Modeling the biogeochemistry of sulfur in beech (Fagus sylvatica L.) stands of the Vienna Woods

In this article, we describe the setup and the application of a novel Sulfur Dynamics Model (SDM), with the aim to identify and quantify processes, which are crucial for the understanding of the biogeochemical sulfur (S) cycle of forest ecosystems. The simulator takes into account abiotic processes as well as biotic interactions between atmosphere, plant and soil. We applied the model to two Austrian beech stands where deposition of S and soil solution chemistry were monitored intensively over a two-year period. Under consideration of high historic loads and the more recent recession of atmospheric S deposition, we found a suitable model configuration where it was possible to assign both intra-annual fluctuations of the SO(4)-S in soil solution and long-time trends in the stream discharge to specific S transformation processes. We identified the interplay of microbial immobilization (the microbial conversion of solute sulfate to organic soil S) and mineralization as key driver of short-term fluctuations in the soil solution. In the long term, the delayed release of historically accumulated S is driven mainly by the slow mineralization of S rich plant biomass, recalcitrant to decomposition. Adsorption and desorption processes seem to play only a negligible role on our investigated stands. We conclude that our proposed model which is based on the current understanding of S biogeochemistry is sufficient to describe S dynamics on the investigated forest stands. The code file (SAS) for all model functions will be provided by the authors after request.