Soil Nitrogen Mineralization in Forested Headwater Stream Watersheds in Relation to Riparian Buffers

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Soil Nitrogen Mineralization in Forested
Headwater Stream Watersheds in Relation to
Riparian Buffers Jessica C Taylor1, Daniel Vogt1,
Robert Edmonds1, Richard Bigley2 1University of
Washington College of Forest Resources 2Washington
State Department of Natural Resources
Introduction Headwater streams (1st order) are
the initiation points and primary sources of
water feeding a drainage network (Stanford 1996).
Headwaters are generally fed by spring water or
snow melt and act as a vital link between
terrestrial and aquatic systems. In western
Washington more than 50 of the stream mileage is
made up of these 1st order reaches, yet little
knowledge of the influence of forest management
on riparian zones exists relative to parameters
such as stream discharge, nutrient export, or
riparian nitrogen fluxes. Biogeochemical nitrogen
cycles are important in riparian zones where
hill-slope hydrologic flows and saturated soils
meet. For example, nitrogen retention could be
affected by forest management in these upper
watersheds. The projected increase of
harvesting on state-owned lands and the lack of
state regulation concerning the presence of
headwater stream buffers emphasizes the
importance of understanding how these systems
function. This information will be crucial for
responsible stewardship and resource protection.

• Study Design and Methods
• Site Waddell Creek watershed in Capitol State
  Forest near Olympia, WA. Soils in this site
  are classified as Ultisols.
• Treatments
• Control 3 sites located in 70 yr old second
  growth Douglas fir forest
• With Riparian Buffers 3 sites harvested
  within 5 years
• Without Buffers 3 sites harvested within 5
  years
• Data Collection
• Nitrogen mineralization analysis, using the in
  situ core method (Raison et al., 1987) and KCl
  extractions
• Total soil Carbon to Nitrogen ratios
• Water chemistry and stream hydrographs
  produced with data collected from pressure
  transducers (automated data loggers) and
  V- notch weirs.

Jeff Grizzel, WA DNR

• Research Objectives
• Characterize nitrogen mineralization rates for
  different forest management treatments.
• Quantify stream discharge and the water
  chemistry.
• Investigate the influence of forested riparian
  buffers on soil nitrogen mineralization and
  stream N concentrations.

• Preliminary Results and Analyses
• There is less immobilization of nitrate in the
  streams during fall and winter flows due to less
  vegetative uptake and lower microbial activity.
  This causes peaks in total nitrate
  concentrations.
• The higher nitrate concentrations in the buffered
  streams could be due to the presence of Alder
  trees which fix their own nitrogen.
• Winter net soil nitrogen mineralization was low
  due to snow cover for entire sampling period.
  Similarly, rates were low in the summer due to
  the drier climate conditions. Subsequently during
  these seasons microbial activity decreases
  compared to the relatively active spring and fall
  periods.

Pressure Transducer
V-Notch Weir
Installation of soil cores
Project funded by
Typical Control Headwater Stream