NITRATE LEACHING UNDER FURROW IRRIGATION AS AFFECTED BY CROP SEQUENCE AND TILLAGE

The potential for NO3-N leaching after alfalfa (Medicago sativa L.) in irrigated crop production depends on cropping sequence and tillage practices. A 2-yr field experiment in south-central Idaho compared the NO3-N leached following alfalfa of a conventional tillage bean-bean (Phaseolus vulgaris L.) rotation with a silage corn (Zea mays L.)-winter wheat (Triticum aestivum L.) rotation in a conventional tillage or no-till system. Nitrate leaching was determined by: (i) sampling the soil solution below the root zone (1.2 and 1.5 m) using ceramic-tipped samplers and calculating the N movement from the water balance, and (ii) measuring the change in soil NO3-N at 1.35 to 4.5 m from soil samples taken in the fall and spring to 4.5 m. During the second growing season, average soil solution NO3-N concentrations (below the root zone) were 28, 4, and 10 mg L(-1) for the bean-bean, corn-wheat no-till, and corn-wheat tilled treatments, respectively. The soil NO3-N in 1.35 to 3.3 m at the end of the study was 80 kg N ha(-1) higher for the bean-bean treatment than for the corn-wheat treatments. The NO3-N that moved below 1.35 m during the 2 yr was 53 kg ha(-1) higher for the bean-bean than for the corn-wheat treatments. The soil NO3-N in the 1.35 to 3.3 m depth after 2 yr was 21 kg ha(-1) higher for the corn-wheat under conventional tillage than under the no-till system.