Root‐Zone Mineral Nitrogen Changes as Affected by Crop Sequence and Tillage

Crop sequence and tillage affect soil mineral N (NH 4 plus NO 3 ) and NO 3 leaching below the root zone following alfalfa ( Medicago sativa L.). A 2‐yr field experiment was conducted in south‐central Idaho to determine the effect on soil NO 3 levels of a corn ( Zea mays L.)‐wheat ( Triticum aestivum L.) rotation compared with a bean ( Phaseolus vulgaris L.)‐bean rotation and to demonstrate improved N utilization with a corn‐wheat rotation. Alfalfa, growing on an irrigated Portneuf silt loam (coarse‐silty, mixed, mesic Durixerollic Calciorthid), was killed in October 1989 with herbicide. Treatments were: (i) BT‐BT: conventional tilled 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 soil 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 kg N ha −1 ) than corn (252 kg N ha −1 , average of CT‐WT and CNT‐WNT treatments) in 1990 and later than winter wheat uptake in 1991. Most wheat N uptake had occurred by late June when bean uptake was just starting. 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 NO 3 levels, and reduce NO 3 leaching potential.