Decades of plowing have depleted organic carbon stocks in many agricultural soils. Conversion of plowed fields to pasture has the potential to reverse this process, recapturing organic matter that was lost under more intensive cropping systems. Temperate pastures in the northeast USA are highly productive and could act as significant sinks for carbon dioxide. However, such pastures have relatively high assimilate partitioning to shoots, the majority of which is removed as hay or consumed by grazing animals. In addition, the ability of pastures to sequester carbon dioxide decreases over time as previously depleted stocks are replenished and the soil returns to equilibrium conditions. Eddy covariance systems were used to quantify carbon dioxide fluxes over two fields in Central Pennsylvania that have been managed as pastures for at least 40 years. Flux measurements over six site-years suggested that the pastures were acting as a net carbon dioxide sink of -152 g CO₂ m^-2 y^-1 (range 296 to -808). However, when biomass removal was taken into account the pastures were a net source to the atmosphere of 313 g CO₂ m^-2 y^-1 (range 722 to -84). Returning manure from the hay that was consumed off site would have partially replenished the lost carbon, but the pastures would have remained net sources. Heavy utilization of the biomass produced on these mature pastures prevented them from acting as carbon sinks.