2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 8
Presentation Time: 10:00 AM


WEEKS, Edwin P., Water Resources Discipline, U. S. Geological Survey, Box 25046, MS 413, Denver Federal Center, Lakewood, CO 80225 and MCMAHON, Peter B., Colorado Water Science Center, U. S. Geological Survey, Box 25046, Denver Federal Center, Lakewood, CO 80225, epweeks@usgs.gov

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.