THE INFLUENCE OF ROCK TYPE, FOREST COMMUNITY, AND TOPOGRAPHIC POSITION ON SOIL NUTRIENTS AND SOLUTE FLUXES IN THE LUQUILLO MOUNTAINS OF PUERTO RICO

Tropical small mountainous rivers (SMRs) have garnered attention given their ability to transport a disproportionate quantity of silicate weathering products to the ocean. However, within this geomorphic setting there can be substantial variation in lithology, biota, and topography, all of which may influence soil properties and thus the fluxes of riverine solutes. Surprisingly, only a few studies have simultaneously tested the effects of these factors in the same observational framework. As a first step towards linking landscape-level controls on soil properties with solute export, we explored the influence of parent material, forest community composition, and topographic position on several soil properties relevant to ecosystem functioning and our understanding of silicate weathering at the Luquillo LTER and CZO in Puerto Rico. We sampled soils across a full factorial combination of two parent materials (granodiorite and volcaniclastics), two forest types (Tabonuco and Colorado) and three topographic positions (ridge, midslope, valley). Here we present three types of dependent variables: soil elemental chemistry, soil elemental losses relative to parent material, and the fraction of atmospheric Sr held in the soil exchangeable cation pools (as measured by ⁸⁷Sr/⁸⁶Sr). We find that while some factors (e.g. soil P or Si content) are driven by differences in parent material, topographic position is more important for other properties (e.g. degree of soil elemental depletion relative to parent material). Furthermore, individual catenas vary in their fraction atmospherically-derived Sr despite similarities in parent material, topography and forest type. Finally, storm sampling of an experimental watershed within the study area for ⁸⁷Sr/⁸⁶Sr revealed a flow dependent shift between atmospheric and lithologic sources thereby confirming the integral role of these two end-member sources on weathering fluxes.