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

Concerns about the environmental impacts of N management in agroecosys tems have increased interest in promoting N efficiency and greater uti lization of animal manures, green manures, and legumes in crop rotatio ns. Predicting N dynamics of different soil management in short-term s tudies 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 eff ects of residue management on the mineralization and plant availabilit y of N. The treatments, established in 1931, included wheat straw burn ing, incorporation of straw, manure, legume plant residue, or inorgani c 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. Ry egrass (Lolium perenne L.) was grown as an indicator plant for 120 day s 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 contro l rate, ryegrass dry matter yield (DMY) and N uptake from the manure-t reated soil were more than 50% higher than from other residue treatmen ts. When adequate nutrients were applied, soil treated long-term with manure had no DMY advantage over long-term inorganic N treatments. Bur ning of wheat straw did not significantly influence DMY, although yiel ds from the burned plots tended to be lower. Large accumulations of ex tractable nitrate (> 12 mg-N kg-1) in the lower portion of the manure- treated soil suggest a potential for nitrate groundwater contamination .