TOXICITY FACTORS IN ACIDIC SOILS: ATTEMPTS TO EVALUATE SEPARATELY THE TOXIC EFFECTS OF EXCESSIVE A13+ AND H+ AND INSUFFICIENT CA2+ AND MG2+ UPON ROOT ELONGATION.

Authors: Kinraide, Thomas

Submitted to: European Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2003
Publication Date: 6/20/2003
Citation: KINRAIDE, T.B. TOXICITY FACTORS IN ACIDIC SOILS: ATTEMPTS TO EVALUATE SEPARATELY THE TOXIC EFFECTS OF EXCESSIVE A13+ AND H+ AND INSUFFICIENT CA2+ AND MG2+ UPON ROOT ELONGATION.. EUROPEAN JOURNAL OF SOIL SCIENCE. 2003. V. 54. P. 323-333.

Interpretive Summary: The forest soils in temperate, humid regions can be very acidic, especially when natural acidification is augmented by depositions from acidic atmospheric pollutants. Forest soil acidification is important because of its possible contribution to forest decline and to the altered chemistry of streams and lakes. In general, acidic soils have higher aluminum and acidity (Al3+ and H+) and lower calcium and magnesium (Ca2+ and Mg2+) than less acidic soils. The combination of high Al3+ and H+ and low Ca2+ and Mg generally inhibits root growth and plant health. The objective of this study was to determine the role and relative importance of these toxicity factors and to consider some implications for mechanisms of tolerance to acidic soils. The results of the study lead to the following conclusions. H+ intoxication and Ca2+ or Mg2+ insufficiency limit growth in very acidic forest soils. Direct Al3+ intoxication could not be confirmed, but if it occurs, it would achieve a maximum at intermediate soil acidities. Indirect effects of Al3+ include an induced deficiency of Ca2+ or Mg2+ and alleviation of H+ toxicity. These results suggest an alternative to Al3+ exclusion as the means of Al3+ tolerance.

Technical Abstract: High Al3+ and H+ and low Ca2+ and Mg2+ 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, publish 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. Al3+ activities ({Al3+}) in soil solutions peaked at pH 4.1 and achieved a broad maximum between pH 4.1 and 4.8 at the PM surface. {Al3+} declined at highe and lower pH values. Plant intoxication (assessed by root elongation) correlated negatively with soil-solution {Al3+}, but not PM-surface {Al3+} independent of Ca2+ and Mg2+ effects. Intoxication did not correlate with soil-solution {Ca2+} and {Mg2+} but did correlate with PM-surface activitie of those ions. Consequently, intrinsic Al3+ intoxication was not confirmed din these soils, but Al3+ appeared to be an extrinsic toxicant by driving d cell-surface {Ca2+}and {Mg2+}. H+ was unlikely to have been an intrinsic toxicant above pH 4.2. In contrast, solution-culture experiments, where problems of intercorrelation among {Al3+}, {H+}, and {Ca2+} can be avoided, clearly demonstrated multiple extrinsic and intrinsic effects of each ion, including intrinsic intoxication by Al3+. These findings have implications for the mechanisms by which plants tolerate very acidic soils.