On the potential CO2 release from tundra soils in a changing climate

About 30% of the carbon in terrestrial ecosystems is stored in northern wetlands and boreal forest regions. Prevailing cold and wet soil conditions have largely been responsible for this carbon accumulation. It has been suggested that a warmer and drier climate in these regions might increase the decomposition rate and, hence, release more CO2 to the atmosphere than at present. This study reports on the spatial variability and temperature dependence of the potential carbon release after incubating highly organic soils from the European Arctic and Siberia at different temperatures. We found that the decay potential, measured as CO2 production in laboratory experiments, differed strongly within and among sites, particularly at higher soil temperatures. Furthermore, both the decay potential and its temperature response decreased significantly with depth in the soil, presumably because the older soils at deeper layers contained higher proportions of recalcitrant carbon than the younger soil organic matter at the surface. These results have implications for global models of potential feedbacks on climate change inferred from changes in the carbon balance of northern wetlands and tundra. Firstly, because the decay potential of the organic matter varies locally as well as regionally, predictions of how the tundra carbon balance may change will be unreliable if these are based on measurements at a few sites only. Secondly, any increase in CO2 production may be transitional as both the carbon flux and its temperature sensitivity decrease when the most easily degradable organic material near the soil surface has decomposed. Consequently, it is crucial to account for transient responses and regional differences in the models of potential feedbacks on climate change from changed carbon cycling in northern terrestrial ecosystems. (C) 1999 Elsevier Science B.V.