Denitrification in Ryegrass and Winter Wheat Cropping Systems of Western Oregon
Climatic and edaphic conditions in the Willamette Valley of Oregon should be conducive to denitrification, yet N fertilizer balances have generally shown little unaccounted for N. This study is the first effort to directly measure denitrification in cropping systems of Western Oregon. Denitrification was measured in winter wheat (Triticum aestivum L.) and ryegrass (Lolium multiflorum Lam.) cropping systems over a 2-yr period using in situ cores with the acetylene block technique. Soil respiration rates, NO-3 concentrations, water contents, and temperatures were also measured. Denitrification rates and NO-3 concentrations were generally higher in winter wheat, but soil respiration was often greater in ryegrass. These soil parameters were all lognormally distributed; soil water content was normally distributed. Microbial activity was greatest in the late spring and often showed a secondary peak in late fall. Denitrification rates were most highly correlated with soil water contents (r2 = 0.29, p < 0.01) and soil respiration rates (r2 = 0.13, p < 0.01). About 43% of the variability in denitrification rates could be explained using multiple regression. Overall, losses of N through denitrification were small, about 1.7 kg N ha−1 yr−1 and 0.7 kg N ha−1 yr−1 in the winter wheat and ryegrass systems, respectively.
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