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Free convection and turbulent fluxes over complex terrain

Rafael Eigenmann — Wed, 22 May 2013 12:03:11 +0200

The impact of complex terrain on the land-atmosphere exchange is investigated in this thesis. Here, free convection, a very effective vertical transport mechanism as turbulence is predominantly driven by buoyant forces, is explicitly addressed. Recently, it was shown for certain situations over complex terrain that free convective injections of surface layer air masses into the atmospheric boundary layer (ABL) can alter the ABL properties significantly. This study aims at the general identification and description of such situations of near-ground free convection conditions (FCCs) over complex terrain. For this purpose, data obtained during the COPS (Convective and Orographically induced Precipitation Study) field campaign in summer 2007 were used. Within this project, several surface flux measurement stations were installed, mainly in valleys and on mountaintops of the Black Forest, southwestern Germany. Turbulent fluxes were calculated with the eddy-covariance (EC) method and were used to detect FCCs with the help of a stability parameter. The flux measurements were further combined with ABL profiling measurements (Sodar/RASS) and a large-eddy simulation (LES) model in order to investigate the impact of FCCs on ABL properties. The effect of complex terrain on the energy balance closure and on spatial and temporal flux differences was also studied with these flux data. FCCs were detected on about 25% of the days during the three month COPS experiment. In situations of weak synoptic forcing, thermally driven orographic (e.g. valley winds) or local wind systems developed over the complex terrain due to heating differences. During the adaption of these wind systems to changing heating differences (e.g. during the reversal of the valley wind from down- to up-valley winds in the morning), the horizontal wind vanished. If, at the same time, the buoyancy flux was positive and enhanced, buoyant forces exceeded the usually prevailing shear forces in the surface layer and FCCs were detected. Moreover, it was demonstrated that FCCs are not restricted to the COPS region. Also, a data set of Nam Co station on the Tibetan Plateau showed FCCs during the reversal of a thermally driven land-lake breeze. However, at this high-altitude site, FCCs were more often detected in the afternoon compared to the COPS region due to the frequent change of heating differences during cloud cover periods. The Sodar/RASS as well as the LES model showed the presence of coherent updraft structures in the developing early-morning convective boundary layer (CBL) in the Kinzig valley (Black Forest) during FCCs. Spectral analysis of the EC data in these situations indicated the existence of large-eddy turbulent scales – typical for thermal updrafts in the CBL – already close to the ground. An ensemble and time mean analysis of the simulated flow field in the valley further confirmed that the Sodar/RASS was located preferably in an updraft region during FCCs. In a CBL over flat homogeneous terrain, the locations of convective structures would occur randomly. However, over the complex orography of the Kinzig valley, the updraft structures were found to develop in quasi-stationary patterns at specific locations relative to the surrounding mountain ridges. The model further showed that the flux through the valley boundary layer is mainly determined by the flux within these coherent updrafts. In combination with the Sodar/RASS observations, the model also showed that these updrafts deeply penetrated into the stably stratified valley boundary layer up to approximately the height of the surrounding mountains leading to an effective upward counter-gradient transport of surface layer air mass properties during FCCs. The analysis of the turbulent fluxes at the different COPS sites showed that the flux values were strongly determined by varying land surface characteristics. Also an increase of the Bowen ratio with increasing altitude could be detected. These findings are in accordance with former studies in this area. As expected, the energy balance was found to be unclosed on average during the entire COPS period, with values of the residual typical for heterogeneous landscapes. However, regarding only the periods with FCCs, no residual occurred on average. This is due to the fact that the landscape heterogeneity is of minor importance in case of the more vertical oriented exchange regime during FCCs, so that missing advective flux components became strongly reduced in these situations. Moreover, it was found that in comparable periods with no FCCs, flux components were missing with exactly the proportions of the buoyancy flux ratio, thus suggesting a correction of the energy balance according to the buoyancy flux ratio approach. These results support recent publications on the energy balance closure problem.

Interactions between hydrology and biogeochemistry within riparian wetlands

Sven Frei — Wed, 15 May 2013 10:21:39 +0200

Interactions between hydrology and biogeochemistry at various spatio-temporal scales are important control mechanisms within terrestrial and aquatic ecosystems and exist among different compartments and transition interfaces. Understanding the fundamental mechanistic couplings between hydrological and biogeochemical processes and how these couplings feed back into ecosystem services and functions is an interdisciplinary challenge that must be addressed especially in the context of humanly mediated climate change. Riparian wetlands, as a transition zone between terrestrial and aquatic ecosystems, occupy large fractions of terrestrial ecosystems and provide important ecohydrological services. Due to their anoxic environments, riparian wetlands are able to store significant amounts of carbon as peat and act as an effective nutrient sink e.g. for sulfur, phosphorous and nitrogen. Riparian wetlands are characterized by highly dynamical interactions between hydrologically controlled transport mechanisms and biogeochemically controlled substrate availability, which governs nutrient cycling as well as the sink and source functions of wetlands. Generally, these interactions and their potential implications on ecosystem functions are only poorly understood. The representation of the tight couplings between hydrology and biogeochemistry in mechanistic models is a very challenging task because they have revealed a complexity which is often beyond the capabilities of current models. The objective of this thesis is to investigate interactions between hydrology and biogeochemistry in riparian wetlands and to understand their potential implications for internal biogeochemical process distributions and solute mobilization. Additionally, one major focus of the thesis is the attempt to represent such fundamental couplings in a process-based, hydrological/biogeochemical modeling approach. To this end, this thesis uses a combination of field and virtual experiments, as well as catchment-scale numerical modeling, performed for the Lehstenbach catchment, which was exemplarily chosen as main study site. Results from the virtual experiments show very complex small-scale hydrological dynamics within the riparian areas. Here, runoff generation processes are strongly influenced by the spatial structure of the wetland-typical micro-topography (hummocks and hollows). Surface flow is episodically generated by a highly dynamical, threshold-controlled process where extended surface flow networks drain large fractions of the wetland's area. During intensive rainstorm events these surface flow networks, which contribute to stream discharge due to a fill and spill mechanism, dominate runoff generation. These fast flow components are characterized by very low residence times (minutes to hours) and once they are activated, the surface flow networks are able to rapidly mobilize large amounts of solutes, like nitrate or dissolved organic carbon (DOC), out of the wetlands by bypassing deeper anoxic layers. The importance of fast flow components for the catchment-scale mobilization of DOC was further confirmed by field investigations and catchment-scale numerical modeling. High frequency measurements of DOC in runoff of the Lehstenbach catchment revealed that DOC export is subject to substantial short term variations at an hourly to daily timescale. During intense rainstorms, DOC concentrations are up to ten times higher (up to 40 mg/L) compared to low flow conditions (~3-5 mg/L). Short term variations together with the dramatic rise of DOC concentrations in runoff during rainstorms can be explained by the episodically activation of fast flow components in the wetland areas. At the catchment-scale, application of a hydraulic mixing-cell (HMC) methodology in combination with numerical modeling has revealed that fast flow components like saturated overland flow are exclusively generated in the wetland areas during intensive rainstorm events. On an annual basis, exemplarily for the hydrological year 2001, the HMC analysis quantified the relative contribution of saturated overland flow related to the total discharge with 19.5%, which highlights the importance of riparian wetlands for catchment-scale runoff generation. Virtual experiments, additionally show that distinct shifts between surface and subsurface flow dominance, as a result of small-scale micro-topographic driven runoff generation in the wetlands, are responsible for very complex three-dimensional subsurface flow patterns showing a wide range of subsurface residence times. To investigate how these micro-topography induced subsurface flow patterns, together with the non-uniform hydrological and biogeogeochemical boundary conditions, affect the internal re-distribution and transformation of redox-sensitive species (like nitrate, sulfate or iron) a coupled hydrological/biogeogeochemical model was developed. In the model, wetland-typical biogeochemical processes are represented in a sequential stream tube approach where redox-sensitive processes are implemented as kinetic reactions. Simulations show the formation of local hot spots for redox-sensitive processes within the subsurface as a result of the complex subsurface flow paths and the transport-limited availability of electron acceptors and donors. Formation of hot spots was simulated for all key reduction processes including iron(III)-/sulfate reduction and denitrification as well as for the corresponding re-oxidation processes. These results offer a new perspective on hydrologically controlled biogeochemical transformation processes in riparian wetlands, which provides a dynamic framework to explain process heterogeneity in wetland soils and variability in process rates over space and time. Findings from this thesis clearly prove how useful interdisciplinary approaches are in understanding processes and mechanisms in ecosystems and how important functions of ecosystems are affected by couplings among those. However, a lot of knowledge gaps still exist in understanding the nature of dependency between water and nutrient cycles across scales and how these interacting cycles feed back into humanly-mediated climate change in ecosystems. Development of new interdisciplinary methodologies and frameworks as well as an integrated way of thinking across the boundaries of the different environmental disciplines is necessary to address the grand challenges associated with climate change.

