The dynamic pattern of soil water transport is a major factor in determining the chemistry of streamwater. In the acidified Birkenes catchment (southernmost Norway) the streamwater chemistry is, to a first approximation, explained by mixing solutions from the forest floor, the B horizon and the deep peat, in various proportions depending on the hydrological conditions. Paradoxically, a direct physical contact between the forest floor and the B horizon on the one hand and the stream on the other is lacking, as the stream banks largely consist of peats. To investigate this paradox, soils and their levels of exchangeable cations were studied in a 100 m × 100 m grid. Results indicate that the exchange sites of the surface peat along the stream are significantly enriched in Al, probably due to return flow of Al‐rich B horizon water. This view is supported by the similarity of the solution chemistry in surface peats and B horizons. Exchangeable base cations dominate in the forest floor upslope. Forest floor solutions, an important component of streamwater during intensive storms, are depleted in Al and may bypass the A‐enriched surface peats via ephemeral flow channels. A parallel study in a pristine catchment in mid‐Norway shows a similar accumulation of Al in return flow areas. This indicates that acid deposition is not a prerequisite for elevated levels of exchangeable Al in the surface organic layers of return flow areas.

The Al3+ saturation of soil solutions of acid forest soils (defined as pAl3+‐3pH) is highly variable with time and soil depth under field conditions. We hypothesized that the release of Al from soil solids into the soil solution is kinetically restricted. This hypothesis was tested by evaluating soil solution data from a field site and in an experiment with large undisturbed soil columns from two acid forest soils. Soil columns of 40‐cm diameter and 90‐cm height were established in six replicates at 10°C and irrigated in a steady‐state flow with an artificial acid throughfall (pH 3.5 and 2.8) at rates of 1, 4, and 12 mm d−1. Soil solutions from the columns as well as in the field were taken by suction lysimeters from various depths and the Al3+ saturation of the soil solutions was related to the extractable soil Al pools and water flow rates. Under field conditions, undersaturation was related to high water flow rates through the soil profile at both the 20‐ and 90‐cm depths. This was attributed to preferential flow in the soil profile. In contrast with the field results, Al3+ saturation of soil solutions from the column experiments was not affected by flow rate despite a large variation in flow rate, acid input, and ionic background. In the column study, the Al3+ saturation differed between soils, soil horizons, and replicates; increased with soil depth; and was undersaturated with respect to gibbsite in the upper soil profile. The Al3+ saturation of soil solutions was spatially highly variable both in the field and in the column experiment corresponding to the heterogeneity 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 kinetic constraints is rather uncertain when short‐term variations and different soil horizons are considered.

Summary The predicted activity of Al in the soil solutions of acid forest soils often differs from that observed in the field. We have investigated the influence of soil structure and flow rate of the soil solution on the aluminum release to explain this divergence. Disturbed and undisturbed samples of soil were collected from the A and B horizons of a dystric cambisol at Waldstein (Fichtelgebirge, Germany). The samples were irrigated with solutions mixed according to field data on throughfall or soil solution composition with pH 3.5 with flow rates of 4 mm d−1, 12 mm d−1 and 36 mm d−1. The percolates were analysed for major ions. Resulting relations between pH and pAl were compared with batch experiments. In neither the A horizon nor in the B horizon did soil structure influence the relation between pH and pAl. The apparent equilibrium between pH and pAl was described as the pKapp value with pKapp= pAl—a pH (where a is an empirical constant). It was found that the pKapp values for the column percolates were in the range of variation of those found in batch experiments. Flow rate had no influence on pKapp at 4 and 12 mm d−1. At 36 mm d−1 a significant increase of pKapp was observed. This relative undersaturation of Al was more pronounced in the A horizon than in the B horizon. When flow is fast Al release into the percolating soil solution might be limited by diffusion.