ROCK-DERIVED MICRONUTRIENT FLUXES AND WEATHERING IN MOUNTAINOUS PANAMA RIVERS

The rock-derived micronutrients Molybdenum (Mo) and Vanadium (V), are important in both terrestrial and ocean ecosystems as essential cofactors in nitrogenase, the enzyme used by microorganisms to fix atmospheric nitrogen, as well as in metalloenzymes required in nitrate reductase. As global nitrogen fixation rates respond increasing atmospheric pCO₂, the weathering, transport, and availability of these micronutrients become essential, because Mo limitation on nitrogen fixation has been documented in diverse ecosystems, including tropical soils in Panama. Therefore, a thorough understanding of the carbon and nitrogen cycles may depend more on the availability of trace metals such as Mo and V than previously realized. Many investigations have examined the effect of trace metal fertilization on biological nitrogen fixation. However, the transformation of these elements as they pass across ecosystem boundaries is less well understood. This work explores the effect of landuse and lithology on the weathering, transport, and availability of these biologically important elements in small (40-153 hectares) single-landuse catchments representing forested and pasture landuses. Additionally, a larger paired watershed study of the Rio Chagres (414km² of old growth forest) and the adjacent Rio Pacora (330 km² of primarily subsistence agriculture) in central Panama will be discussed. Water samples were collected from precipitation, forest throughfall, soil water, groundwater, soil seeps, and first-order through high-order streams. In order to explore changes in fractionation and possible biological availability of these metals through the catchment, unfiltered samples were collected in addition to field-filtered samples for both dissolved and colloidal fractions. These data will be combined with precipitation and flow data to describe the weathering and fluxes of Mo and V through the hydrologic reservoirs in the catchments.