I recently ran a forest regression model, this model showed me that increasing soil pH increases sulphur isotope variation across the landscape. I am not how this is possible and if it makes sense
Sulfur has a natural variation in the proportions of its isotopes (consider volcanoes, for example, which act as natural isotope fractionators). So the variation due to biochemical action is expected.
The United States Geological Survey (USGS) discusses the use of sulfur (and oxygen) isotope ratio to determine the source of sulfur, temperature and pH in forests.
The National Center for Biotechnology Information (NCBI) discuses the fractionation of sulfur isotopes in beech forests: "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." This seems to me to explain the results of your tests.