Kurzbeschreibung
(Englisch)
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Atmospheric CO2 enrichment and climatic warming as well as N deposition affect input and output of carbon and nitrogen in soils. This experiment will assess quasi steady state signals of these fluxes and pools by using 'experiments by nature', i.e. established gradients of temperature and N input, the major drivers of NPP and the soil C balance. We will test the hypothesis that soil respiration (R) is driven by net primary production rather than temperature (T) per se. We will further test the hypothesis that enhanced nitrogen input (here naturally simulated by stands composed of nitrogen-fixing trees) will facilitate greater carbon sequestration. By selecting topography-driven IPCC T-gradients across identical bedrock chemistry and macroclimate and high vs. low N input (Alnus, Robinia vs. control) we will thus complement data obtained by other projects which employ shorter-term manipulative tests. The work will be conducted in the Swiss Jura Mountains and the Central Alps in part using existing infrastructure.
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Partner und Internationale Organisationen
(Englisch)
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AT, BA, BE, BG, CH, CZ, DE,DK, EE, ES, FI, GR, HU, IE, IL, IT, LT, NL, NO, PT, RO, SE, SI, SK, TR, UK
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Abstract
(Englisch)
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Soil respiration (Rs) plays a key role in any ecosystem carbon (C) balance consideration. Using established temperature (T) responses of respiration in plants, soils and whole ecosystems in biogeochemical models, substantial increases in soil C release could be expected as a positive feedback to climate warming. While Rs is indeed responding to T in the short term, in the longer run though, we suggest that Rs is driven by substrate availability. In a first chapter we hypothesized that cumulative annual Rs at contrasting T reflects the difference in the production of short-lived biomass. Our second objective was to link Rs responses to the nitrogen (N) cycle. Globally increasing soil N availability implies unknown consequences on the C cycle and storage. There is a controversial discussion about a N-induced C sink in forest ecosystems. A basic question to be explored is, whether or not high rates of N-input increase the rate of C-cycling by accelerating both productivity and respiratory C-release. We thus tested the hypothesis, that high-N-input Alnus forests facilitate higher soil C release, which, however, still remains in proportion to substrate availability. Our third objective addressed the issue of N-overflow responses that may lead to N-based greenhouse gas emission. Current rates of atmospheric N deposition have the potential to saturate the biological demand for N within many ecosystems over time. We thus hypothesized, that naturally high N-input to forest soils are enhancing soil N transformation, resulting in higher soil N2O emissions. Here we report data obtained from long-term established gradients of T and N-availability, so-called 'experiments by nature'. The present study takes advantage of a mean annual T difference of 6K across a 1200 m change in elevation from the Central Swiss Alps to the Swiss Plateau. Within each elevation we chose forest stands with contrasting N availability: symbiotically N2-fixing species (Alnus) and non N2-fixing species. The sum of the short-lived NPP fractions (canopy leaf, understory and fine root litter) did not significantly differ across the thermal gradient of 6K, irrespective of the doubling in annual forest wood production from high to low elevation (largely explained by the length of the growing season). Concurrently, cumulative annual Rs did not differ significantly between elevations. Annual soil CO2 release thus largely reflected the input of labile C and not T, despite Rs showing the well-known short-term T response within each site. These results caution against assuming strong positive effects of climatic warming on Rs, but support a close substrate relatedness of Rs. We further found Rs to remain in proportion to forest litter production, irrespective of N-availability, supporting our productivity-based explanation of Rs. N2-fixation enhanced N2O emission, suggesting that high ecosystem N inputs may cause significant losses of the highly potent greenhouse gas N2O.
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