The fate of carbon in grasslands under carbon dioxide enrichment

The concentration of carbon dioxide (CO₂) in the Earth's atmosphere is rising rapidly, with the potential to alter many ecosystem processes. Elevated CO₂ often stimulates photosynthesis, creating the possibility that the terrestrial biosphere will sequester carbon in response to rising atmospheric CO₂ concentration, partly offsetting emissions from fossil-fuel combustion, cement manufacture, and deforestation. However, the responses of intact ecosystems to elevated CO₂ concentration, particularly the below- ground responses, are not well understood. Here we present an annual budget focusing on below-ground carbon cycling for two grassland ecosystems exposed to elevated CO₂ concentrations. Three years of experimental CO₂ doubling increased ecosystem carbon uptake, but greatly increased carbon partitioning to rapidly cycling carbon pools below ground. This provides an explanation for the imbalance observed in numerous CO₂ experiments, where the carbon increment from increased photosynthesis is greater than the increments in ecosystem carbon stocks. The shift in ecosystem carbon partitioning suggests that elevated CO₂ concentration causes a greater increase in carbon cycling than in carbon storage in grasslands.