Fakultät Naturwissenschaften

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Biologie, Ernährungs-wissenschaften und Lebensmittelwissenschaften sind die Schwerpunkte der Fakultät. Die Forschung befasst sich mit Schlüsselthemen der Life Sciences.

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Carbon and nitrogen transformations in alpine ecosystems of the Eastern Alps, Austria
(2007) Koch, Oliver; Kandeler, Ellen
This thesis investigated net CH4 and net CO2 emissions from sites in the alpine region of the Eastern Alps, Austria. Four mature alpine sites (one dry meadow and three fen sites) were selected and the influence of abiotic (radiation, temperature, soil water conditions) and biotic (above-ground standing plant biomass) environmental controls on diurnal and seasonal emission patterns were studied. For a better understanding of the response of soil C- and N pools to global warming, the temperature sensitivity of activities involved in C- and N cycling were determined. The first part of the thesis dealt with net methane fluxes measured over a period of 24 months. During snow-free periods, average methane emissions of the fen sites ranged between 19 and 116 mg CH4 m-2 d-1. Mean emissions during snow periods were much lower, being 18 to 59% of annual fluxes. The alpine dry meadow functioned as a small methane sink during snow-free periods (-2.1 mg CH4 m-2 d-1 (2003); -1.0 mg CH4 m-2 d-1 (2004)). The diurnal and seasonal methane uptake of the dry meadow was positively related to soil temperature and negatively related to water-filled pore space (wfps). In the fen, the seasonal methane fluxes were related to soil temperature and groundwater table. The live above-ground standing plant biomass contributed to net methane fluxes only at those sites with higher water table positions. This study provided evidence that alpine fens acted as methane sources throughout the year, whereas an alpine meadow site acted as a net methane sink during snow-free periods. In the second part of the thesis the CO2 balance was estimated based on diurnal flux measurements and on the influence of photosynthetic active radiation (PAR), plant green area index (GAI), soil temperature and wfps. The daylight net ecosystem CO2 emission rate was influenced by PAR and GAI throughout snow-free seasons. The seasonal net CO2 emission rate at night was positively related to soil temperature, while low wfps reduced flux rates at the meadow and at the driest fen study site but reinforced carbon loss at the wetter fen sites. The daily average ecosystem net CO2 gain during snow-free periods at the meadow was 3.5 g CO2 m-2 d-1 and at the fen sites between 1.5 and 3.4 g CO2 m-2 d-1. The mean average daily CO2 emission during snow periods was low, being -0.9 g CO2 m-2 d-1 for the meadow and between -0.2 and -0.7 g CO2 m-2 d-1 for all fen sites. All sites function as significant annual net carbon sinks, with a net carbon gain from 50 to 121 g C m-2 a-1 (averaged over both years), irrespective of water balance. The results indicate that alpine fen sites, that have built up a large carbon stock in the past, are not expected to gain a further carbon surplus compared with meadows under the current climate. Temperature is important for regulating biological activities. The third part of the thesis focused on temperature sensitivity of soil C mineralization, N mineralization and potential enzyme activities involved in the C- and N cycle (ß-glucosidase, ß-xylosidase, N-acetyl-ß-glucosaminidase, tyrosine aminopeptidase, leucine aminopeptidase) over a temperature range of 0-30°C. The objective was to calculate Q10 values and relative temperature sensitivities (RTS) and to quantify seasonal (summer, autumn, winter) and site-specific factors. The Q10 values of C mineralization were significantly higher (average 2.0) than for N mineralization (average 1.7). The Q10 values of both activities were significantly negatively related to soil organic matter quality. In contrast, the chemical soil properties, microbial biomass and sampling date did not influence Q10 values. Analysis of RTS showed that the temperature sensitivity increased with decreasing temperature. The C- and N mineralization and potential aminopeptidase activities (tyrosine, leucine) showed an almost constant temperature dependence over 0-30°C. In contrast, ß-glucosidase, ß-xylosidase and N-acetyl-ß-glucosaminidase showed a distinctive increase in temperature sensitivity with decreasing temperature. Low temperature at the winter sampling date caused a greater increase in the RTS of all activities than in autumn and summer. Our results indicate a disproportion of the RTS for potential enzyme activities of the C- and N cycle and a disproportion of the RTS for easily degradable C compounds (ß-glucose, ß-xylose) compared with the C mineralization of soil organic matter. Thus, temperature may play an important role in regulating the decay of different soil organic matter fractions.
The effect of forest cover changes on the regional climate conditions in Europe during the period 1986–2015
