Old soil carbon losses increase with ecosystem respiration in experimentally thawed tundra

Old soil carbon (C) respired to the atmosphere as a result of permafrost thaw has the potential to become a large positive feedback to climate change. As permafrost thaws, quantifying old soil contributions to ecosystem respiration (R_eco) and understanding how these contributions change with warming is necessary to estimate the size of this positive feedback. We used naturally occurring C isotopes (δ¹³C and Δ¹⁴C) to partition R_eco into plant, young soil and old soil sources in a subarctic air and soil warming experiment over three years. We found that old soil contributions to R_eco increased with soil temperature and R_eco flux. However, the increase in the soil warming treatment was smaller than expected because experimentally warming the soils increased plant contributions to R_eco by 30%. On the basis of these data, an increase in mean annual temperature from −5 to 0 °C will increase old soil C losses from moist acidic tundra by 35–55 g C m⁻² during the growing season. The largest losses will probably occur where the plant response to warming is minimal.