Soil Nitrogen Concentrations Along the Elwha
River: Implications for Dam Restoration
Justin Hill, senior capstone project, Environmental Science and
Resource Management, University of Washington, Seattle, WA
0.120
Average soil nitrogen concentration (%)
Introduction
In 2011 world’s largest dam removal project began
as two hydroelectric dams on the Elwha River in
Olympic National Park were removed to open the
waterway to salmon, and to restore the riparian ecosystem that had been significantly altered by the
dams for a century. In October 2012, Lake Mills was
drained, leaving behind a 483 acres of sparsely vegetated sediment. The former lake is being revegetated by the National Park Service in order to stabilize
ecosystem processes, establish native forests, and
minimize invasive plant species (Chenoweth et al.,
2011). The objective of this research is to quantify
the amount of soil nitrogen in Lake Mills and to determine if the amount is sufficient for restoration of
native vegetation.
0.080
0.060
0.040
0.020
0.000
Figure 4: The former Lake Mills reservoir is located within
Olympic National Park and is intersected by the Elwha River.
0-20
20-50
50-100
100-200
200-300
300-400
400-500
500-600
Soil depth (cm)
Figure 6: Graph of average nitrogen concentration percentages in
Lake Mills by soil depth.
Results and Discussion
xLow soil nitrogen will slow the growth of restoration plant species, especially
late successional conifers.
xNitrogen-fixing plants native to the Pacific Northwest such as red alder trees,
snowbrush shrubs, and lupine herbs can increase soil nitrogen over time.
Figure 1: Photo of Lake Mills before the removal of the Elwha
dams.
xSlow growing species that are able to tolerate low nutrient conditions, like
salal, can prevent erosion by stabilizing the soil, and prevent the establishment
of invasive plants.
Materials and Methods
In July 2013, 165 soil samples were obtained from
41 plots in Lake Mills using hand tools and a bulk
density corer at depths of 0-20cm, 20- 50cm, 50100cm, 1-2m, 2-3m, 4-5m, and 5-6m. The samples
were then run through a carbon, hydrogen and nitrogen analyzer, and the concentrations of nitrogen
measured. Bulk densities were obtained by oven
drying and weighing bulk density corer samples of
known volume, or by obtaining a volumetric sample
whose volume was determined by displacement.
Percent coarse and fine fractions were determined
and coarse fraction (>2mm) was considered to be
zero % nitrogen.
Conclusions
xNitrogen is an essential nutrient, and the most frequently limiting nutrient in
Pacific Northwest soils for plants.
xDetermining the amount of spatial distribution of soil nitrogen in Lake Mills
will aid the park service in the restoration of the Elwha River ecosystem; an
effort being undertaken to restore salmon, prevent erosion and enhance the
scenic beauty of Olympic National Park.
References
0-50cm
Figure 2: Digital elevation model of
the Lake Mills study site. Red dots
indicate the 41 plots where soil
samples were collected.
50-100cm
Figure 5: Map of nitrogen concentration
percentages in Lake Mills to a depth of
1m. Values between plots were interpolated using kriging in ArcGIS.
xBoth total nitrogen content and the concentration in Lake Mills soil are low, particularly in
central portions of the reservoir furthest from neighboring forests.
xThe total nitrogen content in Lake Mills was calculated to be 4183 kg ha -1. An actively
growing Douglas-fir forest with below 5000 kg ha-1 has an inadequate supply of nitrogen
(Heilman et al. 1981).
Figure 3: Photo of restoration plants in Lake
Mills, July 2013. Yellowing foliage is an indicator
of nitrogen deficiency.
0.100
xAt the most critical soil depth for plant growth, 0-50cm, the mean nitrogen concentration
was 0.05%. The mean nitrogen concentration of glacial sediments in forests near former
Lake Mills are 0.4% (Mussman , 2006).
Cole, D.W., & Gessel, S.P. (1973). Nitrogen balances
in forest ecosystems of the pacific northwest. Soil
Biol. Biochem. Vol 5. pp 19-34.
Chenoweth, J., S.A. Acker and M.L. McHenry. (2011).
Revegetation and restoration plan for lake mills and
lake aldwell. Olympic National Park and the Lower
Elwha Klallam Tribe. Port Angeles, WA.
Mussman, E. (2006). Stabilization and pedogenesis
of reservoir sediments following dam removal on the
Elwha River. Master’s Thesis. University of Washington, Seattle, WA.
Acknowledgements
xDr. Joshua Chenoweth and The National Park
Service for permitting this research.
xDr. Darlene Zabowski and Seth Wing for their
guidance.
Figure 7: Photo of Justin Hill
in Lake Mills. Photo credit:
Crescent Calimpong.