Rhizodeposition stimulated by elevated CO₂ in a semiarid grassland

• Rhizodeposition, or the addition of C from roots to soil C pools, is expected to increase if net primary production is stimulated and some excess C is allocated below-ground. We investigated the effects of 5 yrs of elevated CO₂ on below-ground C dynamics in a native, C₃-C ₄ grassland ecosystem in Colorado, USA. • Cylinder harvests following each growing season and monolith excavation at the end of the experiment provided data on root biomass, root C:N ratios, and root and soil δ¹³C values. We applied an isotopic mixing model to quantify new soil C inputs on elevated and ambient CO₂ treatments. • Root biomass increased by 23% and root C:N ratios increased by 26% after 5 yrs of elevated CO₂. Species-specific differences were found in root residence times, which ranged from 6 to 8 yrs. • Rhizodeposition was roughly doubled in elevated compared with ambient CO₂ chambers, at 83 ± 16 versus 35 ± 9 g C m^-2 yr^-1 over the last 4 yrs of the experiment (t-test, P = 0.006). Net C sequestration will depend on how decomposition rates are altered by elevated CO₂.