Soil Gas Diffusivity Controls N ₂ O and N ₂ Emissions and their Ratio

Abstract

Core Ideas Relative gas diffusivity controls both N 2 O and N 2 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 N 2 O and N 2 fluxes is essential to ensure that agricultural land management is environmentally and economically sustainable. This study determined how varying soil relative gas diffusivity ( D p / D o ) affected cumulative N 2 O and N 2 fluxes under simulated ruminant urinary‐N deposition. Using repacked soil cores, the effects of varying soil bulk density (ρ b ; from 1.1 to 1.5 Mg m −3 ) and soil matric potential (ψ; −10 to −0.2 kPa) on D p / D o were examined in a Templeton silt loam soil (Udic Haplustept) following the application of simulated ruminant urine (700 kg N ha −1 ). Fluxes of N 2 O and N 2 , soil inorganic N, pH, and dissolved organic C (DOC) dynamics were monitored over 35 d. Soil D p / D o declined as soil bulk density and soil moisture increased. Soil N 2 O emissions increased exponentially as D p / D o decreased until D p / D o equaled 0.005, where upon N 2 O fluxes decreased rapidly due to complete denitrification, such that N 2 fluxes reached a maximum of 60% of N applied at a D p / D o of <0.005. Regression analysis showed that D p / D o was better able to explain the variation in N 2 O and N 2 fluxes than water‐filled pore space (WFPS) because it accounted for the interaction of soil ρ b and ψ. This study demonstrates that soil D p / D o can explain cumulative N 2 O and N 2 emissions from agricultural soils. Under grazed pasture systems, potential exists to reduce the emissions of the greenhouse gas N 2 O and significant economic losses of N as N 2 if soil management and irrigation can be maintained to maximize D p / D o .