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ORIGINAL RESEARCH

Nitrous Oxide Fluxes from Cultivated Areas and Rangeland: U.S. High Plains

U.S. Geological Survey, Box 25046, MS 413, Lakewood, CO 80225
^* Corresponding author (epweeks{at}usgs.gov).

Received 8 November 2006.

Concentration profiles of N₂O, a greenhouse gas, and the conservative trace gases SF₆ and the chlorofluorocarbons CFC-11, CFC-12, CFC-113, and were measured periodically through thick vadose zones at nine sites in the U.S. High Plains. The CFC and SF₆ measurements were used to calibrate a one-dimensional gas diffusion model, using the parameter identification program UCODE. The calibrated model was used with N₂O measurements to estimate average annual N₂O flux from both the root zone and the deep vadose zone to the atmosphere. Estimates of root-zone N₂O fluxes from three rangeland sites ranged from near 0 to about 0.2 kg N₂O-N ha^–1 yr^–1, values near the low end of the ranges determined for native grass from other studies. Estimates of root-zone N₂O fluxes from two fields planted to corn (Zea mays L.) of about 2 to 6 kg N₂O-N ha^–1 yr^–1 are similar to those determined for corn in other studies. Estimates of N₂O flux from Conservation Reserve grassland converted from irrigated corn indicate that production of N₂O is substantially reduced following conversion from cropland. Small N₂O fluxes from the water table or from deep in the vadose zone occurred at three sites, ranging from 0.004 to 0.02 kg N₂O-N ha^–1 yr^–1. Our estimates of N₂O flux represent space- and time-averaged values that should be useful to more fully evaluate the significance of instantaneous point flux measurements.

Abbreviations: CAL, Central High Plains Conservation Reserve Program site • CFC, chlorofluorocarbon • CHP, Central High Plains • CNG, Central High Plains rangeland site • EEC, equivalent equilibrium concentration in water-vapor-saturated air • GNT, Northern High Plains irrigated corn site at Grant, NB • IMP, Northern High Plains rangeland site • JRW, Southern High Plains irrigated cotton site in Cochran County, TX • MPL, Southern High Plains irrigated cotton site at Maple, TX • MWR, Southern High Plains rangeland site • NHP, Northern High Plains • SHP, Southern High Plains • UMA, Northern High Plains irrigated corn site at Yuma, CO