Managing Nitrogen Contaminated Soils:Benefits of N 2 –Fixing Alfalfa

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 (NO 3 ) 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‐N 2 –fixing alfalfa would remove more inorganic soil N than standard, N 2 –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 N 2 –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, N 2 –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 N 2 –fixing alfalfa. During 2 yr when inorganic N uptake was estimated by the 15 N natural abundance technique, N 2 –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 NO 3 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 N 2 –fixing alfalfa.