Evaluating Soil Oxygen as a Control on N2O Emissions from Ruminant Urine Patches under Different Irrigation Frequencies

Abstract:

Urine patches from grazing ruminant animals are a significant source of nitrous oxide (N₂O) emissions, and irrigation is increasingly used to improve forage quality and yield for grazing cattle. The objective of this study was to test whether irrigation frequency influenced N₂O emissions from urine patches on a free-draining grazed pasture soil. It was hypothesized that greater irrigation frequency would increase soil moisture thereby lowering soil oxygen (O₂), and that these O₂-limited conditions would increase the potential for N₂O to be reduced to nitrogen gas (N₂), resulting in lower N₂O emissions. A field trial tested the effects of two irrigation frequencies and urine deposition on N₂O fluxes measured daily for 35 days. Denitrification potential measurements using the acetylene inhibition technique were completed to infer N₂O/(N₂O+N₂) ratios, and soil O₂ concentrations were measured continuously at three depths within the soil profile. While a more frequent irrigation treatment resulted in a lower N₂O/(N₂O+N₂) ratio, this did not give rise to lower N₂O emissions. Nitrous oxide fluxes were not influenced by irrigation frequency, and approximately 0.09% of the nitrogen applied as urine was emitted as N₂O from both irrigation treatments. Neither N₂O nor soil O₂ varied with individual irrigation events. Soil O₂ ranged from 17 to 20% expect following urine deposition, where it temporarily decreased to 13%. Soil O₂ measurements failed to explain N₂O emissions, but a relationship was derived between N₂O fluxes and estimates of soil gas diffusivity (Dp/Do). This work is the first to show how soil O₂ concentrations vary under a urine patch and under different irrigation treatments, and supports Dp/Do as robust predictor of N₂O emissions in situ.