Dairy farm effluent effects on urine patch nitrous oxide and carbon dioxide emissions

Dairy farm effluent (DFE) comprises animal feces, urine, and wash-down water collected at the milking shed. This is collected daily during the milking season and sprayed onto grazed dairy pastures. Urine patches in grazed pastures make a significant contribution to anthropogenic N₂O emissions. The DFE could potentially mitigate N₂O emissions by influencing the N₂O to dinitrogen (N₂) ratio, since it contains water-soluble carbon (WSC). Alternatively, DFE may enhance N₂O emissions from urine patches. The application of DFE may also provide a substrate for the production of CO₂ in pasture soils. The effects of DFE on the CO₂ and N₂O emissions from urine patches are unknown. Thus a laboratory experiment was performed where repeated DFE applications were made to repacked soil cores. Dairy farm effluent was applied at 0, 7, or 14 d after urine deposition. The urine was applied once on Day 0. Urine contained ¹⁵N-enriched urea. Measurements of N₂O, N₂, and carbon dioxide (CO₂) fluxes, soil pH, and soil inorganic N concentrations were made. After 43 d the DFE had not mitigated N₂O fluxes from urine patches. A small increase in the N₂O flux occurred from the urine-treated soils where DFE was applied 1 wk after urine deposition. The amount of WSC applied in the DFE proved to be insignificant compared with the amount of soil C released as CO₂ following urine application. The priming of soil C in urine patches has implications for the understanding of soil C processes in grazed pasture ecosystems and the budgeting of C within these ecosystems