HR: 0830h
AN: B21F-0784    [PDF]
TI: Unexpected Dominance of Parent-Material Strontium in a Tropical Forest on Highly Weathered Soils.
AU: * Bern, C R
EM: Carleton.Bern@colorado.edu
AF: Institute of Arctic and Alpine Research, Campus Box 450 University of Colorado, Boulder, CO 80309-0450 United States
AU: * Bern, C R
EM: Carleton.Bern@colorado.edu
AF: Dept of Ecology and Evolution, Campus Box 334 University of Colorado, Boulder, CO 80309-0334 United States
AU: Townsend, A R
EM: alan.townsend@colorado.edu
AF: Institute of Arctic and Alpine Research, Campus Box 450 University of Colorado, Boulder, CO 80309-0450 United States
AU: Townsend, A R
EM: alan.townsend@colorado.edu
AF: Dept of Ecology and Evolution, Campus Box 334 University of Colorado, Boulder, CO 80309-0334 United States
AU: Farmer, G L
EM: farmer@terra.colorado.edu
AF: Dept of Geological Sciences, Campus Box 399 University of Colorado, Boulder, CO 80309-0334 United States
AB: Controls over nutrient supplies influence the basic structure and function of terrestrial ecosystems. Major plant nutrients supplied by mineral weathering (Ca, Mg, K) can be severely depleted in the highly weathered soils found in the tropics. Some recent studies have shown that as pools of rock-derived nutrients diminish, a transition occurs in which nutrients supplied by dust and precipitation become increasingly important. A state of near complete reliance on the atmosphere can occur on soils after as little as one million years of development. Such studies have relied heavily on strontium as a proxy for the nutrient elements of interest. We investigated sources of nutrients to a tropical forest in Costa Rica growing on a highly weathered soil derived from basaltic parent material 50-70 Ma in age. Base cations, including the strontium tracer, are severely depleted in the bulk and exchange pools of the upper soil profile. The close proximity of the ocean and rainfall in excess of 5m per year provide substantial inputs of atmospheric nutrients. Despite this, isotope ratios (87Sr/86Sr) indicate that $>$90% of actively cycling Sr is rock-derived. This result cannot be explained by inputs of continental dust, Central American tephra, or decoupling of Sr from the elements it is intended to trace. It places our sites on the opposite end of the transition from what previous studies would predict. Although the precise mechanisms responsible are currently unknown, our data suggest that variations in geomorphological and biological processes across systems with broadly similar climate and geology may lead to considerable variation in the dominant sources of key nutrients.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0315 Biosphere/atmosphere interactions
DE: 0330 Geochemical cycles
SC: Biogeosciences [B]
MN: 2003 Fall Meeting