Plant and Soil Mediation of Elevated CO2 Impacts on Trace Metals

The cycling of trace metals through terrestrial ecosystems is modulated by plant and soil processes. Changes in plant growth and function and soil properties associated with increased atmospheric carbon dioxide (CO2) may therefore also affect the biological storage and stoichiometry of trace metals. We examined CO2 effects on a suite of metal micronutrients and contaminants in forest trees and soils at two free-air CO2 enrichment sites-a loblolly pine forest in North Carolina (Duke) and a sweetgum plantation in Tennessee [Oak Ridge National Laboratory (ORNL)]-and an open-top chamber experiment in a scrub-oak community in Florida [Smithsonian Environmental Research Center (SERC)]. We found that CO2 effects on soil metals were variable across sites; there were significantly higher surface soil metal concentrations with CO2 enrichment at Duke and ORNL (P < 0.05), but a trend of decreased soil metal concentrations at SERC (non-significant). These impacts on metals may be understood in the context of CO2 effects on soil organic matter (SOM); changes in percent SOM with CO2 enrichment were greatest at Duke (18% increase), followed by ORNL (7% increase), with limited effect at SERC (3% increase). There were significant effects of elevated CO2 on foliar metal concentrations at all sites, but the response of foliar metals to CO2 enrichment varied by metal, among sites, and within sites based on plant species, canopy height, and leaf age. Contrary to expectations, we did not find an overall decline in foliar metal concentrations with CO2 enrichment, and some essential plant metals were greater under elevated CO2 (for example, 28% increase in Mn across species and sites). Our results suggest that elevated CO2 impacts on trace metal biogeochemistry can be understood by accounting for both metal function (or lack thereof) in plants and the soil characteristics of the ecosystem.