Tree Species Drive Differences in Soil Acidity and Soil Carbon in the A Horizon Via Variation in Nutrient Stoichiometry of Different Plant Organs.

It is well established that different tree species can have unique influences on soil properties.  However, we still lack a mechanistic understanding of these effects.  Given on-going changes in dominant tree species composition due to climate change and other disturbances, this knowledge gap limits our ability to predict biogeochemical cycling in forests.  To facilitate a more mechanistic understanding of tree species impacts on soil biogeochemistry, we measured tree growth, plant litter fluxes, plant nutrient stoichiometry, and various aspects of soil chemistry in monospecific plots of 14 tree species planted at the same site in 1970-71.  Our analyses reveal a tight positive correlation between exchangeable soil acidity (H⁺, Al³⁺, Fe³⁺) and soil C in the A horizon, indicating that acidity exerts a dominant control over soil organic matter stabilization in the sandy soils at our site.  In turn, soil acidity appears to be largely a function of the nutrient stoichiometry of the overlying tree species.  Two aspects of plant nutrient stoichiometry appear to dominate the species effect on soil acidity at our site: 1) differences in nutrient uptake and storage among species (especially calcium and nitrogen), which influence the balance of cations and anions in soil, and 2) differences in nitrification among species (a proton generating process), which are driven by variation in root litter nitrogen content.  We propose that on similar soils (sandy, nutrient poor), tree species may have predictable influences on soil acidity and soil C based on measurable differences in the nutrient stoichiometry of above- and below-ground plant organs.  Similar studies are needed on additional species and a variety of soil types to test the breadth of relationships observed at our site.