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