© 1991 Heron Publishing—Victoria, Canada
Differential adsorption of Al, Ca, and Mg by roots of red spruce (Picea rubens Sarg.)
Christopher S. Cronan
Department of Botany and Plant Pathology, University of Maine, Orono, Maine 04469, USA / Received May 10, 1990
Summary
The surface adsorption characteristics of red spruce (Picea rubens) roots were examined as a function of changes in external acidity and cation concentrations. Root cation exchange capacity
varied significantly with changes in pH, increasing from 110 μmolc g–1 at pH 3.5 to 155 μmolc g–1 at pH 4.5, and reaching 250 μmolc g–1 at pH 7.0. In general, Al adsorption by spruce roots was much greater than either Ca or Mg adsorption under the same initial
conditions. However, root affinity for the divalent cations was proportionately much more sensitive to pH changes than was
root affinity for Al. The fractions of adsorbed Ca and Mg increased by 50 to 100% as pH increased from 3.5 to 4.5, whereas
the fraction of adsorbed Al remained relatively constant at both initial pH conditions. Competition experiments at pH 3.5
and 4.5 indicated that Al adsorption was strongly favored over Ca adsorption, except at low Al concentrations (~10 μmol l–1), high solution Ca2+/Al3+ ion activity ratios (> 1.5 to 5.0), and at the higher pH. These results suggest that cell wall exchange sites in red spruce
roots will tend to become progressively saturated with Al under the prevailing conditions of many acidic forest soils. To
the extent that root adsorbed Al interferes with the active uptake of Ca and Mg, this process of competitive cation adsorption
can contribute to impaired mineral nutrition in the spruce forest community.
