TOPOGRAPHY, EROSION, AND GEOCHEMICAL PROPERTIES OF TROPICAL SOILS, OSA PENINSULA, COSTA RICA

Soils in many tropical regions are subject to intense weathering as a result of high mean annual temperature and precipitation. Decomposition of primary minerals and leaching of mobile elements may produce soils lacking in rock-derived plant nutrients. The ability of such soils to support diverse and highly productive ecosystems is often attributed to efficient nutrient recycling, and more recently, to low but chronic inputs of some key nutrients from the atmosphere. We investigated the geochemical characteristics of soils at four sites on the Osa Peninsula of Costa Rica. The soils at all sites derive from the same parent-material, basalt of the Nicoya Complex. They are subject to similar intense weathering (MAT 26° C, MAP 4.5 m), and support similar vegetation. Topography and its resultant effect on duration of soil development appear to produce soils of substantially different mineralogical and geochemical properties. Soils on ridgetops and other topographically stable areas are intensely weathered and leached. Soils on rapidly eroding backslopes are at an earlier stage of development and often contain recognizable fragments of parent basalt. The eroding soils retain more of the nutrient elements leached from soils in stable positions. This relation implies a strong topographic control on soil geochemical properties and suggests the possibility that tectonic uplift influences the regional distribution of these soil types. We used strontium as a proxy for cation nutrients to assess their provenance in these forests. Strontium 87/86 isotopic ratios in soil leachates and vegetation from certain highly weathered soils (0.7034-0.7043) corresponded with parent rock (0.7036) and not with atmospheric inputs (0.7088) It appears that rock-derived elements still dominate biogeochemical cycling in these forests despite severe depletion of mobile elements in some soils. This pattern may also be driven by erosion as a soil-renewing process. Topography and rates of erosion appear to be controls on the geochemical evolution of soils on the Osa Peninsula with implications for the nutrition of the forests they support.