Increased levels of aluminium in forest soils: Effects on the stores of soil organic carbon

Acid atmospheric deposition results in increased levels of mobile aluminium (AI) in forest soils. Laboratory studies suggest that increased binding of Al to soil organic matter (SOM) in the forest floor results in decreased mobility of organic matter in soil water, viz. lower concentrations of dissolved organic carbon (DOC). Other laboratory studies indicate decreased decomposition rates of SOM as a result of Al binding. So far, little field evidence supporting these effects of Al on the lability of SOM have been reported. Here we present a field manipulation experiment in mature Norway spruce forest in Norway, where the content of Al in soil and soil water was increased. Increased Al in the forest floor caused a pronounced decrease in the leaching of DOC. Simultaneously, the decomposition rate of SOM decreased by 30% to 40%. This suggests that elevated Al in the forest floor stimulates accumulation of SOM. In a companion paper we present the effect of increased Al on forest vitality.