Soil Gas Diffusivity Controls N2O and N2 Emissions and their Ratio

Abstract

Core Ideas Relative gas diffusivity controls both N2O and N2 emissions. Relative gas diffusivity integrates the effects of soil bulk density and matric potential. Nitrogen use efficiency is likely to be driven by soil physics. 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 (ρb; from 1.1 to 1.5 Mg m−3) and soil matric potential (ψ; −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 ρb and ψ. 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.