Graduate Thesis Or Dissertation

Denitrification and vegetative uptake in a pasture, poplar and native oak riparian buffer area

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  • Managing riparian buffer zones is a potentially important approach to protecting streams from agricultural pollution. This study was conducted to determine if a pasture, a hybrid poplar (Populus trichocarpa x deltoides) stand, or a native oak (Quercus garryana) forest, had the greatest potential to serve as a nutrient buffer zone. The effects of fertilizer treatments and vegetation type on denitrification and vegetative uptake were investigated in the Willamette Valley riparian area of Oak Creek, near Corvallis, Oregon. Field and potential denitrification rates measured in the pasture and poplar stand were between 2 and 83 times greater than in the native forest. Two possible explanations exist: (1) lack of a readily degradable C source may be limiting denitrification in the organic C rich native forest plots, or (2) denitrifying microbial populations may be smaller in the native forest due to a lack of soil NO. While field denitrification rates peaked in the winter and were at a minimum in the summer, denitrification potentials, a reflection of the site's accumulative in situ denitrification over time, were greatest in the fall, suggesting that more favorable conditions for actual denitrification may occur during this season. Vegetative uptake of N (as measured by litterfall N) was greatest in the native oak forest, with values 1.5 and 2 times greater than poplar and pasture sites, respectively, and primarily occurred from early spring through summer To optimize nutrient removal rates, vegetation in riparian buffer zones should be able to take up entering excess nutrients as well as provide a readily degradable C source for denitrifying microorganisms. Because the native oak forest took up more N and the pasture and poplar sites denitrified more N, an integrated riparian buffer area with all three vegetation types may serve as the best riparian nutrient removal solution. In areas devoid of native forest, establishment of a poplar plantation can provide for significant uptake and good denitrification potential in a relatively short time.
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