FIGURE 3. Feedback loops that could be induced by climate change in below-ground ecosystem carbon balances.

The three examples given here are crucial processes in the ecosystem, shown in simplified form. a, Potential interactions between microbial metabolism and the physics of permafrost thawing and carbon release. b, The 'microbial priming effect'. An increase in carbon and energy sources easily utilized by microbes can stimulate the decomposition of 'old' soil carbon, especially in grassland soils. In the context of climate change this effect may have a positive-feedback effect on CO2 increase and global warming. c, Interactions between the carbon and nitrogen cycles shown here could alter expected ecosystem carbon responses to the prevailing trend of climate change. Pink arrows denote effects of terrestrial ecosystems on climate, orange arrows denote effects of climate change on terrestrial ecosystems, and black arrows denote interactions within ecosystems. The background image is a world map of soil organic carbon. (Map reproduced, with permission, from USDA-NRCS, http://soils.usda.gov/use/worldsoils/mapindex/soc.html.)