THE EVOLUTION OF MINERAL WEATHERING-RATE ESTIMATES IN THE HUBBARD BROOK ECOSYSTEM STUDY

The Hubbard Brook Ecosystem Study was founded forty years ago, based on measurement of water, chemical and energy budgets at the small watershed scale. From the beginning it was recognized that one of the most valuable contributions of these multidisciplinary studies would be the ability to estimate mineral weathering rates in a natural field setting, a long standing geochemical problem with important ecological implications. Early studies estimated mineral weathering, or more accurately, chemical denudation, based on assumptions that biological and available soil pools were at steady state. This approach was gradually abandoned as dynamics of forest biomass and soil pools were recognized, resulting in a reduction of mineral weathering rate estimates by an order of magnitude. Refinement of the mass balance approach, through the use of isotopic tracers, mineral stoichiometry, and ratio techniques confirmed the contribution of biological and soil pools to net ecosystem losses, consistent with lower mineral weathering rate estimates. Recent studies have explored the role of direct biologic enhancement of mineral weathering, which may bypass soil exchange pools as a source of plant nutrients, suggesting the possibility of higher mineral weathering rates. Alternate hypotheses propose tight recycling of organic nutrient sources, and destabilization of secondary mineral pools as important sources of nutrients to support forest growth, consistent with lower mineral weathering rate estimates. Resolution of these alternate hypotheses has important implications for understanding the geochemistry of mineral weathering in temperate forest soils, as well as the impacts of anthropogenic disturbances, such as acid deposition and forest harvesting.