NITROUS OXIDE PRODUCTION RATES DERIVED FROM MEASUREMENTS OF N2O IN THE THICK UNSATURATED ZONE OF THE HIGH PLAINS AQUIFER, USA

Increased understanding of N₂O production due to development of semiarid land for irrigated agriculture is needed to better quantify the rate of buildup of this greenhouse gas in the atmosphere. Concentration profiles of N₂O, CFCs, and SF₆ through the thick unsaturated zone have been analyzed for 7 (3 rangeland and 4 irrigated) sites underlain by the High Plains aquifer. Atmospheric concentration histories of CFCs and SF₆ were used with lithologic and moisture content data to calibrate gaseous diffusion parameters for designated layers for each site. These were in turn used with N₂O concentration data to estimate time- and area-integrated rates of N₂O production, both in the soil zone and at depth. Results for the rangeland sites indicate that little or no soil-zone N₂O is being produced at the Texas site, about 10-20 mol N₂O/ha/a at the Nebraska site, and about 25-50 mol/ha/a at the Kansas site. Soil-zone N₂O production at the irrigated sites ranges from 15-30 mol/ha/a at the cotton site in Texas and the corn site in Kansas to about 250-500 mol/ha/a at corn sites in Nebraska and Colorado. N₂O concentration profiles at two irrigated sites indicate production from the vicinity of the water table, ranging from about 0.3 mol/ha/a at the Texas cotton site to about 0.6 mol/ha/a at the Kansas corn site. Data from the Nebraska corn site indicate limited N₂O production in a thin layer at depth in the unsaturated zone, but none at the water table. No production at depth is indicated at the Colorado corn site. Estimates of soil-zone N₂O production are subject to much uncertainty due to their sensitivity to the assumed thickness of the production zone. However, the estimates provide a measure of N₂O production that is integrated in space and time, proving useful to bracket the long-term mean of production that is known to be extremely variable both temporally and spatially. Estimates of production from the water table, on the other hand, are subject to much less uncertainty, as a small rate of production at depth produces a large concentration signal.