Tibet Plateau Atmosphere-Ecology-Glaciology Cluster Joint Kobresia Ecosystem Experiment: Documentation of the second Intensive Observation Period, Summer 2012 in KEMA, Tibet

Tobias Biermann — Tue, 07 May 2013 07:48:21 +0200

Experiment documentation of the second joined Kobresia ecosystem experiment conducted by the Atmosphere-Ecology-Glaciology Cluster within DFG SPP 1372 (Tibetan Plateau)in Kema, Tibet, China. The report provides background information about the field side, conducted measurements and participants.

Applicability of weight-shift microlight aircraft for measuring the turbulent exchange above complex terrain

Stefan Metzger — Thu, 18 Apr 2013 09:11:09 +0200

The possibility to reliably observe the exchange of heat and moisture between the land surface and the atmosphere is vital to our understanding of the regional and global cycling of energy and water. While ground-based flux measurements can be made continuously for long periods, they only represent a small landscape unit. On the other hand, aircraft-based measurements have the ability to directly measure the exchange over large areas. Especially over heterogeneous landscapes the spatio-temporal characteristics of both approaches complement each other. However, complex terrestrial ecosystems are sparsely investigated to date, in particular over topographically structured terrain. This can be attributed to; (i) limitations in the description of boundary layer processes over non-homogenous terrain, and (ii) a lack of applicable measurement platforms and techniques to study these processes. In pursue of a resolution strategy, this dissertation investigates the applicability of weight-shift microlight aircraft (WSMA) to gain new insights in the spatial variability of heat and moisture exchange over complex terrain. WSMA are comparatively cheap in procurement and maintenance, and their unique structure provides exceptional transportability and climb rate. These structural features qualify the WSMA for terrain-following flight over complex and inaccessible terrain, but potentially influence measurements aboard the aircraft. In this dissertation a WSMA with a scientific payload enabling fast measurements of the 3D wind, temperature, water vapor concentration, position, and the radiative flux is used to; (i) Quantify the WSMA wind measurement uncertainty. A novel time-domain procedure is developed, which improves the accuracy of the WSMA wind measurement by 63% for the horizontal- and 72% for the vertical wind components. The resulting precisions are ±0.09 m s−1 and ±0.04 m s−1, and the agreement with ground-based measurements is in the order of ±0.4 m s−1 and ±0.3 m s−1 (root mean square deviation), respectively. (ii) Quantify the WSMA eddy-covariance flux measurement uncertainty. From uncertainty propagation the smallest resolvable changes in friction velocity (0.02 m s−1), and sensible- (5 W m−2) and latent (3 W m−2) heat flux are estimated. In comparison to tower measurements, the WSMA observes higher fluxes (17–21%). The differences are not statistically significant, and can be explained by the tower setup and non-propagating eddies. (iii) Spatially resolve and regionalize the heat and moisture exchange above a complex landscape. Wavelet decomposition of the turbulence data is used to yield a flux observation each 90 m along the flight path. For each flux observation the biophysical surface properties in the flux footprint are determined. An environmental response function between the flux observations and biophysical and meteorological drivers is then inferred using a machine learning technique. This function is used to produce regional maps of the heat and moisture exchange to an accuracy of ≤18% and a precision of ≤5% for individual land covers. Hence this dissertation provides the necessary basis for using WSMA to investigate the mechanisms of turbulent exchange over heterogeneous and topographically structured terrain. Moreover, the developed algorithms are generally applicable to (i) partitioning flux uncertainty and environmental variability, (ii) extrapolating flux measurements, (iii) assessing the spatial representativeness of long-term tower flux measurements, and (iv) designing, constraining and evaluating flux algorithms for remote sensing and numerical modeling applications.