(2024) Breil, Marcus; Schneider, Vanessa K. M.; Pinto, Joaquim G.
Afforestation affects the earth's climate system by changing the biogeochemical and biogeophysical characteristics of the land surface. While the regional effects of afforestation are well understood in the tropics and the high latitudes, its climate impact on the midlatitudes is still the subject of scientific discussions. The general impact of afforestation on the regional climate conditions in Europe during the last decades is investigated in this study. For this purpose, regional climate simulations are performed with different forest cover fractions over Europe. In a first simulation, afforestation in Europe is considered, while this is not the case for a second simulation. We focus on the years 1986–2015, a period in which the forest cover in Europe increased comparatively strongly, accompanied by a strong general warming over the continent. Results show that afforestation has both local and non-local effects on the regional climate system in Europe. Due to an increased transport of turbulent heat (latent + sensible) into the atmosphere, afforestation leads to a significant reduction of the mean local surface temperatures in summer. In northern Europe, mean local surface temperatures were reduced about -0.3 K with afforestation, in central Europe about -0.5 K, and in southern Europe about -0.8 K. During heat periods, this local cooling effect can reach -1.9 K. In winter, afforestation results in a slight local warming in both northern and southern Europe because of the albedo effect of forests. However, this effect is rather small and the mean temperature changes are not significant. In the downwind direction, locally increased evapotranspiration rates with afforestation increase the general cloud cover, which results in a slight non-local warming in winter in several regions of Europe, particularly during cold spells. Thus, afforestation had a discernible impact on the climate change signal in Europe during the period 1986–2015, which may have mitigated the general warming trend in Europe, especially on the local scale in summer.
Profiling the molecular destruction rates of temperature and humidity as well as the turbulent kinetic energy dissipation in the convective boundary layer
(2024) Wulfmeyer, Volker; Senff, Christoph; Späth, Florian; Behrendt, Andreas; Lange, Diego; Banta, Robert M.; Brewer, W. Alan; Wieser, Andreas; Turner, David D.
A simultaneous deployment of Doppler, temperature, and water-vapor lidars is able to provide profiles of molecular destruction rates and turbulent kinetic energy (TKE) dissipation in the convective boundary layer (CBL). Horizontal wind profiles and profiles of vertical wind, temperature, and moisture fluctuations are combined, and transversal temporal autocovariance functions (ACFs) are determined for deriving the dissipation and molecular destruction rates. These are fundamental loss terms in the TKE as well as the potential temperature and mixing ratio variance equations. These ACFs are fitted to their theoretical shapes and coefficients in the inertial subrange. Error bars are estimated by a propagation of noise errors. Sophisticated analyses of the ACFs are performed in order to choose the correct range of lags of the fits for fitting their theoretical shapes in the inertial subrange as well as for minimizing systematic errors due to temporal and spatial averaging and micro- and mesoscale circulations. We demonstrate that we achieve very consistent results of the derived profiles of turbulent variables regardless of whether 1 or 10 s time resolutions are used. We also show that the temporal and spatial length scales of the fluctuations in vertical wind, moisture, and potential temperature are similar with a spatial integral scale of ≈160 m at least in the mixed layer (ML). The profiles of the molecular destruction rates show a maximum in the interfacial layer (IL) and reach values of ϵm≃7×10-4 g2 kg-2 s-1 for mixing ratio and ϵθ≃1.6×10-3 K2 s-1 for potential temperature. In contrast, the maximum of the TKE dissipation is reached in the ML and amounts to ≃10-2 m2 s-3. We also demonstrate that the vertical wind ACF coefficient kw∝w′2‾ and the TKE dissipation ϵ∝w′2‾3/2. For the molecular destruction rates, we show that ϵm∝m′2‾w′2‾1/2 and ϵθ∝θ′2‾w′2‾1/2. These equations can be used for parameterizations of ϵ, ϵm, and ϵθ. All noise error bars are derived by error propagation and are small enough to compare the results with previous observations and large-eddy simulations. The results agree well with previous observations but show more detailed structures in the IL. Consequently, the synergy resulting from this new combination of active remote sensors enables the profiling of turbulent variables such as integral scales, variances, TKE dissipation, and the molecular destruction rates as well as deriving relationships between them. The results can be used for the parameterization of turbulent variables, TKE budget analyses, and the verification of large-eddy simulations.

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