Nitrate‐Nitrogen Leached Below the Root Zone During and Following Alfalfa

Abstract

The nitrate‐nitrogen (NO 3 ‐N) contribution to subsurface drainage water by irrigated alfalfa ( Medicago sativa L.) in crop rotations was evaluated by measuring the soil water flux and NO 3 ‐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 (Durixerollic Calciorthid) with a permeable hardpan, 44 kg NO 3 ‐N ha −1 year −1 moved below the root zone at concentrations between 3 and 15 ppm. During the growing season following alfalfa, 85–96 kg NO 3 ‐N ha −1 year −1 moved below the root zone under nonfertilized bean ( Phaseolus vulgaris ) crops at concentrations between 1 and 83 ppm. The second growing season after alfalfa, 17–29 kg NO 3 ‐N ha −1 year −1 at 3–15 ppm NO 3 ‐N moved below the root zone of nonfertilized bean and wheat ( Triticum aestivum L.) crops. A field planted to corn ( Zea mays L.) and fertilized with 200 and 170 kg N ha −1 the first and second year after alfalfa lost 153 and 108 kg NO 3 ‐N ha −1 , respectively, from leaching. Leachate N concentrations varied from 1 to 64 ppm. Unfertilized corn lost 60 and 17 kg NO 3 ‐N/ha the first and second year after alfalfa, respectively, at leachate concentrations of 1–31 ppm. The NO 3 ‐N concentration in the soil solution below only slightly permeable hardpan areas was between 13 and 67 ppm, but only 10–23 kg ha −1 year −1 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 denitrification and/or dilution takes place at the water table interface since 5.2 ppm NO 3 ‐N was the highest concentration measured in the subsurface drainage water with an average of 3.2 ppm NO 3 ‐N. The NO 3 ‐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.