DENITRIFICATION IN CULTIVATED AND NONCULTIVATED RIPARIAN AREAS OF GRASS CROPPING SYSTEMS

The factors that affect denitrification of fertilizer N were determine d in cultivated riparian (CR) soils cropped to perennial ryegrass (Lol ium perenne L.) and noncultivated naturally vegetated riparian soils ( NCR) of poorly drained grass cropping systems in western Oregon, Denit rification activity in the NCR was low compared with the CR using the Acetylene Inhibition method. The CR soil denitrification activity was consistent over the growing season, averaging between 269 and 280 g N2 O-N ha(-1) d(-1) during the measurement period depending on soil type. Denitrification activity was positively correlated to soil NH4-N leve l in the CR. The greater denitrification activity of the CR likely ref lects fertilizer applications in contrast to no fertilizer addition to the NCR. Nitrate-N levels in the CR averaged 5 to 12 times higher tha n those of the NCR. Nitrate did not appear to encroach from the CR int o the NCR by subsurface water movement. The microbial biomass was four times larger in the NCR soils and may have functioned as a significan t sink for N reducing denitrification activity, Multivariate factor an alysis using orthogonal factor rotation showed that approximately 83% of the observed variance in denitrification was explained by grouping experimental variables by microbial activity, N2O-N emission, temperat ure, and nitrification. The grouping of soil variables was useful in e xplaining the importance of different soil processes in regulating den itrification. These denitrification activity data demonstrate the poss ibility for significant N losses, amounting to 12.5% of the applied fe rtilizer N, from poorly drained cropping systems.