PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

Proceedings-Research on Coniferous Forest Ecosystems-A symposium .
Bellingham, Washington-March 23-24, 197 2
Nutrient budget of a Douglasfir
forest on an experimenta l
watershed in western Oregon
R . L . Fredrikse n
Soil Scientist, Forestry Sciences Laborator y
Pacific Northwest Forest and Range Experiment Statio n
Forest Service, U .S . Department of Agricultur e
Corvallis, Orego n
Abstract
Annual loss of nitrogen, phosphorus, silica, and the cations sodium, potassium, calcium, and magnesiu m
followed the same pattern as annual runoff which is heavily dominated by winter rainstorms arising from th e
Pacific Ocean. Even though 170 and 135 cm of water passed through this Douglas-fir ecosystem for the 2 year s
reported here, this ecosystem conserved nitrogen effectively as indicated by an average annual dissolved nitroge n
outflow of 0.5 kg/ha from an annual average input of 1 .0 kg/ha in precipitation . There was a small annual ne t
loss of phosphorus (0.25 kg/ha) . Cation input in precipitation was less than 10 percent of sources from minera l
weathering-indicating that mineral weathering was the principal source of cations to the system. Average
annual net losses of calcium, sodium, magnesium, and potassium were : 47, 28, 11, and 1 .5 kg/ha, respectively.
Silica loss of 99 kg/ha-yr was the largest of all constituents and came entirely from within the forest system .
Although loss of sediment was low during the period of study, loss of nutrients by soil erosion may become of
major importance over a longer time scale due to widely spaced unsampled catastrophic erosion .
Introduction
The purpose of this study was to measur e
the inputs, losses, and retentions of specifi c
plant nutrients .
Forested watersheds have been used fo r
decades to study the hydrologic cycle .
Bormann and Likens (1967) indicated tha t
small watersheds are suitable for studies of
chemical cycles if the ecosystem is located o n
a watershed with relatively impermeable bed -
rock . Then, all annual chemical fluxes int o
and out of the forest system can be attribute d
to a definable forest ecosystem, and error s
due to deep seepage can be minimized . Small
watersheds containing soil-plant systems hav e
been generally accepted as useful units fo r
ecological research .
Nutrient cycles begin with the establishment
of vegetation, and the quantity o f
nutrients cycled undoubtedly increases as the
forest passes through successional phases . At
all points, along this developmental sequence ,
the supply of nutrients in the soil-plan t
system is regulated by the balance betwee n
(1) the inputs from the atmosphere an d
mineral weathering and (2) the outflow b y
soil erosion or chemicals dissolved in stream
water. A continued level of fertility of th e
system therefore depends upon the retentio n
of nutrients in the cycle from loss by leachin g
and soil erosion .
Nutrient budget studies were begun on two
small watersheds at the H . J. Andrews Experimental
Forest in 1967 . Only the initial results
from one of these and the chemicals in solution
will be reported here . The followin g
questions were asked :
1 . What quantities of nitrogen, phosphorus ,
the cations (sodium, potassium, calcium ,
magnesium), and silica reach the site i n
precipitation?
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