Nitrogen movement and water quality at a poorly-drained agricultural and riparian site in the Pacific Northwest

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Grass seed cropping systems in the Pacific Northwest account for about half of the cool-season forage and turf grass seed production in the world. Grass seed cropping systems are intensely managed with inorganic fertilizers to sustain production. Much of the land where grass seed production occurs in western Oregon is marginally productive for most other crops because of the poorly drained soil conditions. The role of riparian areas bordering grass seed fields in moderating surface water and groundwater quality is not well understood. A study site in western Oregon was instrumented to determine how riparian areas bordering grass seed fields process N and thereby influence water quality. The site consisted of a perennial ryegrass (Lolium perenne L.) seed field, a grass riparian area, and an intermittent creek. Groundwater levels were monitored during the fall of 1995 to the late spring of 1996 and a complete set of samples were collected every two weeks from wells with water and analyzed for NO_3^- and NH_4^+. Total fertilizer applied to the crop was 200 kg N ha^<-1> yr^<-1>. Data indicated a lateral subsurface hydrologic flow path from the cultivated field through the riparian zone, to the creek. Averaged over the season (November 1995 - June 1996), riparian zone A and C horizon groundwater mean NO_3^- concentration was mostly undetectable and less than 3.8 mg L^<-1> in the agricultural field. At all sampling dates, the NO_3^- concentration was higher in the field than in the riparian zone. Seasonal mean groundwater NH_4^+ concentrations for the riparian and field A and C horizons were <__-0.2 mg L^<-1>. Annual crop net N accumulation was 144 kg N ha^<-1>. Riparian soil electrode potentials (E_h) at 25 and 45 cm averaged -200 to -100 mV during the wet cycle, whereas the field ranged between 0 to +50 mV. Collectively, findings indicate that both crop and riparian processes at the Lake Creek site are responsible for reducing shallow groundwater NO_3^- to low levels.
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Nitrogen movement and water quality at a poorly-drained agricultural and riparian site in the Pacific Northwest

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