THE SOLUBILITY OF ALUMINUM IN ACIDIC FOREST SOILS - LONG-TERM CHANGESDUE TO ACID DEPOSITION

Despite the ecological and pedogenic importance of Al, its solubility control in acidic forest soils is poorly understood. Here we discuss t he solubility of Al and its development with time in three acid brown forest soils in The Netherlands, which are under extreme acidification from atmospheric deposition. AH soil solutions (to a 60 cm depth) wer e undersaturated with respect to synthetic gibbsite (Al (OH)3; log K = 9.12 at 8-degrees-C), with the highest degree of undersaturation occu rring in the surface soil. In about one third of the individual soil l ayers a significant positive correlation existed between the activity of Al3+ and H+, but this relationship was far less than cubic. Kinetic ally constrained dissolution of Al is unlikely to explain the disequil ibrium with respect to gibbsite, because undersaturation was highest t hrough summer when water residence times were longest and temperatures greatest. Time series analysis of six year data sets for several soil layers revealed a significant annual decline in soil solution pH and Al solubility (defined as log Al + 3 pH) despite a constant concentrat ion of strong acid anions. The annual decline of both pH and Al solubi lity was greatest in the surface soil and was positively correlated wi th the relative depletion of reactive organically bound soil Al. The r esults support our earlier hypothesis that in strongly acidified fores t soils complexation by solid phase organics controls the solubility o f Al, even in mineral soil layers, relatively low in organic C. The da ta lend no support to the current widespread, and often uncritical use of gibbsite as a model for the Al solubility in highly acidic forest soils (pH < 4.5) of the temperate zone.