Tundra soils contain large amounts of organic carbon (C) that might become available to microbial decomposition as soils warm. To elucidate the C sources currently sustaining CO2 emissions from striped tundra soils (soil respiration) in Northwest Greenland, we studied the seasonal pattern and radiocarbon (C-14) signature of soil respiration and of CO2 within the pore space, respired from roots and non-root associated microbes, and of bulk soil organic matter. Old C pools are present in the topsoil of both barren ridges (1000-5000 yrs) and vegetated troughs (modern to 600 yrs). Before leaf-out, soil respiration was depleted in C-14 relative to atmospheric CO2, root and microbial respiration within the topsoil, demonstrating a substantial contribution of C fixed before 1950. As the growing season progressed, the contribution of older C pools decreased, but remained apparent in the soil respiration from ridges and in pore space CO2. Soil respiration from troughs became dominated by recently fixed C. As the active layer deepens with permafrost thaw, buried C may become an increasingly larger component of soil respiration. Detecting microbial decomposition of older C pools requires continuous monitoring of soil and microbial respiration and better constraints on soil C pools.