The rate of microbial decomposition of soil humic compounds: the effect of acid rain induced Al depletion. Running head: Acid rain and the decomposition of humic substances

The effect of aluminum complexation on the net decomposition rate of purified humic acid (HA) and fulvic acid (FA), extracted from a podzol B horizon in the Netherlands, were studied at room temperature (20°C). Net decomposition rates were determined from C02 evolution from samples as measured by gas-chromatography. Increasing aluminum additions significantly reduced the decomposition rate of HA, of FA ,and of organic matter in podzol B horizons, whereas pH had no notable effect on decompostion rates. Results indicate that at the current high depletion rates of organically-bound soil Al, due to acid rain, the decomposition of soil humic compounds may increase significantly. This may have serious consequences for the ecologically relevant pool of humic compounds in forest soils.