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, C-3-C-4 grassland ecosystem in Colorado, USA. Cylinder harvests following each growing season and I monolith excavation at the end of the experiment provided data on root biomass, root C : N ratios, and root and soil delta(13)C 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.