Grassland species effects on soil CO₂ flux track the effects of elevated CO₂ and nitrogen

• Understanding and predicting the impacts of elevated atmospheric CO₂, elevated nitrogen deposition, and decreases in plant diversity require better understanding of the way in which plant species influence soil CO₂ flux. • In experimental grassland plots where atmospheric CO₂, nitrogen supply, and species composition and diversity were manipulated, species effects on soil CO₂ flux during 19 sampling periods over 2 yr were determined for 16 grassland species. • The average effect of a species on soil CO₂ flux was correlated with biomass of the species grown in monoculture, suggesting that effects of species on soil CO₂ flux are related to the potential productivity of a species and total belowground C allocation. During dry, warm conditions there is a greater effect of elevated atmospheric CO₂ on soil CO₂ flux and during these times deeper-rooted species contribute to soil CO₂ flux more than average. • Although differences in responses to elevated CO₂ and nitrogen among species were not great, decreases in diversity can affect belowground carbon allocation depending on the plant traits of the species that are lost from ecosystems.