Mapping hotspots of soil carbon vulnerability to land use and climatic change.

Don et al. (2009)

The global soil carbon stock of about 1600 Pg C forms an important buffer between previously sequestered carbon and release to the atmosphere. Currently the terrestrial vegetation is taking up carbon at a rate of about 3 Pg C yr-1, part of which is likely to be turned into soil carbon. However, the subsistance of soil carbon is threatened on the short term by ongoing land use change and by climatic change on the longer term. This study aims to present the first results of a new assessment of global hotspots in soil carbon stocks and in soil carbon vulnerability due to low intrinsic soil carbon stability and due to land use pressure and climatic change. Carbon stocks in soils are compiled from regional soil maps, soil profile data and field studies, focusing on areas with high carbon stocks per area (wetlands and forests in low latitudes, soils in high latitudes). The carbon stocks are analysed in relation to and climatic factors (water balance, temperature regime, growing season timing, permafrost and snow cover) and soil intrinsic stabilization mechanisms (clay content, mineralogy) to differentiate between stable and vulnerable carbon stocks. We furthermore use recent meta-analyses of land-use change effects on soil carbon in tropical and temperate regions to quantify the soil carbon losses by land use change in relation to the size of the vulnerable C pool. As a result, we present first regional maps and projections of soil carbon stocks vulnerability to disturbance and climate change. These maps will be combined with scenario maps of likely future land use change to identify hotspots of high soil carbon stock, and high risk for carbon losses. With respect to the impacts of climate change, we assess the main limiting factors of soil carbon decomposition and overlay these with projections of future climate, resulting in an change in the balance between production and decomposition of soil carbon. Using a conceptual model of residence times, we estimate how long it takes for the balance to be restored and how much the carbon pool changes in that period. This study aims to produce important spatial and process-related data synthesis to understand the role of soil carbon in the global carbon cycle during the coming century. The research is part of the EU FP7 project COordination action Carbon Observing System (COCOS).