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

Nitrous oxide acts both as a greenhouse gas and as an agent in the destruction of stratospheric ozone. Its atmospheric concentration is increasing, due partly to expanding agricultural use of N-based fertilizers. Many efforts have been made to quantify N₂O fluxes to the atmosphere from a wide variety of vegetative cover types and fertilizer treatments, but most measurement techniques are handicapped by the extreme temporal and spatial variability of the fluxes. A new approach, based on measurement of concentration profiles of N₂O, CFCs, and SF₆ in thick unsaturated zones, in conjunction with gas-phase diffusion modeling, is proving useful in providing estimates of annual N₂O fluxes from a variety of cultivated and rangeland sites. Application of the method is based on the use of atmospheric concentration histories of CFCs and SF₆, in conjunction with lithologic and moisture content data, to calibrate gaseous diffusion parameters for layers within the unsaturated zone for a given site. The resulting calibrated model is in turn used with N₂O concentration data to estimate time- and area-integrated rates of N₂O production. The method has been applied at nine sites in the High Plains, where the N₂O profiles indicated that production occurs in the soil zone, and, in some cases, at the water table and/or at isolated depths in the unsaturated zone. Soil-zone N₂O fluxes from three rangeland and two grass sites range from about 0 to about 0.5 kg N₂O-N/ha/a, values that are near the low end of those determined from other studies. Soil-zone N₂O fluxes from two cotton fields are also small (0.06 kg N₂O-N/ha/a), while those from two corn fields, of about 5 kg N₂O-N/ha/a, are similar to rates determined in other studies. These estimates of root-zone N₂O flux should be useful to more fully evaluate the significance of commonly made instantaneous point measurements. Nitrous oxide flux from deep in the unsaturated zone occurs at four sites, but the fluxes are small, ranging from 0.004 to 0.01 kg N₂O-N/ha/a. These fluxes are accompanied by high N₂O concentrations at depth, resulting in robust determination of their magnitude. In summary, our methodology provides a means of estimating fluxes that should be useful for evaluating N₂O contributions to the atmosphere in many areas underlain by a thick unsaturated zone.