Preliminary study of N₂O flux over irrigated bermudagrass in a desert environment

The increase of atmospheric nitrous oxide (N₂O) is believed to be related to human activities, including increased agricultural use of nitrogen (N) fertilizers and irrigation. The purpose of this study was to evaluate N₂O flux (F_N) into the atmosphere using chamber and gradient profile methods over an irrigated, N fertilized, bermudagrass (Cynodon dactylon cv. 'Midiron') field in the Sonoran Desert of Arizona. For the gradient profile method it was hypothesized that locally stable atmospheric conditions would enhance N₂O concentration differences sufficiently (more than approximately 4 nl 1^-1) to be resolved by gas chromatographic analysis of air samples collected at two heights (0.05 and 3.3 m) over the field. Significant differences (205 and 30 nl 1^-1) in mean concentration occurred during two sampling intervals in late afternoon and early morning of a 24 h period on 18-19 July 1991. During those intervals the surface layer was stable and relatively large F_N estimated by the chamber method (371 and 48 ng m^-2 s^-1) were comparable with F_N estimated by the gradient profile method (283 and 101 ng m^-2 s^-1). Simulations based on similarity theory indicate that resolution of N₂O concentration differences less than 1 nl 1^-1 was required, but could not be achieved, when the surface layer was unstable and/or F_N was small. Analysis also indicates that uncertainty of F_N estimated by the chamber method was reduced slightly by estimation of temporal variation of vapor pressure in chamber air.