Vinsamlegast notið þetta auðkenni þegar þið vitnið til verksins eða tengið í það: http://hdl.handle.net/1946/26419
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 continuous nature allows to study the full range of global change projections and to detect non-linearities in the response.
Here, I investigated natural (geothermal) soil temperature gradients (+0 - +20 °C) at the ForHot research site (www.forhot.is) in southwest Iceland, where the presence of both shortterm (formed in 2008) and long-term (≥ 50 years) gradients allowed to separate transient (short-term) from permanent (long-term) responses. Natural gradients in N inputs were investigated on islands of Vestmannaeyjar (off the south coast of Iceland) with differing soil age (and thus different N-accumulation time; 50 vs. 1,600 years) and amounts of seabird N inputs.
Firstly, I aimed to evaluate the potential of natural gradients of geothermal soil warming and N inputs as global change laboratories and secondly to assess and compare their short-term and long-term effects on the C dynamics of subarctic grasslands, with focus on plant phenology and SOC sequestration.
The geothermal soil temperature gradients proved to be a valuable complement to other warming techniques to improve our understanding of long-term warming effects on ecosystems, overcoming important difficulties that are typical for climate manipulation experiments, such as limited duration of the warming and a limited number of warming levels.
The length of the growing season (LOS), a phenological parameter of great importance in the C balance of northern ecosystems, extended linearly with increasing soil temperature by on average 15.6 ± 4.7 (SE) days °C-1. This was primarily due to an advance in the start of the growing season, while the end of the growing season was largely unresponsive to warmer soil temperatures. This persistency of the warming-induced extension of LOS, which has recently been questioned, has important implications for the C-sink potential of subarctic grasslands under climate change.
SOC stocks showed a large and linear decline under soil warming (4.1 ± 0.5 SE % °C- 1), both after 5 and ≥ 50 years of warming. Central to the observed SOC loss was a loss of physical stabilization in soil aggregates. This was an important finding and such mechanism is presently not embedded in any Earth System Model, but its inclusion could possibly improve model projections. Further, the similar SOC loss after short- and long-term warming suggested that warming of subarctic grassland soils could cause a rapid positive feedback to climate warming.
In contrast to soil warming, increasing N inputs increased the short-term SOC storage rate of early successional and mature subarctic grasslands (from 0.018 to 0.29 and from 0.30 to 0.44 ton SOC ha-1 yr-1 respectively), as well as the long-term SOC storage rate of the mature grasslands (from 0.12 to 0.16 ton SOC ha-1 yr-1). This suggested that the N-induced C sink of subarctic grasslands could be maintained for many centuries, which contradicts the current hypothesis that the N-deposition driven terrestrial CO2 sink is likely to saturate in the near future.
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