| Summary: | Some components of the below-ground carbon (C) cycle in terrestrial ecosystems are still poorly understood. A better understanding will be necessary to predict adequately the impacts of global change factors on C cycling and storage, especially in high-latitude ecosystems, where much of the C is stored below-ground. In this work some of the mechanisms of the below-ground C cycle in subarctic ecosystems were studied and responses to present and potential future environmental conditions assessed. Using 14C pulse-labelling, C allocation to above-ground biomass, rhizomes, coarse roots, fine roots, hair roots, ericoid mycorrhizas, microbes and dissolved organic C (DOC) was determined repeatedly over the growing season in four of the most common vegetation types of the Scandinavian subarctic: (1) Dry dwarf shrub tundra; (2) Semi-wet mire; (3) Wet mire; and (4) the understorey of subarctic birch forest. Effects of increased temperatures, increased atmospheric CO2 concentrations and both factors in combination on below-ground C allocation, ericoid mycorrhizal colonisation and functioning were studied in an full-factorial open-top chamber experiment. Furthermore, responses of ericoid mycorrhizal colonisation rates to environmental variation during the growing season were investigated. Ecosystem C partitioning varied temporally in all studied ecosystems, possibly indicating changes in growth, nutrient uptake or C storage by vegetation. The relative importance of C pools with "fast" versus "slow" turnover rates varied spatially, among vegetation types. Therefore it is important for global change studies to consider the possible effects of vegetation changes on ecosystem C dynamics. Allocation of recent assimilates to fast-turnover C pools such as hair roots and DOC was particularly high in a dwarf shrub tundra making them quantitatively interesting pools to consider in studies of ecosystem C dynamics. Furthermore, a significant proportion of assimilates in the DOC pool were allocated to P-mobilizing organic acids, ...
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