Heterogeneous chemistry of HONO and surface exchange

Matthias Sörgel — Fri, 22 Mar 2013 10:48:27 +0100

Nitrous acid (HONO) is an important precursor of OH radicals, which are the key oxidizing species in the atmosphere and are therefore called the detergent of the atmosphere. Despite the importance of HONO for atmospheric chemistry and about 30 years of detailed research the exact formation mechanisms of both day- and night-time formation remain unclear. The main formation pathways discussed to date are heterogeneous reactions with nitrogen dioxide as the HONO precursor or microbiological activity in soil. As the ground surface is a major source of HONO, the vertical distribution of HONO is very sensitive to the extent of vertical mixing. Additionally, some uncertainty in comparing laboratory and field measurements might be caused by the not yet clarified role of relative humidity and surface wetness on HONO formation and deposition, respectively. These influences have been investigated in field measurements in the present study. For HONO measurements, LOng Path Absorption Photometers (LOPAPs) have been deployed. During the Diel Oxidant Mechanism In relation to Nitrogen Oxides (DOMINO) campaign, HONO and other reactive trace gases were measured above a pine forest in south west Spain. In line with all recently published work, this study also found a substantial daytime formation of HONO. This so-called additional daytime source or unknown source was found to be slightly correlated (r² = 0.16) with actinic flux. Normalizing this unknown source to nitrogen dioxide mixing ratios improved the correlation (r² = 0.38), which indicates an influence of nitrogen dioxide availability. The coefficient of determination improved further to 0.47 by restricting the data to clear days and rejecting data from advection events. Thus, a fraction of the unknown source might be explained by light-induced nitrogen dioxide conversion but other factors have to be taken into account. Two processes of light-induced nitrogen dioxide conversion, proposed by recent laboratory studies, were shown to be negligible for the semirural conditions during our study. HONO photolysis was found to be the most important primary OH-radical source during DOMINO, contributing 20 % more OH than ozone photolysis integrated over the day. Vertical exchange of HONO was studied at the “Waldstein-Weidenbrunnen” field site of the University of Bayreuth in the Fichtelgebirge Mountains in south east Germany. The simultaneous HONO measurements in and above a forest canopy highlighted the importance of turbulent exchange for the vertical distribution of HONO mixing ratios. The so-called coupling regimes of the forest (with the air layers above) were found to be a very useful micrometeorological concept to study vertical differences of mixing ratios in a forest. They denote which parts of the forest are coupled to the air layer above the canopy and thus take part in turbulent exchange of energy and matter. With this coupling tool it was possible to explain vertical mixing ratio differences by different sources and sinks and the magnitude of the difference by the intensity of vertical exchange. Studying the vertical mixing ratio differences of HONO, an unexpected result was that during late morning and around noon they were close to zero. As the lifetime of HONO below canopy of about 250 to 300 min was a factor of 25 to 30 longer than that above canopy of about 10 min, large mixing ratio differences would have been expected. The lack of these differences could be explained by efficient vertical mixing, which was indicated by a full coupling of the forest or a coupling by sweeps and only intermittent decoupling of the subcanopy during these periods. Around sunset, the whole forest became decoupled from the air layers above. This caused a steep increase in mixing ratio differences up to about 170 ppt due to a faster increase below canopy, indicating local formation below the canopy. HONO and RH are correlated due to their diurnal cycles which are mainly caused by radiation. This diurnal contribution has to be removed from the respective signals in order to extract correlations on other timescales. Singular System Analysis, a tool for time series analysis, has been applied successfully to remove diurnal variations and long-term trends from the HONO and RH time series. Correlations of the higher frequency contributions of the remaining signals were poor but slightly positive. The HONO mixing ratios increase exponentially with RH from about 25 % RH to about 70 % RH. No clear correlation was found between around 70 and 95 % RH. Above 95 % RH, HONO mixing ratios decreased due to HONO uptake in droplets and liquid films. These features are in line with previously proposed mechanisms for interactions of water and HONO on surfaces. The study highlighted the need to assess turbulent transport and surface properties in addition to chemistry for understanding the heterogeneous reactions and processes forming HONO.

Kinetische Untersuchungen der Halogen-Aktivierung einer simulierten Salzpfanne in einer Smogkammer

Natalja Balzer — Fri, 15 Feb 2013 12:02:07 +0100

Reaktive Halogenverbindungen, insbesondere solche von Br und Cl, spielen eine wichtige Rolle beim atmosphärischen Abbau von Ozon und Kohlenwasserstoffen. Auch beeinflussen sie die Radikalzusammensetzung in der Troposphäre und haben somit eine Auswirkung auf klimarelevante Prozesse. Ziel dieser Arbeit war die Untersuchung der Halogenfreisetzung aus einer künstlichen Salzpfanne bei unterschiedlichen umweltrelevanten Bedingungen, wie relative Feuchte (RF) und Konzentrationen von Stickoxiden und Kohlenwasserstoffen, in einer Smogkammer. Diese Methode wurde in Rahmen dieser Arbeit entwickelt, um diese bislang wenig untersuchte Quelle von Halogenen zu untersuchen. Als Modell für die Salzpfanne wurde eine umweltrelevante Mischung von NaCl mit NaBr verwendet. Kinetische Berechnungen lieferten die zeitlichen Verläufe der Cl-Atome und OH-Radikale aus dem Verbrauch der zugesetzten Kohlenwasserstoffe und der Br-Atome aus dem Ozonverbrauch und UV-Absorptionsmessungen die Verläufe der BrO-Radikale. Während Salzpfannen-Experimenten bei einer mittleren RF von 37 % erreichte die Cl-Konzentration ein Maximum von 5 x104 cm-3. Ein rascherer Ozonabbau wurde bei größerer RF und zugleich höherer BrO-Konzentration beobachtet. Bei den Experimenten in Anwesenheit von sekundären organischen Aerosolen (SOA) verlangsamte sich der Ozonabbau um einen Faktor 16. Das BrO-Mischungsverhältnis erreichte in diesem Fall maximal 0.15 ppb im Vergleich zu Experimenten ohne SOA, bei denen das BrO Mischungsverhältnis 6 ppb erreichte. Nach Einwirkung der aus der Salzpfanne freigesetzten Halogene auf das SOA wurde ein erneutes Partikelwachstum beobachtet. Dies könnte eine Hauptsenke für Halogene in der Atmosphäre darstellen und einen neuen Weg zur Partikelbildung.

