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 (13°lC and 14°C) 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 â '5 to 0 °C will increase old soil C losses from moist acidic tundra by 35-55 g C m â '2 during the growing season. The largest losses will probably occur where the plant response to warming is minimal.