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 a reas of intensive fertilization and irrigation which may impact region al CH4 and N2O fluxes. Nitrogen fertilization typically increases yiel ds, but N-use efficiency is generally low. Neither the amount of ferti lizer-N recovered by the forage nor the effect on N2O and CH4 emission s were known. These trace gases are long-lived in the atmosphere and c ontribute to global warming potential and stratospheric ozone depletio n. From 1991 through 1993 studies were conducted to determine the effe ct of N source, and timing of N-fertilization on forage yield, N-uptak e, and trace gas fluxes at the CSU Beef Improvement Center near Sarato ga, Wyoming. Plots were fertilized with 168 kg N ha(-1). Microplots la beled with N-15-fertilizer were established to trace the fate of the a dded N. Weekly fluxes of N2O and CH4 were measured during the snow-fre e periods of the year. Although CH4 was consumed when soils were dryin g, flood irrigation converted the meadow into a net source of CH4. Nit rogen fertilization did not affect CH4 flux but increased N2O emission s. About 5% of the applied N was lost as N2O from spring applied NH4NO 3, far greater than the amount lost as N2O from urea or fall applied N H4NO3. Fertilizer N additions increased forage biomass to a maximum of 14.6 Mg ha(-1) with spring applied NH4NO3. Plant uptake of N-fertiliz er was greater with spring applications (42%), than with fall applicat ions (22%).