Increased belowground carbon allocation reduces soil carbon losses under long-term warming

The response of the carbon cycle in forests to global warming could lead to a positive climate feedback if warming accelerates the mineralization of soil organic carbon (SOC), thereby causing net emissions of CO2 into the atmosphere. In Europe, humus-rich alpine forest soils could be particularly affected by global warming, as a greater rise in temperature is expected in this region than the global average. Here we show that nearly two decades of experimental soil warming (+4°C during the snow-free seasons) in a mountain forest in the Northern Limestone Alps significantly (13% per 1°C warming) and persistently (no change in response over 18 years) increased soil CO2 effluxes. The SOC stocks in the warmed plots decreased compared to controls, yet non-significantly, and quantitatively much less than the surplus carbon outflux from warmed soil suggests. We attribute the increase in soil CO2 efflux primarily to stimulation of root respiration, which was most sensitive to long-term warming.