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TECHNICAL REPORTS

Ground Water Quality

Mitigation of Shallow Groundwater Nitrate in a Poorly Drained Riparian Area and Adjacent Cropland

^a USDA-ARS, 3450 SW Campus Way, Corvallis, OR 97331
^b Dep. LAWR, Univ. of California, Davis, CA 95616
^c Dep. of Crop and Soil Science, Oregon State Univ., Corvallis, OR 97331

^* Corresponding author (griffits{at}onid.orst.edu)

Received for publication May 10, 2006. Riparian ecosystems, through their unique position in the agricultural landscape and ability to influence nutrient cycles, can potentially reduce NO₃ loading to surface and ground waters. The purpose of this study was to determine the fate of NO₃ in shallow groundwater moving along a lateral flowpath from a grass seed cropping system through an undisturbed mixed-species herbaceous riparian area. Soil A (30–45 cm) and C horizon (135–150 cm) NO₃, dissolved oxygen, and nitrous oxide concentrations were significantly higher in the cropping system than the adjacent riparian area. Nitrate concentrations in both horizons of the riparian soil were consistently at or below 0.05 mg N L^–1 while cropping system concentrations ranged from 1 to 12 mg N L^–1. Chloride data suggested that NO₃ dilution occurred from recharge by precipitation. However, a sharp decrease in NO₃/Cl ratios as water moved into the riparian area indicated that additional dilution of NO₃ concentrations was unlikely. Riparian area A horizon soil water had higher dissolved organic carbon than the cropping system and when the riparian soil became saturated, available electron acceptors (O₂, NO₃) were rapidly reduced. Dissolved inorganic carbon was significantly higher in the riparian area than the cropping system for both horizons indicating high biological activity. Carbon limitation in the cropping system may have led to microbial respiration using primarily O₂ and to a lesser degree NO₃. Within 6 m of the riparian/cropping system transition, NO₃ was virtually undetectable.

Abbreviations: DIC, dissolved inorganic carbon • DNRA, dissimilatory nitrate reduction to ammonium • DOC, dissolved organic carbon • DO, dissolved oxygen • ECD, electron capture detector

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