3 METHODS TO ESTIMATE N2O FLUXES AS IMPACTED BY AGRICULTURAL MANAGEMENT

This study examines three methods of evaluating N2O flux and accumulat ion in soil profile over a growing season under three soil management regimes (fallow, fallow with manure addition and cropped to alfalfa). Estimates of N2O flux were made based on measured soil atmosphere conc entration gradients in the top 15 cm and compared to flux estimates ba sed on ex situ cores and micro-meteorological measurements made in par allel studies. All methods indicated strong seasonal trends relating t o precipitation events. The amounts of N2O accumulating the profile de creased in the order alfalfa < fallow < fallow/manure. The amounts of N2O accumulating in the profile ranged from ambient (0.35 mu L L-1) to 490 mu L L-1. Diffusion of N2O to the lower profile was shown to prov ide temporary storage of N2O and 2 thereby provide the opportunity for further reduction to N-2 prior to efflux from the surface. In compari ng the estimates of surface flux, all three methods were of the same o rder of magnitude for the fallow site but profile-based estimates were much lower and much higher for fallow/manure and alfalfa sites, respe ctively. Differences were attributed to the location and timing of car bon addition in each system.