Land uses and land cover changes frequently alter key ecosystem processes such as productivity and decomposition, with potential implications for the carbon cycle. In the southern Great Plains (USA), invasion of grassland systems by mesquite (Prosopis glandulosa), an N₂-fixing woody species, is often managed through prescribed burning. As part of a broader effort to understand the effects of woody encroachment and prescribed fire on ecosystem carbon dynamics, we employed long-term laboratory incubations to quantify soil organic carbon (SOC) pools and their turnover rates in soils collected beneath grasses and woody vegetation in 3 long-term fire treatments (unburned controls, winter fire, and summer fire). Respiration kinetics were fit to parallel first-order decay models to estimate carbon pool (C_active and C_slow) sizes and fluxes. Both fire and vegetation exerted important controls over the magnitude and turnover rates of SOC. Regardless of fire treatment, total SOC, soil total N, and C_slow pools increased by 15– 20% under mesquite and were significantly larger than corresponding pools from soils collected beneath grasses (p< 0.05). Winter and summer fire treatments resulted in increased SOC beneath grasses but not beneath mesquite. The size of the C_active pool was consistent across vegetation types and fire treatments, accounting for 1.5-3% of total SOC. Winter fire treatment soils had significantly higher C_active turnover rates than those from summer fire treatments or controls (p < 0.05), paralleling trends of increased net primary productivity and soil microbial biomass that we have observed following winter fire.  Both woody plant encroachment and attempts to manage encroachment with fire are likely to have significant impacts on the carbon balance of Great Plains grasslands.