The urine patch diffusional area: An important N2O source?

Urine patches contribute greatly to greenhouse gas emissions within livestock grazed ecosystems. The effective area of a ruminant urine patch comprises the wetted area, the diffusional area and the pasture response area. This study specifically assesses the importance of considering the diffusional area for monitoring urine patch N2O emissions. Spatial and temporal changes in N2O emissions and potential drivers of emissions (soil pH, EC, redox potential, dissolved organic carbon and nitrogen, NO3− and NH4+) were measured in sheep urine amended Eutric Cambisol mesocosms, maintained at 50% or 70% water-filled pore space (WFPS). At 70% WFPS, over 10 weeks, the emission factor (EF) was greater when considering the wetted area plus a 9 cm diffusional area (EF = 2.75 ± 0.72% of applied N) than when considering the wetted area alone (EF = 1.44 ± 0.30% of applied N); differences were not statistically significant at 50% WFPS. Redox potential, total extractable N and WFPS contributed significantly to the observed variation in daily N2O fluxes from the urine patch. We conclude that the urine patch diffusional area is an extremely important source of emissions from urine patches. This has implications when measuring EFs, as the lateral diffusion of solutes may be restricted by chamber walls resulting in an underestimate of N2O emissions, particularly at higher soil moisture contents. Site-specific assessments of the urine patch diffusional area should be made, and accounted for, prior to monitoring emissions and calculating emission factors from urine patches applied within chambers.