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

The nitrate‐nitrogen (NO3‐N) contribution to subsurface drainage water by irrigated alfalfa (Medicago sativa L.) in crop rotations was evaluated by measuring the soil water flux and NO3‐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 NO3‐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 NO3‐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 NO3‐N ha−1 year−1 at 3–15 ppm NO3‐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 NO3‐N ha−1, respectively, from leaching. Leachate N concentrations varied from 1 to 64 ppm. Unfertilized corn lost 60 and 17 kg NO3‐N/ha the first and second year after alfalfa, respectively, at leachate concentrations of 1–31 ppm. The NO3‐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 NO3‐N was the highest concentration measured in the subsurface drainage water with an average of 3.2 ppm NO3‐N. The NO3‐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.