Spatial variability of sulfate isotherms in forest soils at different scales and its implications for the modeling of soil sulfate fluxes

To predict reversibility of soil and water acidification under conditions o f decreasing S-input into forested ecosystems, the amount of reversibly bou nd SO42- in the soil as well as its desorption behavior must be considered. This study investigated variability of sulfate isotherms and their spatial scaling. The Langmuir isotherm parameters in two forested catchments (one deciduous, one coniferous stand) and their spatial distribution at two different scal es (site and catchment scale) were investigated. The soil samples (250 cm(3 )) were taken in a systematic grid with spatial distances ranging from 20 x 20 m to 300 X 300 m, Isotherm parameters, soil pH, dithionite- and oxalate -extractable iron and aluminum, crystalline iron oxide, and ent were determ ined. Significant relationships were found between sulfate sorption isotherm para meters and soil chemical properties for each site. It would be useful to be able to replace the experimental determination of isotherm parameters with the less costly measurements of soil chemical properties such as pH, Al- a nd Fe-oxides, and C-org content of the soils. Regression analysis, however, resulted in different parameters for each site. Thus, at sites where this relation is unknown, isotherms have to be determined via soil extraction pr ocedures and cannot be predicted by soil chemical properties. To determine the average isotherms with an accuracy of +/- 0.2 mu mol g(-1) SO42- (sorb) , a sample size of 55 for the site scale and up to 106 for the catchment sc ale is required. The semivariance of the isotherm parameters revealed no spatial patterns. T he influence of isothenll variability on the prediction of SO42--fluxes wit h seepage was investigated using the chemical equilibrium model MAGIC. Fore cast accuracy depends on proper consideration of the variability of sulfate sorption in a catchment.