EFFECTS OF LONG-TERM RESIDUE MANAGEMENT AND NITROGEN-FERTILIZATION ON AVAILABILITY AND PROFILE DISTRIBUTION OF NITROGEN

Concerns about the environmental impacts of N management in agroecosystems have increased interest in promoting N efficiency and greater utilization of animal manures, green manures, and legumes in crop rotations. Predicting N dynamics of different soil management in short-term studies is difficult because soil properties change slowly. A long-term experiment under a winter wheat-fallow system in the semi-arid region of eastern Oregon provided an opportunity to study the cumulative effects of residue management on the mineralization and plant availability of N. The treatments, established in 1931, included wheat straw burning, incorporation of straw, manure, legume plant residue, or inorganic N. To evaluate N availability to plants, a greenhouse pot study was conducted on the treated soils collected from the 0 to 20-cm depth. Ryegrass (Lolium perenne L.) was grown as an indicator plant for 120 days in 1-kg soil samples treated with different rates of inorganic N and harvested and analyzed for total N at 30-day intervals. At the control rate, ryegrass dry matter yield (DMY) and N uptake from the manure-treated soil were more than 50% higher than from other residue treatments. When adequate nutrients were applied, soil treated long-term with manure had no DMY advantage over long-term inorganic N treatments. Burning of wheat straw did not significantly influence DMY, although yields from the burned plots tended to be lower. Large accumulations of extractable nitrate (> 12 mg-N kg-1) in the lower portion of the manure-treated soil suggest a potential for nitrate groundwater contamination.