Rock-derived Micronutrient Transport in the Tropics: Molybdenum Cycling in Deeply-weathered Panama Soils

The rock-derived micronutrient Molybdenum (Mo) is important in both terrestrial and ocean ecosystems as an essential cofactor in nitrogenase, the enzyme used by microorganisms to fix atmospheric nitrogen. As global nitrogen fixation rates respond to increasing atmospheric pCO2, the weathering, transport, and availability of this micronutrient becomes 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 than previously realized, but the weathering and cycling of these elements as they pass across ecosystem boundaries remains poorly understood. This work explores the sources and sinks of Mo in small (36-176ha) single land cover catchments in tropical Panama. Water samples were collected from precipitation, canopy throughfall, soil water, groundwater, soil seeps, and first- order through higher-order streams. Though Mo is considered a “rock-derived” micronutrient, concentrations were higher in precipitation and shallow soil water than in the groundwater and stream waters in contact with underlying rocks and weathered saprolite. Event-based mass balance suggests that Mo is being retained within the catchment ecosystem. The source of Mo in tropical forests may have important implications as the amount of nitrogen fixation changes during tropical forest recovery from a previous land use.