METAL DISTRIBUTION ACROSS DIFFERENT POOLS IN THE ORGANIC LAYER OF A FOREST UNDER ACID DEPOSITION AND ITS CONSEQUENCES FOR THE METAL DYNAMICS

Acid atmospheric deposition can cause losses of metal nutrients from t he organic layer of a soil. The size of these losses depend on the siz es of the different pools in which the metals are present, as these po ols differ in mobility. The metal pools in an organic soil layer of a Douglas fir forest in the Netherlands subjected to acid deposition wer e determined by means of extractions and percolations. Na was mainly d issolved and exchangeably adsorbed, K dissolved, exchangeably adsorbed and present in the soil microbial biomass, Ca exchangeably adsorbed a nd present in organic precipitates, Mg exchangeably adsorbed and prese nt in the soil biomass, and Mn exchangeably adsorbed and present in in organic precipitates. The main part of the metals was exchangeably ads orbed. The adsorption affinity increased in the order Na < K < Mg < Mn approximate to Ca. The vertical distribution of the metals in the org anic layer showed that all metals were continuously lost from the orga nic layer. The differences between the metals in retention and vertica l distribution patterns were in agreement with their differences in de position rate, pool distribution, and exchange affinity. Since the met als were mainly exchangeably adsorbed, and the acidifying cations domi nated the atmospheric deposition, acid deposition and cation exchange must be processes that strongly affect the losses of metals from this organic soil layer.