THE FATE OF CARBON IN GRASSLANDS UNDER CARBON-DIOXIDE ENRICHMENT

The concentration of carbon dioxide (CO2) in the Earth's atmosphere is rising rapidly(1), with the potential to alter many ecosystem process es. Elevated CO2 often stimulates photosynthesis(2), creating the poss ibility that the terrestrial biosphere will sequester carbon in respon se to rising atmospheric CO2 concentration, partly offsetting emission s from fossil-fuel combustion, cement manufacture, and deforestation(3 ,4). However, the responses of intact ecosystems to elevated CO2 conce ntration, particularly the below-ground responses, are not well unders tood. Here we present an annual budget focusing on below-ground carbon cycling for two grassland ecosystems exposed to elevated CO2 concentr ations. Three years of experimental CO2 doubling increased ecosystem c arbon uptake, but greatly increased carbon partitioning to rapidly cyc ling carbon pools below ground, This provides an explanation for the i mbalance observed in numerous CO2 experiments, where the carbon increm ent from increased photosynthesis is greater than the increments in ec osystem carbon stocks. The shift in ecosystem carbon partitioning sugg ests that elevated CO2 concentration causes a greater increase in carb on cycling than in carbon storage in grasslands.