Nitrous Oxide Fluxes, Soil Oxygen, and Denitrification Potential of Urine- and Non-Urine-Treated Soil under Different Irrigation Frequencies

Despite increased use of irrigation to improve forage quality and quantity for grazing cattle (Bos taurus, Linnaeus), there is a lack of data that assess how irrigation practices influence nitrous oxide (N2O) emissions from urine-affected soils. Irrigation effects on soil oxygen (O-2) availability, a primary controller of N2O fluxes, is poorly understood. It was hypothesized that increased irrigation frequency would result in lower N2O emissions by increasing soil moisture and decreasing soil O-2 concentrations. This would favor more N2O reduction to dinitrogen (N-2). We examined effects of high (3-d) versus low (6-d) irrigation frequency with and without bovine urine addition to pasture. Nitrous oxide fluxes were measured daily for 35 d. Soil O-2, temperature, and water content were continuously measured at multiple depths. Inorganic nitrogen, organic carbon, and soil pH were measured at 6-d intervals. Measurements of denitrification enzyme activity with and without acetylene inhibition were used to infer the N2O/(N2O + N-2) ratio. The N2O/(N2O + N-2) ratio was lower under high-compared with low-frequency irrigation, suggesting greater potential for N2O reduction to N-2 with more frequent irrigation. Although N2O fluxes were increased by urine addition, they were not affected by irrigation frequency. Soil O-2 decreased temporarily after urine deposition, but O-2 dynamics did not explain N2O dynamics. Relative soil gas diffusivity (DP/DO) was a better predictor of N2O fluxes than O-2 concentration. On a freedraining soil, increasing irrigation frequency while providing the same total water volume did not enhance N2O emissions under ruminant urine patches in a grazed pasture.