Rhizodeposition and its effects on C fluxes in the soil

Johanna Pausch — Wed, 06 Feb 2013 11:58:39 +0100

Organic compounds released from living roots (rhizodeposits) are easily available sources of energy for microorganisms strongly affecting soil organic matter (SOM) dynamics. Although, rhizodeposition is a key driver of microbially mediated processes in the soils, it still remains the most uncertain component of the terrestrial carbon (C) cycle. The input of C through rhizodeposition occurs in temporal and spatial hotspots. The objective of Study 1 was to determine the dynamics of hotspots of recently assimilated C in ryegrass roots. Shoots were 14CO2 pulse labeled and the allocation patterns at increasing time intervals were visualized by phosphor imaging. We could show a quick translocation of assimilated C to the roots. 14C hotspots were detected at the root tips already 6 hours after labeling. The hotspots remained active for at least 2 days. Eleven days after assimilation the hotspots at the tips had disappeared, and the 14C distribution was much more even than after 6 hours and 2 days. Through the availability of rhizodeposits, hotspots create preferred habitats for microbes. Rhizodeposits are an important source of C and energy for microorganisms stimulating their growth and activity. Thereby, roots can influence the rate of native SOM decomposition in the rhizosphere. This rhizosphere priming effect (RPE) was reported to be plant-species specific. Therefore, we hypothesized that also plant inter-species interactions affect the RPE. In Study 2, we used continuous 13CO2 labeling to investigate the RPE of monocultures and mixtures of typical agricultural crops. The RPE was consistently positive for all cultures with an increase of 43% - 136% above the unplanted soil. Of particular interest was the result that plant inter-species interactions between sunflower and wheat significantly reduced the RPE in contrast to mixtures which included soybean as a legume. It was argued that the RPE of the sunflower-wheat mixture was reduced through a more severe competition for nitrogen (N), whereas, due to the N-rich rhizodeposits of the legume and its lower demand for soil mineral N the RPE of the legume containing mixtures remained unaffected. Besides potential plant-specific differences in the quality and quantity of rhizodeposits, also photosynthesis could control root exudation because of the fast transport of recently assimilated C to belowground pools. Taking both factors into account, in Studies 3 and 4 the effect of limited photosynthesis on the distribution of recently assimilated C, of stored C and of N was investigated. Based on 13C, 14C and 15N labeling of a legume and a non-legume we could demonstrate that high C and N demands of regrowing shoots after clipping led to a remobilization of stored C and N to the shoots. Additionally, recently assimilated C was retained in the regrowing shoots. Shading, in contrast, did not induce a remobilization of stored C, since recently assimilated C obviously covered the demand of the shoots with lower growth rates. For both treatments lower amounts of recently assimilated C were observed in the belowground pools emphasizing the importance of the tight coupling of assimilation and belowground processes. Furthermore, different responses of clipping and shading of the legume and the non-legume could be detected for root-derived CO2. The quantitative importance of rhizodeposition at field scale was determined in Study 5. We proposed a new approach for an improved quantification of rhizodeposition under field conditions taking into account the decomposed fraction of rhizodeposits. Based on a 14CO2 pulse labeling experiment under controlled conditions a rhizodeposition-to-root ratio was calculated and was applied to the root biomass of the field. The root biomass C of maize, sampled in July 2009, was 298±64 kg C ha-1. Gross rhizodeposition was 166±53 kg C ha-1. With aging of SOM, the availability of C for microbial decomposition declines. In Study 6 the availability of younger relative to older C sources was assessed. The natural isotope abundances of 13C and 12C of SOM and CO2 were analyzed after a C3 to C4 vegetation change. The contribution of younger C, originating from the belowground C input by maize in the previous year, and that of older C sources, derived from the former C3 vegetation, to SOM and CO2 was determined. Comparing the proportions of younger and older C in SOM with that in CO2, we found that younger C was 7 times more available for microbial decomposition than older C pools. In summary, this thesis extends the understanding of factors affecting rhizodeposition and of processes occurring at the soil-root interface. Furthermore, it presents a new method to quantify gross rhizodeposition at field scale. Although, we could gain insight in temporal changes of the availability of C pools for microbes, the ecological importance of C fluxes in the rhizosphere requires future research on this topic with regard to spatial and temporal predictions.

Jahresbericht 2010-11 zum Förderprojekt 01879 Untersuchung der Veränderung der Konzentration von Luftbeimengungen und Treibhausgasen im hohen Fichtelgebirge 2007 – 2014

Thomas Foken; Lisa Dirks — Mon, 07 Jan 2013 10:08:37 +0100

no abstract

Beyond productivity- Effects of extreme weather events on ecosystem processes and biotic interactions

Julia Walter — Mon, 12 Nov 2012 10:13:43 +0100

Under global climate change, extreme weather events, such as heat waves, drought or heavy rain spells, are projected to increase in magnitude and frequency. As these may affect vegetation and ecosystems more than gradual shifts in mean climatic parameters, investigating the consequences of extreme weather events recently became an important issue in climate change research. The main focus of most experiments investigating effects of extreme weather events on vegetation is on primary productivity. In our experiment in artificially planted communities, even an extreme drought of 1000-year recurrence did not have effects on above- or below-ground biomass production from 2005-2010. Thus, the main objectives of this thesis were (1) to investigate if extreme weather events have an effect on ecosystem functions beyond productivity, (2) to test if such a high resistance or resilience in response to drought regarding productivity also exists in more naturally grown plant communities and (3) to further elucidate possible mechanisms of the surprisingly large stability of the plant communities. To investigate these objectives, several experimental studies were conducted in artificially planted, as well as in naturally grown grassland communities and consequences of extreme weather events for ecosystem processes, such as decomposition and herbivory were investigated. In a pot experiment, it was studied, if grass plants react improved towards repeated drought when compared to a first drought and thus reveal a kind of drought memory. Such a memory might be one possible, but up until now widely neglected mechanism of resilience. Even though biomass production remained stable in our experiment in artificially planted communities, biomass quality was severely affected by extreme drought, thereby strongly affecting the development of a herbivore caterpillar feeding on drought-exposed leaves. Further, plant compounds of the host plant depended on the composition of the plant community it was grown in. This in turn resulted in strong effects on the larval mortality of herbivores feeding on such plants. In contrast to the study in artificially planted communities, aboveground net primary productivity (ANPP) was reduced in naturally composed grassland in response to extreme rainfall variability, including an extreme drought followed by heavy rainfall. Forage quality was altered by drought. Furthermore, mowing frequency strongly altered forage quality and biomass production, but did not interact with rainfall variability and thus did neither buffer, nor amplify effects of extreme rainfall variability. Despite effects of rainfall variability on ANPP, grassland showed high resilience after drought followed by heavy rain, as effects were large shortly after the extreme event, but did not persist until a second harvest later in the year. In natural grassland, rainfall variability and drought also affected ecosystem processes, here litter decomposition, beyond productivity. Drought followed by heavy rain pulses decreased decomposition rates. Decomposition in more frequently mown meadows was more vulnerable towards drought exposure. Winter warming and additional winter rain had no long-term effect on decomposition. To conclude, projected increases in drought frequency under climate change may inhibit decomposition and alter nutrient and carbon cycling along with soil quality in temperate grassland, whereas a reduction of snow cover leading to more variable soil surface temperatures may counteract increased decomposition under winter warming. In this thesis, an ecological stress memory as one possible mechanism of resilience is defined as any response of a single plant after a stress experience that improves the reaction of the plant towards future stress experience and which is assessed on a whole plant level. This thesis further provides evidence of a drought memory in grass plants: Plants repeatedly subjected to drought showed improved photo-protection and a higher rate of living biomass when compared to plants faced with their first drought. Similarly, tree seedlings exposed to drought in summer revealed higher frost resistance during winter, providing evidence of a long-lasting “cross-stress-memory” . To sum up, the thesis shows that extreme weather events, even though neither severely affecting biomass production in artificially composed, nor in naturally growing communities in the long-term, exert strong influence on physiological or biogeochemical parameters, such as plant compounds or soil biotic activity. These changes in turn modify ecosystem functions beyond productivity, for example herbivory or decomposition, possibly altering biotic interactions and nutrient cycling. Furthermore, the findings imply that plants exhibit a stress memory after stress exposure, which may be one mechanisms leading to a high stability and resilience upon frequent stress.

