Plant diversity enhances ecosystem responses to elevated CO2 and nitrogen deposition

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Abstract
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 unknown1,2. Reduced biodiversity has raised numerous concerns, including the possibility that ecosystem functioning may be affected negatively1,2,3,4, which might be particularly important in the face of other global changes5,6. 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.