1 INTRODUCTION The increasing magnitude of anthropogenic impacts on the Earth over the last century has prompted the scientific community to intensify its efforts to under- stand natural variations in climate and ecosystems. Two topics of particular interest are: 1) areas im- pacted by acid rain, pollution, and logging and 2) the global carbon cycle, especially related to CO 2 forc- ing and climate. This study examines the effects of trees on soil chemistry and mineral weathering. Such research lies at the intersection of these two impor- tant areas because soil minerals are an important nu- trient source for forests, and the weathering rates of Ca and Mg silicate minerals play a key role in regu- lating CO 2 levels over geologic time. Furthermore, by focusing the investigation upon the giant sequoia, considered the largest and one of the longest-lived higher organisms on Earth, we maximized the prob- ability of documenting soil mineralogical changes in response to biota. 2 BACKGROUND In recent years, research driven by questions about the effects of acid deposition has discovered novel ways in which trees affect the chemical and minera- logical properties of soil. For example, van Breemen et al. (2000) found that plants and associated my- corrhizza may mine nutrients directly from minerals, affecting mineral properties while bypassing the bulk soil solution. Another study found evidence for pine and spruce trees directly accessing Ca from apatite grains in the absence of other Ca sources, making these trees well-adapted for soils depleted in base cations, whether through natural or anthropo- genic processes (Blum et al. 2002). Researchers also have studied the effects of trees on silicate mineral weathering rates. The goal of the studies has been to understand these effects because weathering of Ca and Mg silicates is thought to be the major controller of atmospheric CO2 levels on the million-year time scale (Berner 1997). Research- ers have found increased mineral weathering rates in aggrading (young, developing) ecosystems. Cochran & Berner (1996) showed at least an order of magni- tude increase in mineral weathering rates under plants on Hawaiian lava flows (<6000 years old) as compared to plant-free outcrops. In a multiple-year study of an aggrading ecosystem, Bormann et al. (1998) measured significantly higher weathering fluxes of the important plant nutrients Ca and Mg from a mesocosm containing young red pines com- pared to fluxes from an unvegetated mesocosm. While studies of aggrading ecosystems have been helpful, study of a steady state ecosystem seems necessary to provide better data about the effects of trees on mineral weathering rates over geologic time. Some researchers have argued that the rise of higher- order plants increased mineral weathering rates by up to three orders of magnitude over abiotic weath- ering (Schwartzman & Volk 1989). Other research- ers have countered that plants have, at most, in- creased mineral weathering rates by a factor of two, and may have decreased rates of mineral weathering on a geologic time scale (Drever 1994). A similar argument has pointed out that biological enhance- Effects of giant sequoia on soil chemistry J. Moore Department of Geosciences, Pennsylvania State University, University Park, PA USA A.F.White U.S. Geological Survey, Menlo Park, CA USA S.L.Brantley Department of Geosciences, Pennsylvania State University, University Park, PA USA ABSTRACT: The role of plants in governing rates of soil mineral weathering remains unknown. Studies of soil mineral weathering rates conducted in aggrading (young, developing) ecosystems have shown increased weathering in the presence of plants. Here we report preliminary observations from a developmentally steady state ecosystem dominated by giant sequoia (Sequoiadendron giganteum). Significant differences were found in the variation and distribution of bulk oxide composition in soils from giant sequoia root zones compared to soil from a control site outside a sequoia root zone. Sequoia root zone soils exhibited CaO and P 2 O 5 depletion, which may be the result of the loss of apatite and plagioclase feldspar in the soil.