ROOT-ZONE MINERAL NITROGEN CHANGES AS AFFECTED BY CROP SEQUENCE AND TILLAGE

Crop sequence and tillage affect soil mineral N (NH4 plus NO3) and NO3 leaching below the root zone following alfalfa (Medicago sativa L.). A 2-yr field experiment was conducted in south-central Idaho to determ ine the effect on soil NO3 levels of a corn (Zea mays L.)wheat (Tritic um aestivum L.) rotation compared with a bean (Phaseolus vulgaris L.)- bean rotation and to demonstrate improved N utilization with a corn-wh eat rotation. Alfalfa, growing on an irrigated Portnenf silt loam (coa rse-silty, mixed, mesic Durixerollic Calciorthid), was killed in Octob er 1989 with herbicide. Treatments were: (i) BT-BT: conventional tille d bean grown in 1990 and 1991; (ii) CNT-WNT: no-till silage corn grown in 1990, and no-till winter wheat grown in 1990-1991; and (iii) CT-WT : same as CNT-WNT but under conventional tillage. Similar amounts of s oil N were mineralized the first (275 kg N ha(-1)) and second (213 kg N ha(-1)) year after killing the alfalfa in all treatments. The BT-BT treatment had the highest growing-season soil mineral N (up to 251 kg ha(-1), 0-0.45-m depth) because the N uptake by bean was lower (187 kK N ha(-1)) than corn (252 kg N ha(-1), average of CT-WT and CNT-WNT tr eatments) in 1990 and later than winter wheat uptake in 1991. Most whe at N uptake had occurred by late June when bean uptake was just starti ng. A rotation that follows alfalfa with corn or a crop with a similar N uptake pattern, instead of bean, will save N fertilizer, lower soil NO3 levels, and reduce NO3 leaching potential.