PARENT SESSION
Oral Session 110: Biogeochemistry VII: Soil I and Nutrient Cycling I.
Presiding: K Lajtha
Thursday, August 5, 1:30 PM to 5:00 PM, Meeting Room A 106.

Dominance of parent-material strontium in a tropical forest on highly weathered soils.

Bern, Carleton^{*,1, 2}, Townsend, Alan^{1, 2}, Farmer, G.², ¹ Institute for Arctic and Alpine Research, Boulder, CO, USA² University of Colorado, Boulder, CO, USA

ABSTRACT- A new paradigm regarding the ultimate source of rock-derived nutrients to ecosystems has emerged in recent years. It states that in situ weathering inputs of rock-derived nutrients decline as soils age and parent material stocks are depleted, thus increasing in relative importance small atmospheric inputs from dust and sea-salt aerosols. Given sufficient time, atmospheric inputs can come to dominate the system. Under humid tropical conditions that promote fast weathering, atmospheric dominance has been demonstrated on soils as young as one million years. Much of the evidence for the shift from rock-derived to atmospherically-derived base cation nutrients has relied on natural abundance of strontium isotopes (⁸⁷Sr/⁸⁶Sr) to trace base cation sources. We used this tracer to examine sources of base cations to tropical forests in Costa Rica growing on highly weathered soils derived from basalt. Base cations, including strontium, are severely depleted in soil pools when compared to those in the parent material. Close proximity to the ocean and rainfall in excess of 5 m per year provide ample atmospheric inputs of strontium. Despite these facts, isotopic ratios indicate that >90% of the actively cycling strontium is rock-derived. We suggest this pattern is best explained by a small but chronic input of rock-derived strontium via erosion, that out-competes atmospheric inputs. Rapid tectonic uplift and rapid erosion due to climate and biota at our sites combine to produce soils that are relatively young despite characteristics that suggest advanced development. These results suggest the potential for considerable variation in the main sources of rock-derived nutrients across forests growing on highly weathered soils.

Key words: parent material, strontium, atmospheric nutrients