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.