Initial effects of experimental warming on above- and belowground components of net ecosystem CO2 exchange in arctic tundra

The Arctic contains extensive soil carbon reserves that could provide a sub stantial positive feedback to atmospheric CO2 concentrations and global war ming. Evaluation of this hypothesis requires a mechanistic understanding of the in situ responses of individual components of tundra net ecosystem CO2 exchange (NEE) to warming. In this study, we measured NEE, total ecosystem respiration and respiration from below ground in experimentally warmed plo ts within Alaskan acidic tussock tundra. Soil warming of 2-4 degreesC durin g a single growing season caused strong increases in total ecosystem respir ation and belowground respiration from moss-dominated inter-tussock areas, and similar trends from sedge-dominated tussocks. Consequently, the overall effect of the manipulation was to substantially enhance net ecosystem carb on loss during mid-summer. Components of vascular plant biomass were closel y correlated with total ecosystem respiration and belowground respiration i n control plots of both microsites, but not in warmed plots. By contrast, i n the warmed inter-tussock areas, below ground respiration was most closely correlated with organic-layer depth. Warming in tussock areas was associat ed with increased leaf nutrient pools, indicating enhanced rates of soil nu trient mineralisation. Together, these results suggest that warming enhance d net ecosystem CO2 efflux primarily by stimulating decomposition of soil o rganic matter, rather than by increasing plant-associated respiration. Our short-term experiment provides field evidence to support previous growth ch amber and modelling studies indicating that arctic soil C reserves are rela tively sensitive to warming and could supply an initial positive feedback t o rising atmospheric CO2 concentrations/changing climate.