Soil Nitrogen Mineralization in Forested Headwater Stream Watersheds in Relation to Riparian Buffers.
Jessica Taylor, Univ of Washington, 6053 25th Ave NE, Seattle, WA 98115, Daniel Vogt, Univ of Washington, College of Forest Resources, Campus Box 352100, Seattle, WA 98195, Robert Edmonds, Box 352100, University of Washington, University of Washington, College of Forest Resources, Seattle, WA 98195, and Richard Bigley, Washington Dept of Natural Resources, PO Box 47014, Olympia, WA 98504-7014.
Little is known about the effects of forest harvesting on the chemistry of non-fish bearing headwater streams in managed lowland Douglas-fir ecosystems in western Washington. Nutrient exports can have a significant effect on downstream processes including salmonid survival. Terrestrial uplands and streams are linked by riparian buffer zones. The objective of this research was to investigate the connection between forest headwater stream chemistry and soil nitrogen mineralization within riparian forest buffer zones in harvested and unharvested watersheds. Headwater stream chemistry was determined by installing weirs in the streams and collecting water samples monthly. Pressure transducers, a type of data logger, were installed in the stream bed to record the different flow regimes and compile enough data for a complete hydrograph of each stream. Nitrogen mineralization rates were determined using mineralization tubes that were inserted into the top 15 centimeters of the mineral soil. The soil therein was analyzed for ammonium and nitrate compounds. Soil was sampled on either side of the headwater streams within the 50 meter riparian buffer zone and upstream of the weirs. Nine headwater streams were involved: three harvested sites with riparian buffers left intact, three recently harvested sites with no riparian buffers, and three control sites with 70-80 year old second growth forests. In addition to the soil nitrogen mineralization data, monthly stream water samples were analyzed for N (NO3, NH4 and DON), C (TOC, DOC, DIC), pCO2, total P, major ions and pH. Data from this study will be used for the development of a predictive model for headwater stream nutrient concentrations related to soil processes, forest harvesting, and the importance of riparian buffers.