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Title: Managing N contaminated soils: benefits of N2-fixing alfalfa

Author
item Russelle, Michael
item Lamb, Joann
item TURYK, NANCY - UNIVERSITY OF WISCONSIN
item SHAW, BYRON - UNIVERSITY OF WISCONSIN
item PEARSON, BILL - UNIVERSITY OF WISCONSIN

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2007
Publication Date: 5/1/2007
Citation: Russelle, M.P., Lamb, J.F., Turyk, N.B., Shaw, B.H., Pearson, B. 2007. Managing nitrogen contaminated soils: benefits of N2-fixing alfalfa. Agronomy Journal. 99:738-746.

Interpretive Summary: Society continues to be faced with problems of water contamination from a wide variety of sources. One of the most common water ground quality problems is excessive nitrate, which can impair human, livestock, and ecosystem health. In surface water, phosphorus encourages algae blooms and heavy water plant growth, which can lead to fish kills. One source of these nutrients is from abandoned feedlots. We tested an approach to prevent some of the water contamination that can occur in these sites. We grew two types of alfalfa, a normal type that can obtain part of its nitrogen needs from the air, and a special type that cannot. We found that the better choice is normal alfalfa, because it provided better yields (which helps reduce the cost of site cleanup) and more nitrogen and phosphorus removal. Even though alfalfa was quite good at removing nutrients, especially nitrogen, we make suggestions for an improved method to protect water supplies. Farmers and land managers can use these recommendations immediately to help protect water quality.

Technical Abstract: Shallow ground water can be contaminated by nitrate under open livestock feeding lots, and nitrate losses often are intensified after animals are removed. Perennial forage crops may offer an effective, low cost method for remediating abandoned sites and, furthermore, results of earlier research suggested that non-N-fixing alfalfa (Medicago sativa L.) may be more beneficial for cleanup than standard, N-fixing alfalfa. We investigated the N and P uptake by alfalfa, N and P movement in the soil, and impacts on ground water at an abandoned feedlot on a Richford sandy loam (mixed, mesic Psammentic Hapludalfs) in Portage County, Wisconsin. Duplicate plots (30 by 60 m) of N-fixing and non-N-fixing alfalfa were seeded in August 1998; one replicate required reseeding in August 1999. The facultative nature of N fixation was evident, with less fixation occurring when alfalfa absorbed soil and manure N, but uniform dry mass and total N yield across the site. On an area basis, non-N-fixing alfalfa removed the same or less N from the soil and manure than N-fixing alfalfa, due largely to stand decline where inorganic N supply was inadequate for persistence of the non-N-fixing type. Maximum recovery of soil and manure N was about 450 kg N/ha over 3 yr for plots seeded with N-fixing alfalfa in 1998, and less if seeding was delayed. Despite this N removal by alfalfa, ground water nitrate concentrations increased after abandonment by 50 to 80 kg nitrate-N ha-1. Ground water nitrate concentration was elevated within 1 year of feedlot abandonment. Ground water P concentration did not change, but P apparently leached through the soil to the 60- to 90-cm depth. Based on these results, it appears that nitrate leaching losses after feedlot abandonment on coarse-textured soils can be minimized by seeding a recommended cultivar of N-fixing alfalfa with a fast-growing companion crop, using no-till seeding to avoid soil mixing and aeration, if soil compaction does not require remediation.