Denitrification in Cultivated and Noncultivated Riparian Areas of Grass Cropping Systems

The factors that affect denitrification of fertilizer N were determined in cultivated riparian (CR) soils cropped to perennial ryegrass (Lolium perenne L.) and noncultivated naturally vegetated riparian soils (NCR) of poorly drained grass cropping systems in western Oregon. Denitrification 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 N2O‐N ha−1 d−1 during the measurement period depending on soil type. Denitrification activity was positively correlated to soil NH4‐N level in the CR. The greater denitrification activity of the CR likely reflects fertilizer applications in contrast to no fertilizer addition to the NCR. Nitrate‐N levels in the CR averaged 5 to 12 times higher than those of the NCR. Nitrate did not appear to encroach from the CR into the NCR by subsurface water movement. The microbial biomass was four times larger in the NCR soils and may have functioned as a significant sink for N reducing denitrification activity. Multivariate factor analysis 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, temperature, and nitrification. The grouping of soil variables was useful in explaining the importance of different soil processes in regulating denitrification. These denitrification activity data demonstrate the possibility for significant N losses, amounting to 12.5% of the applied fertilizer N, from poorly drained cropping systems.