Submitted to: Grassland International Congress
Publication Type: Abstract only
Publication Acceptance Date: 12/14/2004
Publication Date: 6/26/2005
Citation: Russelle, M.P., Browne, B.A., Turyk, N.B., Pearson, B. 2005. Denitrification under pastures on permeable soils helps protect ground water quality [abstract]. XXth International Grassland Congress, June 26 to July 1, 2005, Dublin, Ireland. p. 692. Interpretive Summary:
Technical Abstract: Pastures have been implicated in ground water contamination by nitrate, especially in humid regions with thin or sandy soils. Lack of evidence that appreciable nitrate leaching was occurring in established Midwestern U.S.A. pastures led us to test the hypothesis that denitrification was preventing or remediating nitrate loading. We studied ground water and soil chemistry at three grazing dairy farms located on soils with high hydraulic conductivity in central Wisconsin, and in a field under corn-soybean management on a nearby confinement dairy farm. Two independent experiments were conducted in a growth chamber on intact soil cores from one paddock, with or without fresh dairy cow excreta applied at the start of each 28- to 31-d incubation period. Ground water samples from the multiport wells indicated that nitrate was leaching at substantially smaller rates than under other agricultural practices in the area. Although differences in soil nitrate concentration were evident between excreta spots and background areas on several sampling dates, no differences in dissolved organic C were detected. The intact soil core experiments provided convincing evidence that urine increased denitrification. Nitrous oxide emission over 4 weeks was 3-fold higher with dung and 9-fold higher with urine than the control soil. Methane emission was 20-fold higher with dung than either urine or no treatment, whereas CO2 emission quadrupled with either excreta. In the field, there was tremendous spatial variability in ground water chemistry (dissolved nutrients and gases). The dissolved denitrified N (measured as dissolved N2 gas in excess of atmospheric N2) was higher as a percentage of total nitrate (nitrate + denitrified N) in groundwater beneath the pasture (n>75 sites per sampling time) than beneath the arable field (n=26 per sampling time), and ancillary measurements (e.g., dissolved organic C and dissolved O2) supported this result. In contrast, dissolved N2O was lower under the pasture than corn. This is the first report of the wide variation in dissolved gas composition in ground water under pastures. The field evidence and results from intact soil cores lend support to the hypothesis that denitrification may remove substantial amounts of N from pastures. Enhanced denitrification may benefit water quality more generally as ground water moves from or toward adjacent arable cropland.