Calcium weathering in forested soils and the effect of different tree species

Soil weathering can be an important mechanism to neutralize acidity in forest soils. Tree species may differ in their effect on or response to soil weathering. We used soil mineral data and the natural strontium isotope ratio Sr-87/Sr-86 as a tracer to identify the effect of tree species on the Ca weathering rate. The tree species studied were sugar maple (Acer saccharum), hemlock (Tsuga Canadensis), American beech (Fagus grandifolia), red maple (Acer rubrum), white ash (Fraxinus Americana) and red oak (Quercus rubra) growing in a forest in northwestern Connecticut, USA. Three replicated sites dominated by one of the six tree species were selected. At sugar maple and hemlock sites the dominant mineral concentrations were determined at three soil depths. At each site soil, soil water and stem wood of the dominant tree species were sampled and analyzed for the Sr-87/Sr-86 ratio, total Sr and Ca content. Atmospheric deposition was collected and analyzed for the same constituents. Optical analysis showed that biotite and plagioclase concentrations were lower in the soil beneath hemlock than beneath sugar maple and suggested species effects on mineral weathering in the upper 10 cm of the mineral soil. These results could not be confirmed with data obtained by the Sr isotope study. Within the sensitivity of the Sr isotope method, we could not detect tree species effects on Ca weathering and calculated Ca weathering rates were low at all sites (< 60 mg m(-2) yr(-1)). We found a positive correlation between Ca weathering and the total Ca concentration in the surface soil. These results indicate that the absolute differences in Ca weathering rate between tree species in these acidic surface soils are small and are more controlled by the soil parent material (plagioclase content) than by tree species.