MINERAL CYCLING IN .4 DOUGLAS FIR FOREST STAND
Phosphorus was selected in addition to the
monovalent potassium and divalent calcium
cations, These three elements can easily be
tagged with #P, §&Rband “Cafor tracer studies.
This paper will report only the radiological
aspects of movementofthe tracers through the
forest floor and surfacesoil, and upinto thetrees,
STUDY AREA
The studyis conducted in close cooperation
with the Fern Lake Program of the Radiation
Biology Laboratory, University of Washington,
Seattle, Washington.” Fern Lake is located
about 50 miles southwest of Seattle, Washing-
ton. The climate is moderate with a mean
annual temperature of 50°F and an average
annual precipitation of 45 in. The study plots
are located close to the lake in a dense stand of
40-y old Douglas fir trees (Pseudotsuga menziesii
(Mirb.) Franco) about 50 ft high. The fairly
level soil is an Alderwood-like gravelly sandy
lcam originated on glacialtill, The forest floor
is about | in. thick and has a 50 per cent moss
cover. The A, horizon is 4—5 in. deep. The pH
throughout the profile is about 5.3, and the
cation exchange capacity decreases from 12 meq/
100 g at the surface to 5 meq/100 g at a depth of
2 ft.)
METHODS
Each plot consists of a central dominant tree
without close competition from other dominants
(See Fig. 1). The forest floor and/or surface
soil of 225 in.? areas arranged aroundthe central
tree were sprayed with tracer solutions of known
specific activities. Tension lysimeters, installed
under the forest floor and 6-8 in. in the surface
soil, collected soil solutions. The tension lysimeter
consists of a porous plate that can be readily
inserted within a forest soil with a minimumof
disturbance to the overlying system. A tension
placed against the porous plate by a controlled
vacuum system eliminates the soil solution and
air interface problem inherent in the drainage of
most lysimeter designs.'§)
Uptake patterns of the tracers by the central
tree were obtained by periodically sampling
young foliage of midcrown positions. A steel
tower erected alongside the tree facilitated
access to the crown. The return movement of
mineral elements was sampled with litterfall
troughs placed radially from the tree, and a
rubber
channel
sealed
around
the «-4
Rainwash solutions from the troughs and -~ §
were led through mixed bed ion exchanger ;
columns which contained porous plastic 1
to prevent clogging of the resins.
Con, $
siphontube flowmeters were used to mit ;
the flow of solutionsat several samplingpo::
Thedischarge of the flowmeter by siphon ac:
cools a positive temperature coefficientre...
which then causes a relay to activate ane.c |
recorder. Incoming precipitation was meas...
by rain gauges placed on two of the ton-Forest floor and surface soil temperatures w4
1364
recorded continuously at one location in
forest stand. The radioactivity ofa few lysin-
solutions was recorded continuously with the +
of throughflow GM-tubes. A control plu:
monitor background activity due to radioa.fallout products in the samples was maint.
throughout the study.
During 1964, 12.5 per cent of thetotal rai.
activity per plot was applied on the forest 3.,
and 87.5 per cent directly on the surface of ++ |
mineral soil. The forest floor was separated a.
lifted off the application area with a board a:
replaced after treatment of the exposed .,
surface.
Thesolutions, exchangeresins and tree crows
were sampledperiodically. The solution volum,
were determined by weighing to prevent cr».
contaminations. Excess radioactive leachs +
were evaporated in the field in a barrel wit ,
floating heating element.!® A mercury sw:::
disconnected the heating element when =.
solution dropped below givenlevel.
One-liter samples were evaporated and wv.
dized in the laboratory and assayed for ra.
activity. The exchange resins were eluted ».
2--3 volumes of 10 per cent HNO. The ehuswere
evaporated
to
dryness,
oxidized a:
assayed for radioactivity. The foliage samiun
were slowly ashed at 450°C, taken up in \
HNO,, filtered and assayed. Calcium in:
foliage samples of 1964 wasprecipitated ou :
the oxalate method!to reduce the radioac:.
ity due to fallout products.
All samples of 1963 were assayed for i,
activity by the Radiation Biology Labora: -
During 1964 the gammaradiation of Rb 4. &
measured using a 4-in. scintillation crystal 2-. |
multichannel analyzer. Data on recovery