Mechanisms of below-ground carbon cycling in subarctic ecosystems

Abstract: 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, apparently as part of a mechanism for circumventing nutrient deficiency. Elevated temperatures led to increased C allocation to hair roots and DOC as well as root ergosterol content (as a proxy for ericoid mycorrhizal colonisation) in a birch forest understorey, apparently as a result of low soil moisture content and plant water stress. Increased atmospheric CO2 lowered leaf ä15N, suggesting an altered role for mycorrhizas in ecosystem N-cycling. Root ergosterol content in a dwarf shrub tundra was correlated with ecosystem photosynthesis two weeks earlier, and the amount of recently-assimilated C in hair roots and ergosterol were highly correlated, suggesting that seasonal variation in mycorrhizal colonisation levels was controlled by ecosystem photosynthesis and C allocation processes. Taken together, the studies reported in this thesis demonstrate the importance of below-ground mechanisms for carbon cycling in the subarctic ecosystems examined. Several of the mechanisms studied were found to be sensitive to temporal or spatial variation in environmental drivers. This suggests that below-ground mechanisms may mediate changes in ecosystem C cycling and storage under the modified environmental conditions of the future.