The global warming trend is exacerbated in the Arctic leading to widespread changes to both marine and terrestrial Arctic ecosystems. These ecosystems comprise a vital part of the global organic matter cycle. One consequence of warming in marine Arctic ecosystems is the increased translocation of terrestrial organic matter via river discharge and coastal erosion into shallow shelf sediments. Once deposited in sediments, substantial amounts of this terrestrial organic matter can be degraded to carbon dioxide and inorganic nutrients. The release of these compounds might affect bottom-up ocean acidification and could potentially alter the chemical composition of Arctic shelf waters, thereby creating cascading effects of marine Arctic ecosystems. In the terrestrial Arctic, warming is leading to an overall greening and to vegetation shifts, especially in the low Arctic. Vegetation shifts are characterized by the encroachment of large shrubs and trees into areas that were previously vegetated by low growing tundra vegetation. There are still major uncertainties on how vegetation changes might affect organic matter cycling in these ecosystems.

This thesis investigated how two observed responses of Arctic ecosystems to warming affect organic matter cycling in these systems: Papers I and II studied whether the influx of terrestrial organic matter into sediments along the Siberian shelf seas influences carbon dioxide, oxygen and nutrient exchange between sediments and shelf waters. Papers III and IV investigated if the encroachment of large deciduous shrubs and coniferous trees affect organic matter storage in mineral permafrost soils and if the quantity and composition of root exudates are different between encroaching and established tundra vegetation. To facilitate an investigation of these research aims an array of experimental and analytical approaches was used including sediment incubations, soil density fractionation, root exudate sampling and subsequent molecular, elemental and isotopic analyses of sediment, soil and root exudate samples. 

The outcome of papers I and II shows that terrestrial material is a major constituent of sediment organic matter in Siberian shelf sediments and contributes to sediment carbon dioxide production and oxygen consumption. Nutrient fluxes show contrasting trends with ammonium and nitrite fluxes being positively correlated with terrestrial organic matter content, but phosphate and dissolved silica being negatively correlated. Put together, these results indicate that increasing inputs of terrestrial organic matter might change the composition and chemistry of shallow shelf sediments by affecting nutrient composition and influencing bottom-up acidification by carbon dioxide emissions. 

The results of papers III and IV indicate that vegetation changes in the low Arctic are likely to affect organic matter cycling in mineral permafrost soils by both changing the way organic matter is stored and by decreasing the diversity of compounds released via root exudation. Paper III shows that soils under large alder shrubs store higher amounts of organic matter in particulate form compared to established tundra vegetation, low birch shrubs and spruce trees. Paper IV revealed that a large proportion of the overall root exudation composition is different between established tundra vegetation and encroaching shrubs and that the exudate diversity might decrease if alder and birch shrubs would displace graminoid vegetation.

The ongoing rise in temperature caused by climate change has already increased the vegetation coverage and altered the vegetation composition at higher latitudes. To date, it is unclear how these changes influence the large carbon stocks in permafrost soils. This PhD thesis focuses on whether a vegetation shift among spruce trees, tussock-forming graminoids, birch, and alder shrubs could affect soil carbon cycling in permafrost soils through plant litter and root exudation. This was achieved by analyzing 1) properties of plant litter, bulk soils and soil organic matter (SOM) fractions, 2) root exudate composition and release rates, 3) concentration-dependent effects of exuded organic acids on soil carbon and nutrient cycling, 4) turnover of photosynthates in different soil horizons.

The results suggest that particularly shifts between alder shrubs and graminoids can affect SOM properties and stability by differences in litter composition, and that biomass production and soil physical properties may also be important contributing factors. Under alder shrubs most carbon was stored as easily degradable particulate organic matter and thus SOM under alder shrubs may be particularly vulnerable to microbial decomposition. The analysis of root exudates showed that Arctic shrubs and graminoids have a distinctly different root exudate metabolome, despite having similar exudation rates of primary metabolites. Therefore, identification of secondary metabolites and their impact on SOM decomposition is required for a better understanding of how plant shifts affect soil carbon cycling in permafrost soils through root exudation. In addition, the comparison of measured root exudation with previous laboratory soil incubations, where root exudates were simulated by e.g. glucose additions, uncovered that in most studies simulated root exudation corresponded to root exudation by living plants of several growing seasons. Comparing the effects of organic acids in soils at such high concentrations with lower and realistic additions revealed that the use of too high concentrations overemphasized soil carbon losses and artificially increased microbial nutrient demand. Furthermore, carbon allocation was plant- and depth-specific with alder shrubs allocating less carbon into O horizons than birch shrubs. Considering temporal and spatial variation in root exudation could therefore improve model predictions on plant-mediated carbon losses.

All in all, this thesis demonstrated that a shift in plant types has the potential to alter soil carbon cycling though plant litter and root exudates but that not all effects will result in soil carbon losses.