How do climate warming and species richness affect CO2 fluxes in experimental grasslands?

• This paper presents the results of 2 yr of CO₂ flux measurements on grassland communities of varying species richness, exposed to either the current or a warmer climate. • We grew experimental plant communities containing one, three or nine grassland species in 12 sunlit, climate-controlled chambers. Half of these chambers were exposed to ambient air temperatures, while the other half were warmed by 3°C. Equal amounts of water were added to heated and unheated communities, implying drier soils if warming increased evapotranspiration. Three main CO₂ fluxes (gross photosynthesis, above-ground and below-ground respiration) were measured multiple times per year and reconstructed hourly or half-hourly by relating them to their most important environmental driver. • While CO₂ outputs through respiration were largely unchanged under warming, CO₂ inputs through photosynthesis were lowered, especially in summer, when heat and drought stress were higher. Above-ground CO₂ fluxes were significantly increased in multispecies communities, as more complementary resource use stimulated productivity. Finally, effects of warming appeared to be smallest in monocultures. • This study shows that in a future warmer climate the CO₂ sink capacity of temperate grasslands could decline, and that such adverse effects are not likely to be mitigated by efforts to maintain or increase species richness. [ABSTRACT FROM AUTHOR]