| Summary: | Sameiginlegt með University of Antwerp - Faculty of Science Leblans, N. I. W. 2016. Natural gradients in temperature and nitrogen: Iceland represents a unique environment to clarify long-term global change effects on carbon dynamics. Joint doctoral thesis at Antwerp University and Agricultural University of Iceland. Global change is one of the greatest challenges of our generation. Surface temperatures are rising as a consequence of anthropogenic greenhouse gas (GHG) emissions, but projections remain highly uncertain as solid knowledge on potent feedback mechanisms from ecosystems to the climate system is limited. One such potentially powerful feedback mechanism is the warming-induced transfer of soil organic carbon (SOC) to the atmosphere, exacerbating the warming. This is especially true for high northern latitude ecosystems (≥ 60° N) where both the highest amount of SOC is stored and where warming is most pronounced. In this region, SOC storage is also strongly linked to nitrogen (N) cycling, as plant productivity, the primary source of SOC, is generally N limited in cold ecosystems. During the past decades, northern temperate and some boreal ecosystems have already been absorbing about 10% of the anthropogenic C emissions due to an N-driven increase in productivity. At high northern latitudes, the N inputs, which have remained relatively low up to now, are expected to increase substantially, potentially transforming northern ecosystems to important contributors of the N-induced C sink in the near future. However, the future evolution of both warminginduced SOC losses and the N-dependent C sink is highly uncertain due to the scarcity of empirical observations of long-term effects. In this thesis, natural gradients were used to obtain empirical observations of longterm warming and N input effects on C dynamics in subarctic grasslands, which cover ca. 10% of the global terrestrial surface. The primary advantage of natural gradients is that equilibrium states can be observed. Further, their typical wide and ...
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