Nitric oxide fluxes from an agricultural soil using a flux-gradient method

Soil emission of nitric oxide may be a significant source of NOx in rural a reas. Agricultural practices may enhance these emissions by addition of nit rogen fertilizers,A system that enables continuous measurement of NO fluxes from agricultural surfaces using the flux-gradient method was developed. H ourly differences in NO concentrations in air sampled at two intake heights (0.6 and 1 m) were determined using a chemiluminescence analyzer. Eddy dif fusivities were determined using wind profiles (cup anemometers), and stabi lity corrections calculated using a 5 cm path sonic anemometer. Fast switch ing of sampling between air intake heights (every 30 s) and determination o f concentration values at a frequency of 2 Hz minimized the errors due to f luctuations in background concentration. Low travel times for air samples i n the tubing (similar to 8 s) were estimated to result in small errors in f lux values (<0.5 ng N m(-2) s(-1)) due to chemical reactions. The overall r esolution of the system was estimated as similar to 1 ng N m(-2) s(-1). NO fluxes from a bare soil were measured quasi-continuously from January to Ju ne 1995 at Elora, Canada, comprising a total of 1833 hourly values. Daily N O fluxes before nitrogen fertilization were small, increasing after nitroge n fertilizer was added (>10 ng N m(-2) s(-1)) Monthly NO fluxes estimated w ere similar to those observed in previous studies. The designed system coul d be easily modified to measure NOx and NO fluxes by using an additional ch emiluminescence analyzer. The system also could be adapted to measure fluxe s sequentially from various plots, enabling testing of agricultural practic es on NO emissions.