PARENT SESSION
Oral Session #76: Plant Ecology: Nutrient Uptake, Movement, Use. Presiding: J. Schade.
Friday, August 10, 2001. 8:00 AM to 11:45 AM. Hall of Ideas G.

Toxicity factors in acidic forest soils. Implications for tolerance mechanisms.

Kinraide, Thomas¹, ¹

ABSTRACT- High Al and H and low Ca and Mg characterize the increasingly acidic soils of temperate forests and many pastures on former forest sites. To assess the relative importance of these potential toxicity factors, published data from soils collected in the United States and Europe were compiled. Activities of ions in the soil solutions and at the surfaces of root-cell plasma membranes (PM) were computed, using electrostatic models. Al activities ({Al}) in soil solutions peaked at pH 4.1 and achieved a broad maximum between pH 4.1 and 4.8 at the PM surface. {Al} declined at higher and lower pH values. Plant intoxication assessed by root elongation) correlated negatively with soil-solution {Al}, but not PM-surface {Al} independent of Ca and Mg effects. Intoxication did not correlate with soil-solution {Ca} and {Mg} but did correlate with PM-surface activities of those ions. Consequently, intrinsic Al intoxication was not confirmed in these soils, but Al appeared to be an extrinsic toxicant by driving down cell-surface {Ca}and {Mg}. H was unlikely to have been an intrinsic toxicant above pH 4.2. In contrast, solution-culture experiments, where problems of intercorrelation among {Al}, {H}, and {Ca2+} can be avoided, clearly demonstrated multiple extrinsic and intrinsic effects of each ion, including intrinsic intoxication by Al. These findings have implications for the mechanisms by which plants tolerate very acidic soils.

KEY WORDS: aluminum, calcium, forest, soil