The Nature of Fluids in Hydrothermal Copper and Molybdenum Ore Deposits - An experimental and analytical study

Linda Lerchbaumer — Mon, 22 Oct 2012 08:39:27 +0200

The evolution of magmatic-hydrothermal fluids in porphyry Cu and porphyry Mo deposits was studied using synthetic and natural fluid inclusions by optical microscopy, microthermometry, Raman spectroscopy, and LA-ICP-MS. The partitioning of Cu between vapor and brine in aqueous NaCl-S ± KCl ± FeCl2-rich fluids was investigated by means of hydrothermal experiments in rapid quench autoclaves at 600-800°C, 70-130 MPa and at both oxidizing and reducing fO2, covering all geologically relevant conditions. Resulting partition coefficients (DCuvap/brine) are between 0.2 and 0.4 for the range of studied S-concentrations, fluid pH, fO2, and P-T conditions. These values indicate that Cu does not partition into the vapor phase at any plausible condition in contrast to data from natural quartz-hosted vapor and brine inclusions which appear to indicate Cu enrichment in the vapor. The formation of such Cu-rich vapor-type fluid inclusions was investigated in hydrothermal re-equilibration experiments. For this purpose, coexisting vapor and brine inclusions of known composition were re-equilibrated in a fluid of slightly different composition and lower pH than the trapped one at 800°C, 70-130 MPa. This procedure led to a dramatic increase in Cu concentrations in the vapor phase from 0.3 ± 0.03 to 5.7 ± 3.3 wt% after re-equilibration and the change of DCuvap/brine from a true value of 0.4 ± 0.05 to an apparent value of 8.3 ± 4.9. This post-entrapment modification can be traced back to the difference in fluid pH between the trapped and the surrounding fluid, inducing diffusion of H+ out of the inclusion and the diffusion of Cu+ (and Na+, Ag+) into the inclusion in order to maintain charge balance. Moreover, the presence of larger amounts of S within vapor inclusions as compared to brine inclusions can bind larger amounts of Cu. The re-equilibration of trails of vapor and brine inclusions in a natural quartz sample in a fluid similar to the trapped one, yet more acidic, showed that this modification process can be reversed, resulting in the loss of major amounts of Cu from natural vapor inclusions. The composition of metal-bearing melts and magmatic fluids of Mo-mineralized granites resembling porphyry Mo deposits was investigated using natural melt-, fluid-, and solid-inclusions in quartz crystals found in miarolitic cavities from minor Mo-occurrences in Colorado and Norway. Melt inclusions from the Treasure Mountain Dome are highly enriched in Mo (4-43 ppm), also melt inclusions from the Drammen and Glitrevann granites (5-32 ppm, and 12 ppm, respectively), resembling highly fractionated melts. Copper concentrations are low (<1-30 ppm) in the melts, but high in the fluids: intermediate density, supercritical fluid inclusions of these three locations host 6-1900 ppm, 8-3500 ppm, and 5-180 ppm Cu, respectively. The comparison of these results with data from economic porphyry Mo and porphyry Cu (Mo, Au) deposits shows no difference in Mo concentrations in the particular melts and fluids. Hence, other factors probably control the evolution of a granitic intrusion into large, economic Mo deposits, minor Mo occurrences, or just into barren plutons. These may be mainly the size, position, and geometry of the parental magma chamber, the multiplicity of intrusions maintaining a constant flux of metalliferous, S-rich, hot melts, and the extent of fluid focusing within small rock volumes forming high-grade ore shells.

Denying access to water? Moral values and commercialization policies in Khartoum governmental water management

Anne-Sophie Beckedorf — Tue, 14 Aug 2012 10:13:04 +0200

This contribution draws on empirical fieldwork carried out in Khartoum/Sudan in 2009/2010 in order to examine the role of value systems in recent commercialization policies of Khartoum governmental water management. The first section provides background information about the current water supply system in Khartoum, which is a necessary precondition to understand current reform processes. The second section singles out three major aspects of commercialization policies and their contestations in greater detail: increases in water prices, increases in water cuts in case of unpaid water bills, and installations of prepaid water meters. The third section summarizes these contestations and argues that value systems are one major reason why current reform processes are not implemented in the way they were perceived.

Copper exposure of freshwater mussels (Anodonta anatina): Some physiological effects

Andhika Puspito Nugroho — Mon, 02 Jul 2012 15:00:53 +0200

Copper (Cu), a transition metal, has the tendency to increase in its concentration in freshwater ecosystems over natural levels, due to industrial and other anthropogenic sources. In water, copper can exist in dissolved form or associated with suspended food particles. Freshwater mussels living at the interface of the free-flowing water and the sediment phase can take up copper directly from the water or by consumption of lower trophic level organisms laden with copper. For mussels, copper is essential at low concentration as cofactor of metalloenzymes involved in growth regulation and development, but it may be toxic at higher levels by disturbing calcium (Ca) homeostasis. The duck mussel Anodonta anatina is a freshwater species found in abundance in limnic and lotic European ecosystems and is used as test organism in ecotoxicological studies. The potential involvement of Cu in the general decline of many European freshwater mussel species is the major motivation for this work. This research aims to study the relevance of Cu exposure pathways on its uptake, distribution, bioaccumulation, and elimination in the freshwater mussel A. anatina and its various potential physiological impacts. The work is started with raising Cu-loaded algae using the stable isotope 63Cu as marker for feeding of mussels without affecting the nutritional value of the algal food. In these latter experiments, mussels are exposed to 63Cu via water or via food to investigate the relative importance of Cu uptake to its distribution and accumulation among the mussel’s organs. Its consequences on calcium homeostasis, soluble carbohydrate and protein levels in various tissues, metallothionein induction, glutathione levels, activities of antioxidative enzymes and glutathione reductase, and on lipid peroxidation are examined. In the algal experiment, Parachlorella kessleri is grown at six 63Cu concentrations (0, 5.9, 11.7, 23.5, 47, and 94 µmol L-1) for 4 days, starting from day 3. When exposed to Cu at a level of up to 6 µmol L-1, P. kessleri is largely unchanged in its nutritional values; so this concentration is used to grow 63Cu-carrying food for mussel experiment. Concentrations above 6 µmol L-1 decrease significantly in the algal growth and alter the other physiological parameters. Three groups of 21 mussels each are used, one as control and two for exposure, receiving copper as the stable isotope 63Cu via the water at 0.3 µmol L-1 or via the food (1.5 mg L-1 freeze-dried Cu-loaded algae, equivalent to 0.06 µmol L-1 Cu) for 24 days, followed by 12 days of depuration. For analysis, three mussels each are taken randomly from every group at days 0, 6, 12, 18, 24, 30, and 36. The mussels are anaesthetized and hemolymph and extrapallial fluid are sampled before the mussels are dissected into gills, mantle, kidney, digestive gland, foot, adductors, intestines, and the remainder (gonads, heart, and labial palps). During copper exposure, the levels of exogenous copper (63Cu) and total Cu increase in all body compartments. Uptake via the water leads to higher Cu levels than via the food, but in relative terms food uptake is more efficient taking the five-fold lower nominal concentration of copper into consideration. Upon exposure via the water, the metal is compartmentalized mainly in the mantle, the gills, and the digestive gland, upon exposure via the food the major recipients are the digestive gland and the intestines. Upon depuration for two weeks, copper is quickly but not completely eliminated. Simultaneously with increasing Cu levels, Ca levels are increased in all body compartments, accompanied by decreases in soluble carbohydrates and proteins in the gills, mantle, digestive gland, and kidney. At the same time, Cu exposure results in increases in malondialdehyde levels, decreases in glutathione levels, strong increases in metallothionein levels, and changes in the activities of the antioxidative enzymes superoxide dismutase, catalase, and glutathione peroxidise, and of glutathione reductase in the gills, mantle, digestive gland, and kidney. During depuration, most parameters tend to normalize but do not return to control values. In conclusion, the overall pictures suggest that the considerable physiological stress elicited by low-level copper exposure may contribute to the factors involved in the decline of many European freshwater mussels.

