ALUMINUM IN SOIL SOLUTIONS OF FOREST SOILS - INFLUENCE OF WATER-FLOW AND SOIL ALUMINUM POOLS

The Al3+ saturation of soil solutions of acid forest soils (defined as pAl(3+)-3pH) is highly variable with time and soil depth under field conditions. We hypothesized that the release of Al from soil solids in to the soil solution is kinetically restricted. This hypothesis was te sted by evaluating soil solution data from a field site and in an expe riment with large undisturbed soil columns from two acid forest soils. Soil columns of 40-cm diameter and 90-cm height were established in s is replicates at 10 degrees C and irrigated in a steady-state flow wit h an artificial acid throughfall (pH 3.5 and 2.8) at rates of 1, 4, an d 12 mm d(-1). Soil solutions from the columns as well as in the field mere taken by suction lysimeters from various depths and the Al3+ sat uration of the soil solutions was related to the extractable soil AL p ools and water now rates. Under field conditions, undersaturation was related to high,vater now rates through the soil profile at both the 2 0- and 90-cm depths. This was attributed to preferential flow in the s oil profile. In contrast with the field results, Al3+ saturation of so il solutions from the column experiments was not affected by flow rate despite a large variation in flow rate, acid input, and ionic backgro und. In the column study, the Al3+ saturation differed between soils, soil horizons, and replicates; increased with sob depth; and was under saturated with respect to gibbsite in the upper soil profile. The Al3 saturation of soil solutions was spatially highly variable both in th e field and in the column experiment corresponding to the heterogeneit y of soil extractable Al pools. The relation of soil extractable Al to the Al3+ saturation was different for the two soils. The modeling of soil solution Al and pH under equilibrium assumptions and without kine tic constraints is rather uncertain when short-term variations and dif ferent soil horizons are considered.