Effects of N management on N2O and CH4 fluxes and N-15 - Recovery in an irrigated mountain meadow

Forage production in irrigated mountain meadows plays a vital role in the livestock industry in Colorado and Wyoming. Mountain meadows are areas of intensive fertilization and irrigation which may impact regional CH4 and N2O fluxes. Nitrogen fertilization typically increases yields, but N-use efficiency is generally low. Neither the amount of fertilizer-N recovered by the forage nor the effect on N2O and CH4 emissions were known. These trace gases are long-lived in the atmosphere and contribute to global warming potential and stratospheric ozone depletion. From 1991 through 1993 studies were conducted to determine the effect of N source, and timing of N-fertilization on forage yield, N-uptake, and trace gas fluxes at the CSU Beef Improvement Center near Saratoga, Wyoming. Plots were fertilized with 168 kg N ha(-1). Microplots labeled with N-15-fertilizer were established to trace the fate of the added N. Weekly fluxes of N2O and CH4 were measured during the snow-free periods of the year. Although CH4 was consumed when soils were drying, flood irrigation converted the meadow into a net source of CH4. Nitrogen fertilization did not affect CH4 flux but increased N2O emissions. About 5% of the applied N was lost as N2O from spring applied NH4NO3, far greater than the amount lost as N2O from urea or fall applied NH4NO3. Fertilizer N additions increased forage biomass to a maximum of 14.6 Mg ha(-1) with spring applied NH4NO3. Plant uptake of N-fertilizer was greater with spring applications (42%), than with fall applications (22%).