Fractionation of sulfur (S) in beech (Fagus sylvatica) forest soils in relation to distance from the stem base as useful tool for modeling S biogeochemistry

The investigation of the fractionation of S compounds in forest soils is a powerful tool for interpreting S dynamics and S biogeochemistry in forest ecosystems. Beech stands on high pH (nutrient-rich) sites on Flysch and on low pH (nutrient-poor) sites on Molasse were selected for testing the influence of stemflow, which represents a high input of water and dissolved elements to the soil, on spatial patterns of sulfur (S) fractions. Soil cores were taken at six distances from a beech stem per site at 55 cm uphill and at 27, 55, 100, 150 and 300 cm downhill from the stem. The cores were divided into the mineral soil horizons 0-3, 3-10, 10-20, 20-30 and 30-50 cm. Soil samples were characterized for pH, Corg, pedogenic Al and Fe oxides and S fractions. Sequential extraction by NH4Cl, NH4H2PO4 and HCl yielded readily available sulfate-S (RAS), adsorbed sulfate-S (AS) and HCl-soluble sulfate-S (HCS). Organic sulfur (OS) was estimated as the difference between total sulfur (ToS) and inorganic sulfur (RAS + AS + HCS). Organic sulfur was further divided into ester sulfate-S (ES, HI-reduction) and carbon bonded sulfur (CS). On Flysch, RAS represented 3-6%, AS 2-12%, HCS 0-8% and OS 81-95% of ToS. On Molasse, RAS amounted 1-6%, AS 1-60%, HCS 0-8% and OS 37-95% of ToS. Spatial S distribution patterns with respect to the distance from the tree stem base could be clearly observed at all investigated sites. The presented data is a contribution to current reports on negative input-output S budgets of forest watersheds, suggesting that mineralization of OS on nutrient rich soils and desorption of historic AS on nutrient-poor soils are the dominant S sources, which have to be considered in future modeling of sulfur.