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Soil & Crop Management

Managing Nitrogen Contaminated Soils

Benefits of N₂–Fixing Alfalfa

^a USDA-ARS-US Dairy Forage Research Center (Minnesota Cluster), Saint Paul, MN 55108
^b USDA-ARS, 1991 Upper Buford Cir., Saint Paul, MN 55108
^c Center for Watershed Science and Education, College of Natural Resources, Univ. of Wisconsin–Stevens Point, Stevens Point, WI 54481
^d Ground Water Quality Bureau, New Mexico Environ. Dep., P.O. Box 26110, Santa Fe, MN 87502

^* Corresponding author (russelle{at}umn.edu)

Received for publication December 1, 2005. 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.

Abbreviations: DM, dry matter • fNdfa, fraction of N derived from the atmosphere • fNdfs, fraction of N derived from the soil • Ndfa, mass of N derived from the air, kg N ha^–1 • Ndfs, mass of N derived from the soil, kg N ha^–1

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