IMPACTS OF ACID DEPOSITION ON CONCENTRATIONS AND FLUXES OF SOLUTES IN ACID SANDY FOREST SOILS IN THE NETHERLANDS

This article summarizes the most important impacts of acid atmospheric deposition on the soil solution chemistry of acid sandy forest soils in the Netherlands, by comparing and interpreting data from soil solution monitoring studies (18 stands) and a national soil solution survey (150 stands). Major conclusions are: (i) in forest subsoils there is almost a 1:1 relationship between the concentrations of SO4 and NO3 and those of H and Al, (ii) presumed critical Al concentrations (0.2 mol(c) m-3) and Al/Ca ratios (1.0 mol mol-1) are generally exceeded below 20 cm soil depth whereas NH4/K ratios sometimes exceed a presumed critical value (5 mol mol-1) in the upper 20 cm, (iii) SO4, NO3, Al and H concentrations decrease in the order: spruce forests > pine forests > deciduous forests, (iv) forest soils are invariably saturated with SO4 (leaching equals deposition), (v) there is generally a relative large retention of N although there are indications that forest soils become N-saturated at total N inputs above 4.0 kmol(c) ha-1 yr-1, (vi) NH4 leaching is generally negligible but may become substantial above an NH4 input of 3.0 kmol(c) ha-1 yr-1, and (vii) Al mobilization is the major buffer mechanism neutralizing the acidity produced from N transformations. Conclusions (i), (iv) and (vi) imply that a decrease in atmospheric inputs of S and N will give a nearly equivalent decrease in (H and) Al leaching.