Rates and pathways of nitrous oxide production in a shortgrass steppe

Abstract

Most of the small external inputs of N to the Shortgrass steppe appear to be conserved. One pathway of loss is the emission of nitrous oxide, which we estimate to account for 2.5–9.0% of annual wet deposition inputs of N. These estimates were determined from an N₂O emission model based on field data which describe the temporal variability of N₂O produced from nitrification and denitrification from two slope positions. Soil water and temperature models were used to translate records of air temperature and precipitation between 1950 and 1984 into variables appropriate to drive the gas flux model, and annual N₂O fluxes were estimated for that period. The mean annual fluxes were 80 g N ha⁻¹ for a midslope location and 160 g N ha⁻¹ for a swale. Fluxes were higher in wet years than in dry, ranging from 73 to 100 g N ha⁻¹y⁻¹at the midslope, but the variability was not high. N₂O fluxes were also estimated from cattle urine patches and these fluxes while high within a urine patch, did not contribute significantly to a regional budget. Laboratory experiments using C₂H₂ to inhibit nitrifiers suggested that 60–80% of N₂O was produced as a result of nitrification, with denitrification being less important, in contrast to our earlier findings to the contrary. Intrasite and intraseasonal variations in N₂O flux were coupled to variations in mineral N dynamics, with high rates of N₂O flux occurring with high rates of inorganic N turnover. We computed a mean flux of 104 g N ha⁻¹ y⁻¹ from the shortgrass landscape, and a flux of 2.6 × 109 g N y⁻ from all shortgrass steppe (25 × 106 ha).