Nitrous oxide fluxes and soil oxygen dynamics of soil treated with cow urine

Ruminant urine deposition onto pasture creates hot-spots where emissions of nitrous oxide (N₂O) are produced by aerobic and anaerobic microbial pathways. However, limited measurements of in situ soil oxygen (O₂)-N₂O relationships hinder the prediction of N₂O emissions from urine-affected soil. This study tested whether soil O₂ concentration or relative diffusivity of O₂ (Dp/DO) could explain N₂O emissions from urine patches. Using a randomized plot design, N₂O emissions were measured daily from a perennial ryegrass (Lolium perenne L.) pasture for 56 d following bovine (Bos taurus) urine deposition to an imperfectly drained silty loam soil. Soil O₂, volumetric water content, pH, conductivity, and extractable N and C were measured in urine-amended and non-amended soil. Values of water-filled pore space (WFPS) and Dp/DO were modeled. When data from treatments were pooled together, daily mean Dp/DO explained 73% of the total variance in mean daily N₂O flux, compared with 65, < 60, and < 20% for WFPS, O₂ and other measured variables, respectively. Soil pH, O₂, volumetric water content, WFPS and Dp/DO all explained more of the variance in the urineamended compared with the non-amended soil. Daily N₂O fluxes increased substantially at Dp/DO values around 0.006, which was consistent with past laboratory studies. These results demonstrate for the first time an O₂ diffusion threshold for elevated N₂O fluxes in the field, expressed as Dp/DO ≈ 0.006. Further studies should examine the consistency of this threshold under varying N and C substrates and a range of soil pH