PARENT SESSION
Oral Session #76: Ecosystem Ecology: Larger scale processes, geomorphology, soils.
Presiding: W. Straw
Thursday, August 8. 9:00 AM to 11:30 AM. Gila Meeting Room, TCC.
Substantial weathering-derived sources of strontium in a lowland tropical rainforest on 40 million-year-old soils.
Bern, Carleton*,1,2, Townsend, Alan1,2, Farmer, G. Lang1, Cleveland, Cory1,2, 1 University of Colorado, Boulder, CO2 Institute for Arctic and Alpine Research, Boulder, CO
ABSTRACT- Traditional conceptual models of base cation cycling in terrestrial ecosystems suggest that weathering of primary minerals dominates the supply of newly available nutrients, with atmospheric inputs playing a secondary role. In contrast, recent evidence from the Hawaiian islands showed that as soils age, atmospheric inputs become increasingly important, reaching a point of complete dominance by soil ages in the low millions of years. Thus, one might expect the atmosphere to be the major - if not only - source of newly available calcium, magnesium and potassium in tropical forests on old, highly weathered soils, especially in coastal forests where ocean-derived inputs should be high. We tested this hypothesis in lowland, wet primary tropical forests on the Osa Peninsula of Costa Rica. All sites are within 5km of the coast, and span a range of soil types and ages from relatively young, Quaternary Mollisols, to ~3-4 million-year-old Ultisols, to highly weathered 40+ million-year-old Oxisols. We used strontium isotopes (87Sr/86Sr) to distinguish between local parent material and atmospheric sources of base cations; strontium is not an essential nutrient for plant growth, but is often used as a proxy for calcium and magnesium cycles. Surprisingly, results showed that soil and foliar values for Sr in the oldest site still reflect a strong influence of the mostly basaltic parent material, despite the proximity of the ocean, rainfall that exceeds 5m per year, and Kr/Ar dates for the parent material that range from 40-78 million years old. These data suggest that even seemingly subtle changes in parent material composition across tropical forests may lead to significant variation in the relative importance of atmospheric vs. weathering sources of rock-derived nutrients.
KEY WORDS: strontium, cation cycling, tropics