The arctic region is particularly vulnerable to climate change, which gives rise to wide-spread vegetation changes. Altered vegetation patterns may in turn affect the carbon balance and cause larger-scale climate feedbacks. In particular, climate-induced increased soil nitrogen (N) availability may enhance plant productivity and alter species composition. As the arctic climate continues to warm, two main processes contribute to increased N availability in arctic soils: In upper soil layers, warmer temperatures accelerate decomposition and N mineralisation. In the deep-soil, permafrost thaw releases previously inaccessible plant-available N. Yet, the potential for this newly available N to trigger vegetation change depends on whether plants can access this N, vertically, spatially, temporally, and in competition with other plants and microorganisms. Understanding the patterns and processes of plant species-specific N uptake constitutes an important step towards disentangling the underlying drivers of arctic vegetation change.
This thesis investigates plant species-specific uptake of newly available N across multiple dimensions of N release (depth, space, time) and according to species characteristics, plantmicrobe competition and environmental change. The work is based on three field experiments in high and low arctic tundra ecosystems in Greenland. Throughout all studies, stable isotope labelling was used to simulate naturally occurring processes of N release and to track plant and microbial N uptake and turnover over time.
This work demonstrates that arctic plants successfully acquire both surface-released and permafrost-released N. Thus, arctic plants can take advantage of both increased decomposition and permafrost thaw for new N supply. While most plants prefer top-soil N, the ability to access deep-soil N pools renders permafrost-released N an important new nutrient source to arctic plants. Even in sloping terrain, plants can capture permafrost-N both locally and downslope from the point-of-release, which may contribute to landscape-scale plant community change. Tight coupling between plant and microbial N cycling and potential changes in the timing and strength of plant-microbe competition exert strong controls over plant responses to climateinduced N release. Plant species differ widely in their nutrient acquisition strategies with respect to foraging depth, timing of uptake, accumulation, storage and redistribution of N. This may lead to divergent patterns of plant community change over shorter and longer time scales.
In summary, this thesis reveals that arctic plants can bridge the temporal and spatial asynchronies between climate-induced N release and uptake and overcome the constraints of competition by adopting different N uptake strategies. The potential to take advantage of newly available N could have important implications for plant growth, species composition, and thereby carbon dynamics. By illustrating the intricate links between N release, plant speciesspecific competitive advantages and microbial competition, this work advances our understanding of the processes, which shape the patterns of long-term and landscape-scale plant community change across the Arctic.