In many forest ecosystems, the rate of release of Ca and other mineral nutrients through chemical weathering far exceeds the rate of input from the atmosphere. Thus, weathering is the primary mechanism by which mineral nutrient losses in streamwater leaching or from forest harvesting are replaced. Recent research has focused attention on the role of Ca and other basic cations (CB) in the acidification of freshwaters, ecosystem recovery from acidification, long-term forest productivity, fish mortality and the decline of red spruce in montane forests. Our poor understanding of the mechanisms and rates of weathering release of CB limits our ability to critically evaluate observed and predicted changes in element cycles. This study is aimed at determining the mechanisms and rates of weathering in two base- poor forest watersheds in New Hampshire. The project is designed to study weathering on the scale of individual minerals, the soil profile and the watershed. Several methods of estimating weathering rates will be used and compared, including geochemical mass balances, mineral depletion estimates, watershed element balances and Sr isotope analysis. By evaluating the contributions of individual minerals to watershed weathering rates and studying the patterns of weathering within the soil profile, our ability to predict changes in CB cycling and forest productivity under future atmospheric conditions will be improved.
