Soils are at risk of leaking 'old carbon' back into the atmosphere in a high-emissions world. Exposing alpine forests to increased levels of carbon dioxide stimulates the breakdown of carbon that has been locked up in the soil for years, according to a new study in the Swiss Alps.
Frank Hagedorn, of the Swiss Federal Institute of Forest, Snow and Landscape Research, and colleagues monitored the impact of elevated carbon dioxide, over a five-year period, on the leaching of 'old' carbon from the forest soil to the surrounding soil water. Although usually only a small percentage of carbon in the soil enters the groundwater, this increased by 20 per cent after the five-year experiment. Over 80 per cent of the carbon that leached into the groundwater originated not from newly acquired carbon sources, such as fresh plant litter, but from organic matter stored in the soil before the experiment began.
This suggests that high levels of carbon dioxide can accelerate the breakdown of old carbon in soils, some of which might escape back into the atmosphere as well as entering the soil water pool. The results throw into question how effective these soils will be at storing carbon in the future.