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 (¹⁴C) 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 ¹⁴C 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.