Document: ALA-3-99-120

Biogeochemical patterns along gradients in soil type and land use history in southwestern Costa Rica: Comparisons to the central Amazon.

TOWNSEND, A.R.*, C.C.CLEVELAND, M.E.LEFER, G.P.ASNER and B.CONSTANCE

University of Colorado, Boulder, CO 80309 USA ¹

Abstract:
Costa Rica's Osa Peninsula presents a unique opportunity to explore biogeochemical dynamics in sites that include pristine moist tropical forests, active cattle pastures, and secondary forests with a range of prior land use histories. All of these vegetation types can be found on soils that range from old, highly weathered ultisols, to younger, nutrient- and organic matter-rich alluvial mollisols, to highly sandy soils derived from former beach ridges. We collected a variety of biogeochemical data in sites representing both soil type and land use gradients, including: total soil C, N and P, soil exchangeable nitrate, ammonium, phosphate, calcium, magnesium and potassium, foliar concentrations of N, P and all base cations in pasture grasses and selected forest leaves, and soil P fractions as determined by a modified Hedley procedure. Like many tropical systems on old soils, sites on ultisols suggest very low levels of plant-available P, while the lowland alluvial soils are comparatively rich in P and in all other nutrients analyzed. Like a set of forest-pasture age gradients we have previously studied in the central Amazon, we found strong decreases in labile forms of P with pasture age on the ultisols, but did not find a similar pattern in alluvial pastures. Moreover, unlike the Amazonian sites, there was little suggestion of significant decreases in total soil P with pasture age. In part, this difference may be due to the fact that repeated pasture burning is not common in this region of Costa Rica. Finally,unlike the old oxisols in the Amazonian sites, the oldest soils in the Costa Rican sites showed relatively high levels of all base cations, as did pasture grasses growing on these soils. Although the ultisols are highly weathered and therefore might be expected to be poor in all rock-derived nutrients, the high base cation levels are probably due to a combination of the original parent material and the proximity of all our sites to the Pacific Ocean.

Keywords: biogeochemical dynamics

This abstract is being presented at: 1:15 PM in session:
Oral Session #13: N Fixation and Biochemical Patterns.