Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2010
Publication Date: 1/25/2011
Citation: Griffith, S.M., Davis, J.H., Wigington, P.J. 2011. Surface water and groundwater nitrogen dynamics in a well drained riparian forest within a poorly drainged agricultural landscape. Journal of Environmental Quality. 40:505-516.
Interpretive Summary: Forested areas along streams can play an important role in water quality. This study was conducted to determine the extent a forested riparian area reduced nutrient concentrations flowing from an adjacent up slope grass seed production field. The study site was on the Calapooia River in the Willamette Valley, Oregon, USA. We found that in spite of relatively high levels of nutrient phosphate-P (0.16 ppm) in the groundwater of the cropping system at a depth of two to three meters, the Calapooia River phosphate-P concentrations were at a level consistently below 0.05 ppm. Groundwater nitrate-N concentrations were low in the upper levels of the soil of the seed field and were associated with low activity of soil N biological processes. In the riparian forest, the water table fluctuated greatly and nitrate-N concentrations were relatively consistent with depth at about 2 ppm. Nitrate-N concentrations in the river were consistently below 2 ppm. This study’s data indicate that the mixing of the river water with the hillslope shallow groundwater from the field significantly contributed to the dilution of nutrients. High nutrient concentrations next to the river may indicate upwelling of deeper, higher nitrate groundwater.
Technical Abstract: Riparian ecosystems, through their unique position in the agricultural landscape and ability to influence nutrient cycles, can potentially reduce nutrient loading to surface and ground waters. The primary purpose of this study was to determine the efficacy of a well-drained, mixed-deciduous riparian forest to buffer a river from N and P originating from a poorly drained grass seed cropping system. The study site was on the Calapooia River in the Willamette Valley, Oregon, USA. Water table elevations indicated that during the wet winters, water flowed from the hillslope to the river. The water table in the cropping system did not fluctuate much during the water year but the riparian forest water table responded quickly to inputs from precipitation and changes in river height. There were three to five punctuated, high-precipitation events per year where water moved from the river back into the hillslope aquifer. In spite of relatively high levels of phosphate (0.16 mg PO4--P L-1) in the groundwater of the cropping system at a depth of 2.1 to 2.6 m, Calapooia River PO4- concentrations were consistently below 0.05 mg PO4--P L-1. Groundwater NO3- concentrations were low in the surface wells of the grass seed cropping system (0.2-0.4 mg NO3- - N L-1) and were associated with relatively low rates of mineralization (0.09 ± 0.02 kg N ha-1 d-1) and nitrification (0.10 ± 0.02 kg N ha-1 d-1). At the depth of 1.5 to 2.6 m, NO3- concentrations in the cropping system were significantly higher (4.8-6.7 mg NO3-- N L-1). Groundwater movement was very slow in the cropping system, allowing NO3- to accumulate in the subsoil. In the riparian forest, the water table fluctuated widely and NO3- concentrations were relatively consistent with depth (0.6-1.8 mg NO3-- N L-1). Nitrate concentrations in the river were consistently below 2 mg NO3-- N L-1. Mixing of water (hillslope and river) within the soil column probably acted to homogenize NO3- concentrations in the riparian forest. The highest groundwater NO3-concentrations on site were in wells that were installed directly next to the river where concentrations reached up to 30 mg NO3-- N L-1 and decreased significantly over the water year. High concentrations next to the river may indicate upwelling of deeper, high NO3- groundwater.