Structure and Reactivity of Terrestrial and Extraterrestrial Pyrrhotite

Dennis Harries — Mon, 02 Jul 2012 09:22:00 +0200

Pyrrhotite (Fe1-xS) is a non-stoichiometric iron monosulfide common in terrestrial rocks, ore deposits, and many extraterrestrial materials. The non-stoichiometry due to metal vacancies relates to a variety of composition-dependent crystallographic superstructures, but little of the existing structural and microstructural complexity has been explored yet. This thesis investigates the occurrences and nature of pyrrhotite superstructures, examines the related nano- and microstructural phenomena, and explores their effects on chemical reactivity. The goal is to comprehend the relations of the nanoscale real structure of pyrrhotite to its physicochemical properties. A central tool in these studies is analytical transmission electron microscopy (TEM), which has been extensively used to study terrestrial and extraterrestrial samples. In three studies, published or submitted as scientific research articles, it is shown that structural complexity of pyrrhotites is a widespread feature in terrestrial and extraterrestrial materials and is strongly interrelated with its physicochemical properties and environments of formation and alteration. A new model based on translation interface modulation is being introduced to provide a realistic description of the structural state of natural NC-pyrrhotites. Novel insights into the thermodynamically stable phase assemblages in the Fe-S system at ambient temperatures are presented and the crystallography and connected thermochemistry of pyrrhotites are deployed to reach new conclusions about the petrogenetic history of chondritic meteorites and the alteration processes they were involved in. Finally, an experimental alteration study reveals for the fist time quantitatively that the vacancy superstructures and anisotropy of pyrrhotites have tremendous effects on their kinetic behaviors during dissolution under acidic and oxidizing conditions. Intrinsic reactivity differences between 4C- and NC-pyrrhotite are clearly resolved and discusses in the framework of the newly established structure model.

Whole-air relaxed eddy accumulation for the measurement of isotope and trace-gas fluxes

Johannes Ruppert; Michael Riederer; Willi A. Brand; Thomas Foken — Thu, 21 Jun 2012 08:59:39 +0200

Measuring the isotopic composition of trace gas fluxes can provide additional information on ecosystem gas exchange, when ecosystem processes, like assimilation, discriminate against heavier isotopes. In the case of CO2 exchange, different mass-balances for bulk CO2 and its 13CO2 or CO18O isotopes can be used to separate respiration from photosynthetic assimilation. Up to now, detectors for direct isotope measurements in the field lack the precision needed for fast eddy covariance (EC) flux measurements. The collection of updraft and downdraft whole-air samples using the relaxed eddy accumulation technique (REA) allows simultaneously determining trace gas concentrations and isotope ratios by high precision laboratory analysis. At the same time whole-air REA relaxes several of the technical problems related to REA sampling on traps. In tests using air from a tank the complete whole-air REA sampling system and its foil balloon bag reservoirs showed no signs of contamination after cleaning. The standard deviations of δ13C and δ18O isotope ratios were only slightly higher than the precision specified for the laboratory analysis procedure. First experiment results showed that isotopic differences (up-drafts−downdrafts) were large enough to yield signal to noise ratios greater than five when applying hyperbolic deadbands during REA sampling (HREA). The performance of the instrument and the HREA sampling method are investigated by simulation of the sampling process for bulk CO2, which serves as proxy scalar. Measurements by whole-air HREA in combination with high precision isotope analysis can quantify the isofluxes of 13CO2 and CO18O. Furthermore, additional information is collected on the scalar correlation of bulk CO2 and its stable isotopes, which represents the relatively short timescale of updrafts and downdrafts in the turbulent exchange above an ecosystem. This information is essential to check the scalar similarity assumptions made in the HREA and EC/flask method for the quan-tification of isofluxes.

Major element diffusion in garnet and the exsolution of majoritic garnet from aluminous enstatite in Earth's Upper Mantle

