maintained to maximize Dp/Do