The nitrate-nitrogen (N0,-N) contribution to subsurface drainage water by irrigated alfalfa (Medic:ago satire L.) in crop rotations was evaluated by measuring the soil water flux and NON-N concentration below the root zone of alfalfa and crops following alfalfa with and without additional nitrogen fertilization. Under alfalfa grown on Portneuf silt loam (Durixeroilic Calciorthid) with a permeable hardpan, 44 kg NO,-N he year ' moved below the root zone at concentrations betWeen 3 and 15 ppm. During the growing season following alfalfa, 85-96 kg NO,-N he year' moved below the root zone under nonferdlized bean (Phaseolus vulgaris) crops at concentrations between 1 and 83 ppm. The second growing season after alfalfa, 17-29 kg NO,-N he year' at 3-15 ppm NO,-N moved below the root zone of nonfertilized bean and wheat (Tridezun aeitiVillit L.) crops. A field planted to corn (Zea mays L.) and fertilized with 200 and 170 kg N ha-' the first and second year after alfalfa lost 153 and 108 kg NO-N he, respectively, from leaching. Leachate N concentrations varied from 1 to 64 ppm. Unfertilized corn lost 60 and 17 kg NO,-N/ha the first and second year after alfalfa, respectively, at leachate concentrations of 1-31 ppm. The NO,-N concentration in the soil solution below only slightly permeable hardpan areas was between 13 and 67 ppm, but only 10-23 kg he year' moved below the root zone because of the lower water flux through the hardpan. Comparing these results with previous data for the same area suggests that considerable denibifkation and/or dilution takes place at the water table interface since 5.2 ppm NO,-N was the highest concentration measured in the subsurface drainage water with an average of 3.2 ppm NO,-N. The NO,-N contributed by alfalfa in the crop rotation was estimated to equal just half of that accounted for in the subsurface drainage in a previous study on the same irrigated tract.' Contribution from the USDA-SEA-ARS; Univ. The potential health hazards of nitrate nitrogen (NO,-N) in surface and subsurface water has drawn increased public attention to agricultural practices as a possible source of nonpoint source NO 3-N water pollution. Because high NO,-N concentrations in drinking water are considered to pose health hazards to livestock and humans, particularly infants, the U.S. Public Health Service has set a maximum of 10 ppm NO,-N as the standard for drinking water (13). The high NO3-N levels found in some areas may come from any of several sources, and the sources must be idenified if management practices are to be changed to reduce nonpoint NO,-N sources entering subsurface, and eventually, surface waters. Nitrate leaching rates are a function of irrigation and precipitation rate and timing, nitrogen fertilizer application rate and timing (1, 6), and the crops grown (9, 12).Alfalfa (Medicago sativa L.) removes water and NO,-N from deeper in the soil profile than other crops (2,7,11), and is an excellent NO 3 scavenger for reducing the amount of NO3-N leaching following a high N fertilized crop, o...