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

USDA-ARS, Soil and Water Management Research Unit, 3793 N 3600 E, Kimberly, ID 83341

*Corresponding author.

ABSTRACT

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 determine the effect on soil NO₃ 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 NO₃ levels, and reduce NO₃ leaching potential.

Received for publication September 1, 1993.

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