Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/31/2007
Publication Date: 6/30/2007
Citation: Davis, J.H., Griffith, S.M., Horwath, W.R., Steiner, J.J., Myrold, D.D. 2007. Mitigation of Shallow Groundwater Nitrate in a Poorly Drained Riparian Area and Adjacent Cropland. Journal of Environmental Quality.36:628-637. Interpretive Summary: Riparian ecosystems have many ecological roles in agricultural landscapes. One important role is the ability of riparian zones to filter excess nutrients flowing from upland agricultural sources through riparian shallow groundwater to streams. This often results in higher water quality entering streams. Often site-specific riparian management policy lacks scientific data to support policy goals; hence science must provide scientific data to make environmental policy more relevant to site-specific management goals. Data concerning riparian water quality function in poorly drained landscapes of western Oregon are scant. This study was designed to try to better understand how a poorly drained riparian area in Willamette Valley, Oregon functions with respect to mitigating nitrate pollution from an adjacent cultivated farm field. Data from this study suggests that both dilution from an abundance of winter precipitation and biological consumption by plants and soil organisms were responsible for significant decreases in shallow groundwater nitrate nitrogen and improving stream water quality and a riparian width of 17 meters or less was sufficient to produce this response.
Technical Abstract: Riparian ecosystems, through their unique positions in the agricultural landscape and ability to influence nutrient cycles, have the potential to mitigate nutrient loading to surface and ground waters. The purpose of this study was to determine the fate of NO3 in shallow ground water moving along a lateral flowpath from a grass seed cropping system through an undisturbed herbaceous riparian area. Soil A (30-45 cm) and C horizon (135-150 cm) NO3 and dissolved oxygen concentrations were significantly higher in the cropping system than the adjacent riparian area. Nitrate concentrations in both horizons of the riparian soil were consistently close to the detection limit (0.05 mg N L-1) while cropping system concentrations ranged from 1 to 12 mg N L-1. Riparian soil water Cl data suggested that some dilution of NO3 occurred from seasonal recharge by precipitation. However, a sharp decrease in the NO3/Cl ratios as water moved into the riparian area from the cropping system indicated that dilution of NO3 concentrations from upwelling or divergence along the lateral flowpath did not likely occur. The riparian area A horizon soil water had higher soluble organic carbon than the cropping system and has also been shown to support higher soil microbial biomass so that when the riparian soil became saturated, available electron donors (O2, NO3) were rapidly reduced. Carbon limitations in the cropping system likely led to microbial respiration using primarily O2 and to a lesser degree NO3. Data suggested that a riparian area width of 17m was sufficient for soil water NO3 removal from this landscape.