A simple routine method for assessing the SO42- remobilization potential of forest soils for sulfate-sulfur from atmospheric deposition

A simple method to assess the SO42- remobilization potential of forest soil s for accumulated sulfate-sulfur from atmospheric deposition is presented. On the basis of three parallel batch extractions, (i) the total potential o f sulfate mobilization equal to the pool of inorganic sulfate (PTOT: extrac tion with 0.5 M. NH4F), (ii) the maximum potential of sulfate mobilization under conditions of a strong decrease of the sulfate concentration in the s eepage water to pre-industrial values (PMAX: extraction with deionized wate r at a solution:soil ratio of 50:1), and (iii) the present potential of sul fate mobilization at current sulfate concentrations in the seepage water (P REAL: extraction with deionized water at a solution:soil ratio of 5:1) can easily be assessed for any soil. The method was tested with 25 horizons of different Southern German forest soils. On average, the maximum potential o f SO42- mobilization of the studied soils was only 60 % of PTOT; indicating an irreversible retention of the remaining S pool under site conditions. B ased on the maximum SO42-. mobilization potential PMAX and the ratio PREAL/ PMAX, an evaluation scheme characterizing the intensity and type of SO42- m obilization from forest soils is presented. For most profiles, the amounts of extractable SO42- ranged between 5 and >100 kmole(c) ha(-1) m(-1) soil d epth, indicating a long-lasting future SO42- loading of forest soil seepage water by remobilization of soil-accumulated SO42-. A significant portion o f the soil SO42- pool will not be mobilized unless SO42- concentrations in the seepage water decrease considerably.