Solid phase-solution-root interactions in soils subjected to acid deposition

Abstract

In managed forests, biomass utilization means a discoupling of the otherwise closed ion-cycle. The rate of proton production caused by the utilization of the timber, however, is of the same magnitude as the rate of proton consumption during silicate weathering. Managed forests can thus be in a steady state and stable. The input of acidity in forest ecosystems due to air pollution will in most cases exceed the rate of proton consumption by silicate weathering and thus result in soil acidification. Acidity can be accumulated as organic acids (mainly phenoles) and as cationic acids, that is, ions of sparingly soluble oxides (Al, Fe, Mn and heavy metals). The lower the pH , the higher is the solubility and toxicity of the acids existing. Owing to its high concentration and solubility, AlOOH produces the most important cation acid. A soil is composed of microcompartments in which different reactions can occur at the same time. Also proton production can be spatially inhomogeneous for the following reasons: (i) a considerable fraction of the input of acidity due to dry deposition of SO ₂ is buffered by the leaves of the trees. It reaches the soil via the roots during ion uptake and thus acidifies the soil close to the root tips. (ii) In acid soils, where the burrowing animals are missing, the soil organic matter formed from the root decomposition accumulates on the aggregate surfaces. It is thus in direct contact with the living roots. If during a temporal discoupling of the ion cycle (nitrification push) nitric acid is formed, this can acidify also, especially the soil close to the root surface. Thus in the direct vicinity of the roots, much higher Al-concentrations have to be expected than those which can be measured in equilibrium soil solution or in lysimeter solutions. A direct effect of Al-toxicity on the root system of trees is the die-back of the young roots, which has been shown both in vitro and in field experiments to be the result of action of Al ions. The mechanism of this action was found to be the inhibition of uptake of Ca ions into the matrix of the cell walls, which changes the macromolecular and physical properties of the pectin molecules which form the primary cell wall of meristemic and parenchymatic tissues in the root tip region.