Managing Nitrogen Contaminated Soils:Benefits of N2–Fixing Alfalfa

Abstract

Perennial forage crops offer an effective, low‐cost method for remediating excess soil N. Where it is agronomically adapted, alfalfa (Medicago sativa L.) is a particularly desirable species for remediation of excess soil N because it has high dry matter (DM) yield and N uptake potential, it can absorb nitrate (NO3) from depths beyond those attainable by most annual crops, and its market value is usually higher than grass forages. On the basis of previous research, we hypothesized that non‐N2–fixing alfalfa would remove more inorganic soil N than standard, N2–fixing alfalfa and tested this hypothesis at an abandoned barnyard on a Richford sandy loam (mixed, mesic Psammentic Hapludalfs) in central Wisconsin, USA. Duplicate plots (30 by 60 m) of both N2–fixing and nonfixing alfalfa were seeded in August 1998 and 1999. Nonfixing alfalfa produced lower DM and N yield and showed greater variability than standard, N2–fixing alfalfa. Yield, N concentration, and stand declined in the nonfixing type where inorganic N supply was inadequate. Average maximum estimated recovery of soil and manure N was about 200 kg N ha−1 annually for plots seeded with N2–fixing alfalfa. During 2 yr when inorganic N uptake was estimated by the 15N natural abundance technique, N2–fixing alfalfa removed nearly 60% more soil and manure N than nonfixing alfalfa, but weeds in the nonfixing alfalfa plots made up the difference. Neither alfalfa prevented ground water contamination by NO3 and concentrations increased similarly under both alfalfas. Because declines in yield and protein concentration may be expected for nonfixing alfalfa on sites with patchy available N distribution, economic remediation of these sites will be promoted by using an adapted cultivar of N2–fixing alfalfa.