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 C₄ grasses that had been exposed for 2–3 growing seasons to a continuous gradient in CO₂ from 250 to 500 μmol 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.