Willem L. van Mierlo — Thu, 26 Apr 2012 09:22:29 +0200

Majorite is a high pressure polymorph of enstatite with the garnet structure. The amount of enstatite that can be dissolved in garnet as a majorite component increases significantly with pressure, and therefore, majoritic garnet is thought to be a major constituent of the Earth's transition zone. The transport properties of majoritic garnet are, however, not well constrained at the moment. The magnitude of the diffusivity of the majorite component in garnet influences our understanding of the homogenization time scale of Earth's mantle. This is important in subduction zone settings, where the subducting oceanic crust will form a majorite inhomogeneity in the transition zone because of its higher aluminium content. Reaction kinetics in the dry transition zone are diffusion controlled and therefore an improved dataset on the diffusivity of the majorite component in garnet will enable us to better understand the of role of disequilibrium in subduction zones. This dissertation therefore reports the results of diffusion experiments on garnet. Diffusion experiments have been conducted with diffusion couples of majoritic garnet – Dora Maira pyrope, Dora Maira pyrope and Ötztal almandine and Ötztal almandine and majoritic garnet in a multi-anvil press between 1400 – 1900 °C and 12 – 20 GPa. The diffusion experiments with the majoritic garnet – Dora Maira pyrope garnet couples show that the diffusion of the majorite component in garnet is very slow, comparable to the diffusivity of silicon in wadsleyite and ringwoodite. The activation energy, activation volume and the pre-exponential for diffusion of the majorite component in garnet were determined to be 241 ± 54 kJ mol-1, 3.3 ± 0.1 cm3 mol-1 and 2.3 x 10-7 cm2 s-1, respectively. The diffusivity of the majorite component in garnet was determined to be 2-3 orders of magnitude slower than the self-diffusivity of Mg, Fe and Ca in garnet at the same conditions. Also Fe – Mg interdiffusion appeared to be significantly faster in majoritic garnet than in almandine garnet. Comparison with diffusion data on wadsleyite and ringwoodite shows that the diffusivity of the majorite component in garnet is very similar to that of the silicon self-diffusivity in the high-pressure polymorphs of olivine. The diffusion data obtained in this PhD has been used to determine whether solid state diffusion can homogenize the mantle after subduction. The diffusion distance of majoritic garnet has been calculated assuming grain boundary diffusion is the main mechanism, and it can be concluded that solid state diffusion is not able to homogenize the mantle. Next to this, a numerical model has been developed that determines whether diffusion of the majorite component is fast enough such that enstatite can dissolve into garnet during subduction. The results show that there will be a significant delay in case of the lower lithospheric mantle of the subducted slab. Due to its lower tempeture, the oceanic crust can, however, only dissolve a fraction of its pyroxene content and metastable pyroxene is thus expected to be present during subduction into the transition zone. The metastable presence of pyroxene leads to the question to what will happen to its aluminium contents as it is expected to get exsolved as garnet. Experiments were performed on aluminous enstatite at 1700 °C and 15 GPa. It is shown that majoritic garnet exsolves with the dominant topotactic relation being [001]clinoenstatite parallel to <111>garnet. Also a high density of stacking faults were observed with a displacement vector of R = ½[1 1 1] which can be explained by the transformation of HP high-clinoenstaite to low-clinoenstatite. Using the aluminium concentration profiles in clinoenstatite directly adjacent to the garnet precipitates the aluminium diffusivity in HP high-clinoenstatite was determined to be at least 6 x 10-11 cm2 s-1 at 1700 °C and 15 GPa. Comparison with data in diopside shows there is a discrepancy between diffusion data at high pressure and at low pressure, which might indicate a strong dependence of Al diffusivity in clinopyroxene on Ca contents or a change in diffusion mechanism. The results of the experiments conducted in this PhD study show that the low diffusivity of components in the Earth may severely hamper reaction kinetics in the Earth in the case where mass transport is required.

Klimawanderweg auf der Landesgartenschau in Bamberg 2012

Thomas Foken — Tue, 10 Apr 2012 12:12:33 +0200

The role of life history traits for coexistence and forest recovery after disturbance – a modelling perspective. Towards a better understanding of species-rich forests

Claudia Dislich — Tue, 10 Apr 2012 11:13:01 +0200

Tropical forests are well known for their exceptional species richness – high diversity of plant species constitutes the basis for an equivalently rich fauna. An astonishing variety of plant life strategies has evolved, manifesting itself also in different compositions of life history traits in trees. This thesis investigates the role of tree life history traits (growth, mortality and recruitment) on different processes structuring species-rich forests. Our study system is a montane rainforest located in the Tropical Andes hotspot of biodiversity in southern Ecuador. Here, we find a mosaic of steep ridges and deeply incised valleys, covered with predominantly broadleaf forest. Forest structure and species composition differ considerably depending on altitude and topographic position. The forest cover is frequently interrupted by scars of landslides, which constitute an important type of natural disturbance in this ecosystem. We utilize ecological models as tools to gain deeper insights into key processes driving the maintenance of tree species richness and affecting forest recovery after landslides. The first part of this thesis concerns the question of species coexistence. We develop a theoretical model to analyze how different trade-offs between life history traits (tree growth, seed dispersal, tree mortality) affect tree species coexistence. We find that the considered trade-offs alone are not sufficient to explain long-term species coexistence. Additional 'stabilizing' mechanisms seem to be indispensable to facilitate coexistence in species-rich forests. Such mechanisms could result from biotic interactions that alter the relation between inter- and intra-specific competition depending on (local) species abundances (e.g. density-dependent mortality). Other possible coexistence mechanisms likely to be relevant to our particular study system are driven by external, abiotic factors like a complex topography resulting in locally differing habitat types (each supporting a different set of species), or the character of a prevailing disturbance regime (e.g. shallow landslides). In the second part of the thesis, we investigate the growth dynamics of the ridge forest in our study system. To this end, we utilize the process-based forest growth model FORMIND. We show that after calibration, the model successfully reproduces forest dynamics on different levels of complexity (e.g. basal area and stem size distribution). We then use this forest model to investigate the influence of landslide disturbances on forest dynamics both on the local scale of a single landslide and on the landscape scale. On landslide sites, changes in environmental conditions might lead to changes in different tree life history traits. We analyze scenarios with changes in different traits (tree recruitment, tree growth, tree mortality) and find that while tree biomass can recover within the first hundred years after a landslide, the time until forest structure and species composition is restored is considerably longer (approximately 200 years). Changes in different traits result in differing spatial distributions of tree biomass: reduced tree growth leads to a more homogeneous distribution of biomass, whereas reduced recruitment and increased mortality yield a more heterogeneous biomass distribution ('patchy' vegetation). On the landscape level, overall forest biomass is substantially reduced by landslides (8-14%), compared to only 2-3% of the area marked by visible traces of landslides. Thus this particular type of disturbance considerably influences the total forest carbon balance. In a complementary investigation we study abiotic and biotic factors that potentially trigger landslide occurrence in our study system. For this, we develop an extension of a standard physically-based model of slope stability. We find that due to the predominantly shallow tree roots, some of the observed landslides might be triggered by the vegetation itself. This thesis demonstrates that ecological models are useful tools to gain deeper insights into important processes shaping forest communities. They can be applied for theoretical questions such as the question of species coexistence, as well as for more applied, management related questions like predicting forest recovery after disturbances.

The Arctic Turbulence Experiment 2009 - additional laser Scintillometer measurement campaign 2009 at the Bayelva catchment on Svalbard: Technical documentation and visualization of the near surface measurements during the ARCTEX-2009 campaign, August, 10th to August, 20th 2009

Johannes Lüers; Jörg Bareiss; Martin Wagner — Fri, 23 Mar 2012 12:38:53 +0100

Accurate quantification of turbulent fluxes between the surface and the atmospheric boundary layer in polar environments, characterized by frequent change of weather and exchange conditions (stable to very stable or intermittent; rapid, short term neutral to unstable stratified conditions) is a fundamental problem in soil-snow-ice-vegetation-atmosphere interaction processes. The observed rapid climate warming in the Arctic requires improvements in the permafrost and carbon cycle monitoring. To address these problems, it is essential to improve the databases with high-quality in-situ measurements of turbulent fluxes above tundra landscape surfaces applying the Eddy-Covariance method and the laser scintillometry. Results from the Arctic Turbulence Experiment 2006 on Svalbard helped to better understand physical exchange processes of energy and matter transport and to improve instrumentation standards as well as quality assessment techniques (Lüers and Bareiss 2010, 2011; http://www.arctex.uni-bayreuth.de). Therefore, the primary goal of this additional laser scintillometer measurement campaign is to estimate the flux contributions covering typical tundra surfaces across the Bayelva catchment during a summer season south-west of the Ny-Ålesund village, Kongsfjord, Svalbard. This effort makes it possible to define the spatial context of the fluxes, and to include land use features of the surrounding terrain in the quality assessment of all observations in the Bayelva catchment over the last 10 years performed by the Alfred Wegener Institute for Polar and Marine Research (AWI).

