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.

Global warming increases the vegetation cover and leads to shifts in vegetation types in the Arctic. An increase in the vegetation cover might substantially enhance carbon dioxide (CO₂) emissions from northern permafrost soils, since root exudation of labile carbon and nitrogen can stimulate soil organic matter (SOM) decomposition via the rhizosphere priming effect. The current understanding of Arctic rhizosphere priming largely rests on soil incubation studies that simulate root exudation by adding various organic substrates in varying concentrations to soils. How the specific exudates of different plants influence rhizosphere priming is unclear as Arctic plant root exudate release rates and composition are largely unknown. Using targeted and non-targeted liquid chromatography–mass spectrometry, we compared the exudate composition and exudation rates of total organic carbon, 7 organic acids, 14 amino acids and 9 carbohydrates from three abundant and functionally different tundra plants (Betula glandulosa, Alnus viridis and Eriophorum vaginatum). While organic carbon and primary metabolites exudation were similar among the studied plants despite their different nitrogen acquisition strategies, distinct differences between the plant species were found in the overall root exudate composition. Between 80 and 94 % of the root exudate metabolome was not shared among the three plants. Our findings indicate that a change in vegetation types across the Arctic will primarily alter the release of secondary plant metabolites into the soil and thereby could alter soil microbial processes. Our observations further suggest that previous laboratory experiments studying priming frequently oversaturated microorganisms with labile substrates compared to natural conditions; this highlights the need for more realistic priming studies. Our data on root exudation provide critical background information for improving laboratory experiments.

Permafrost soils constitute a large part of the terrestrial carbon pool that is vulnerable to future climate warming. Continued warming of the low Arctic is also leading to the encroachment of large shrubs and trees into tundra ecosystems with effects on microbial community composition, organic matter cycling and physical soil parameters. To date it is still largely unknown how such vegetation shifts affect soil organic matter cycling in permafrost soils on short and long timescales. Here, we investigated differences in soil organic matter properties under graminoid tussock (Eriophorum vaginatum), birch shrub (Betula glandulosa), spruce tree (Picea mariana) and alder shrub (Alnus viridis) vegetation by density fractionation and subsequent measurements of organic carbon, total nitrogen, δ¹³C, and lignin phenol biomarker contents. Particulate organic matter constituted 1.3–11.3% of soil weight and stored between 29 and 89% of the total soil lignin, 12–60% of organic carbon and 6–40% of total nitrogen. The contribution of particulate organic matter generally decreased with soil depth. Soils under Alnus viridis showed significantly higher amounts of particulate organic matter and stored more lignin, organic carbon and total nitrogen in particulate form in all soil depths. Sites dominated by Eriophorum vaginatum exhibited higher lignin content and lower degradation state in the subsoil, which was associated with water saturation and low active layer depth. We conclude that the effect of vegetation changes on soil organic matter cycling is dependent on plant species with the encroachment of Alnus viridis shrubs potentially increasing the deposition of particulate organic matter into permafrost soils.