Nitrous oxide fluxes, soil oxygen, and denitrification potential of urine- and non-urine-treated soil under different irrigation frequencies

© American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. 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 (O2) 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 O2 concentrations. This would favor more N2O reduction to dinitrogen (N2). 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 O2, 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 + N2) ratio. The N2O/(N2O + N2) ratio was lower under high- compared with low-frequency irrigation, suggesting greater potential for N2O reduction to N2 with more frequent irrigation. Although N2O fluxes were increased by urine addition, they were not affected by irrigation frequency. Soil O2 decreased temporarily after urine deposition, but O2 dynamics did not explain N2O dynamics. Relative soil gas diffusivity (DP/DO) was a better predictor of N2O fluxes than O2 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