© 1986 Heron Publishing—Victoria, Canada
Effects of pretreatment conditions on ammonium and nitrate uptake by Douglas-fir seedlings
P. T. Rygiewicz (1, 2) and C. S. Bledsoe (1, 3)
1. College of Forest Resources AR-10, University of Washington, Seattle, Washington 98195, USA / 2. U.S. Environmental Protection Agency, Environmental Research Laboratory, Corvallis, OR 97333, USA / 3. Author to whom reprint requests should be addressed / Received January 14, 1986
Summary
Effects of pretreatment solutions containing varying concentrations of calcium, potassium, ammonium and nitrate were evaluated
by measuring fluxes of the same ions during a subsequent 4-hour uptake by Douglas-fir seedlings. Maximal rate of ammonium
uptake (11 μeq g–1 root dry weight h–1) was about 5 times faster than that of nitrate (2 μeq g–1 root dry weight h–1). Ammonium uptake was most rapid after pretreatment with low potassium levels and was unaffected by ammonium pretreatment.
Nitrate uptake was most rapid after pretreatment with high levels of nitrate and low levels of potassium. Calcium uptake was
greater when nitrate replaced ammonium as the N source. High calcium pretreatment levels depressed subsequent calcium uptake
or resulted in calcium release in both ammonium and nitrate experiments. Potassium efflux occurred with both N sources, but
the release was less during nitrate uptake than during ammonium uptake. Efflux of potassium is probably associated with the
high potassium status of the seedlings and the exchange between potassium and other cations. High levels of potassium in the
pretreatment solutions enhanced potassium efflux and caused a reduction in the subsequent rates of uptake of both ammonium
and nitrate. Length of pretreatment and seedling size generally did not affect uptake rates.
