Plant diversity enhances ecosystem responses to elevated CO₂ and nitrogen deposition

Human actions are causing declines in plant biodiversity, increases in atmospheric CO₂ concentrations and increases in nitrogen deposition; however, the interactive effects of these factors on ecosystem processes are unknown. Reduced biodiversity has raised numerous concerns, including the possibility that ecosystem functioning may be affected negatively, which might be particularly important in the face of other global changes. Here we present results of a grassland field experiment in Minnesota, USA, that tests the hypothesis that plant diversity and composition influence the enhancement of biomass and carbon acquisition in ecosystems subjected to elevated atmospheric CO₂ concentrations and nitrogen deposition. The study experimentally controlled plant diversity (1, 4, 9 or 16 species), soil nitrogen (unamended versus deposition of 4 g of nitrogen per m² per yr) and atmospheric CO₂ concentrations using free-air CO₂ enrichment (ambient, 368 μmol mol^-1, versus elevated, 560 μmol mol^-1). We found that the enhanced biomass accumulation in response to elevated levels of CO₂ or nitrogen, or their combination, is less in species-poor than in species-rich assemblages.