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This article in JEQ

  1. Vol. 37 No. 4, p. 1456-1467
    Received: Feb 7, 2007

    * Corresponding author(s): bbrowne@uwsp.edu


Collateral Geochemical Impacts of Agricultural Nitrogen Enrichment from 1963 to 1985: A Southern Wisconsin Ground Water Depth Profile

  1. Bryant A. Browne *,
  2. George J. Kraft,
  3. Juliane M. Bowling,
  4. William M. DeVita and
  5. David J. Mechenich
  1. College of Natural Resources, Univ. of Wisconsin, Stevens Point, WI 54481


In this study, we used chlorofluorocarbon (CFC) age-dating to investigate the geochemistry of N enrichment within a bedrock aquifer depth profile beneath a south central Wisconsin agricultural landscape. Measurement of N2O and excess N2 allowed us to reconstruct the total NO3 and total nitrogen (TN) leached to ground water and was essential for tracing the separate influences of soil nitrification and ground water denitrification in the collateral geochemical chronology. We identify four geochemical impacts due to a steady ground water N enrichment trajectory (39 ± 2.2 μmol L−1 yr−1, r 2 = 0.96) over two decades (1963–1985) of rapidly escalating N use. First, as a by-product of soil nitrification, N2O entered ground water at a stable (r 2 = 0.99) mole ratio of 0.24 ± 0.007 mole% (N2O–N/NO3–N). The gathering of excess N2O in ground water is a potential concern relative to greenhouse gas emissions and stratospheric ozone depletion after it discharges to surface water. Second, excess N2 measurements revealed that NO3 was a prominent, mobile, labile electron acceptor comparable in importance to O2. Denitrification transformed 36 ± 15 mole% (mol mol−1 × 100) of the total N within the profile to N2 gas, delaying exceedance of the NO3 drinking water standard by approximately 6 yr. Third, soil acids produced from nitrification substantially increased the concentrations of major, dolomitic ions (Ca, Mg, HCO3 ) in ground water relative to pre-enrichment conditions. By 1985, concentrations approximately doubled; by 2006, CFC age-date projections suggest concentrations may have tripled. Finally, the nitrification induced mobilization of Ca may have caused a co-release of P from Ca-rich soil surfaces. Dissolved P increased from an approximate background value of 0.02 mg L−1 in 1963 to 0.07 mg L−1 in 1985. The CFC age-date projections suggest the concentration could have reached 0.11 mg L−1 in ground water recharge by 2006. These results highlight an intersection of the N and P cycles potentially important for managing the quality of ground water discharged to surface water.

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