CO2 enrichment increases element concentrations in grass mixtures by changing species abundances

Atmospheric carbon dioxide (CO2) enrichment may increase plant growth more than the uptake of chemical elements from soil. Increased CO2 also may alter element levels in biomass from multi-species vegetation by changing plant species abundances. We measured concentrations of ten elements in aboveground tissues of three C-4 grasses that had been exposed for 2-3 growing seasons to a continuous gradient in CO2 from 250 to 500 mu mol mol(-1). The grasses, Bouteloua curtipendula, Schizachyrium scoparium, and Sorghastrum nutans, are competitive dominants in assemblages of tallgrass prairie vegetation growing on each of three soil types along a field CO2 gradient in central Texas, USA. Our objective was to determine whether CO2 influences element concentrations in grass mixtures by changing concentrations in individual species or shifting species abundances. Increased CO2 had little effect on element concentrations in grasses compared to differences observed among grass species and soils. Increasing CO2 from the pre-Industrial to elevated levels reduced the phosphorus concentration in grasses grown on a clay and sandy loam soil. Concentrations of most other elements did not respond to CO2 treatment. Cover of the mid-grass Bouteloua declined at higher CO2 levels as cover of the taller grass Sorghastrum increased. Concentrations of several elements were lower in Bouteloua than Sorghastrum; hence, this exchange of species at higher CO2 increased element concentrations in grass assemblages. Potential consequences include an improvement in the nutritional quality of plants for herbivores. Results highlight the underappreciated impact that CO2 enrichment may have on ecosystem functioning by changing plant composition.