Applying regional climate change projections for spatio-temporal risk analyses of vector-borne diseases

Dominik Fischer — Tue, 21 Feb 2012 10:46:29 +0100

Bei vorliegender Dissertation handelt es sich um eine Abhandlung zu vektor-assoziierten Krankheiten in Zeiten des Klimawandels. Bei vektor-assoziierten Krankheiten wird ein Pathogen durch einen Vektor (Überträger), auf ein Wirtstier übertragen. Als solche Vektoren agieren meist Arthropoden. Klimatische Veränderungen beeinflussen vektor-assoziierte Krankheiten insbesondere dadurch, dass Arthropoden ihre Körpertemperatur nicht selbst regeln können und zudem bestimmte Temperaturansprüche zur Pathogenentwicklung im Vektor erfüllt sein müssen. Das Klimaänderungssignal des 21. Jahrhunderts wird von Klimamodellen in verschiedenen räumlichen und zeitlichen Auflösungen wiedergegeben. Die Projektionen beruhen auf Emissionsszenarien klimawirksamer Treibhausgase. In der Arbeit werden die Einsatzmöglichkeiten von regionalen Klimamodellen zur Gefährdungsabschätzung anhand verschiedener Fallbeispiele aufgezeigt. Deren Nutzen und Einsatzmöglichkeiten werden einführend aufgeführt. Für die Risikoanalysen werden regionalen Klimamodelle REMO und COSMO-CLM angewandt, die durch dynamisches „Downscaling“ globaler Modelle generiert wurden. Beide sind in ihrem neuesten Prozesslauf in das globale Modell ECHAM5 eingebettet. Der direkte Übertrag bekannter Temperaturansprüche von Vektor und/oder Pathogen auf künftig zu erwartende Bedingungen stellt den ersten methodologischen Schwerpunkt dieser Arbeit dar. Eine Amplifikation des Dengue-Virus im Überträger der Stechmücke Aedes aegypti könnte demnach zunächst in Südeuropa, im weiteren Verlauf des 21. Jhd. aber auch in weiteren europäischen Regionen möglich sein. Weiterhin verdeutlichen die Ergebnisse, dass sich auch das Zeitfenster einer potentiellen Übertragung des Dengue-Virus verlängern kann. Durch das Überlagern der bekannten Temperaturansprüchen von Sandmücken (Gattung Phlebotomus) und der von ihnen übertragbaren Erreger - Leishmania infantum Komplex - können potentielle Regionen Deutschlands identifiziert werden, in denen einer autochthone Übertragung der Leishmaniose möglich ist. Es ist zu erwarten, dass ein solches Risiko zunächst in südwestlichen und westlichen Regionen Deutschlands, im späteren Verlaufe des des 21. Jhd. jedoch auch für eher nördlich und östlich gelegene Regionen bestehen wird. Der zweite innerhalb dieser Arbeit gewählte methodologische Ansatz zeigt die Einsatzmöglichkeiten regionaler Klimaprojektion für die bioklimatische Nischenmodellierung von Krankheitsüberträgern auf. Die anhand statistischer Verfahren ermittelte bioklimatische Nische der jeweiligen Art wird hierbei auf zukünftig zu erwartende klimatische Bedingungen übertragen. Anhand dieser Analyse kann aufgezeigt werden, dass sich die klimatische Eignung für die invasive Stechmücke Aedes albopictus (Überträger mehrere human-pathogener Viren) ausgehend von westlichen Regionen Europas über Mitteleuropa und schließlich Osteuropas erhöhen wird. Der Transfer der ermittelten spezifischen klimatischen Nische ausgewählter Sandmücken-Arten (u.a. Überträger der zum Leishmania-Komplex zählenenden Pathogenen) auf künftige Bedingungen lässt vermuten, dass deren klimatische Eignung in Mitteleuropa - abgesehen von alpinen Regionen - zunehmen wird. Künftige potenzielle Ausbreitungswege der Sandmücken in einer sich verändernden Umwelt, werden via “least-cost analysis“ ermittelt. Die Ergebnisse deuten darauf hin, dass aufgrund der eingeschränkten natürlichen Ausbreitungsfähigkeit, einige der künftig potenziell geeigneten Lebensräume nicht erreicht werden. In den verschiedenen Fallstudien kann gezeigt werden, dass die zu erwartenden klimatischen Veränderungen im 21. Jhd. eine mögliche Ausbreitung der in dieser Arbeit adressierten Vektoren und vektor-assoziierter Krankheiten in Europa begünstigen werden. Als einheitliche Tendenz kann speziell für Mitteleuropa festgehalten werden, dass sich die Gefährdung, Ende des 21.Jhd. erhöhen wird. Dies begründet sich höchstwahrscheinlich durch die projizierte raschere Erwärmung in der zweiten Jahrhunderthälfte. Abschließend bleibt jedoch festzuhalten, dass es neben klimatischen Veränderungen weitere Faktoren für die Ausbreitung bzw. Neuetablierung von Vektoren und den damit verbundenen übertragbaren Infektionskrankheiten ausschlaggebend sind. Der Einfluss einzelner Faktoren auf die Etablierung bzw. Ausbreitung vektor-assoziierte Krankheiten variiert auf raum-zeitlichen Skalen. Für die ermittelten klimatisch-abgeleiteten Risikogebiete sollten in Folgestudien auf kleineren Skalen wirksam werdenden Faktoren integriert werden. Diese Ergebnisse können wiederum die Entwicklung von Surveillance- und Monitoringprogramme unterstützen, um somit Maßnahmen gegen die Ausbreitung von vektor-assoziierten Krankheiten initiieren zu können.

Turnover and fluxes of carbon and nitrogen in a spruce forest under natural and extreme meteorological conditions

Kerstin Schulze — Thu, 09 Feb 2012 09:01:20 +0100

Climate models predict an increase in the intensity and frequency of extreme meteorological climate events like extended summer droughts, heavy rainfall or intensive frost periods with largely unknown effects on microbial activity and pysico-chemical soil properties and their impact on availability of soil organic matter. The influence of drying/rewetting (A/W) and freezing/thawing (G/A) events on solution chemistry and leaching losses of soils is barely known. This thesis aimed to study the effects of A/W and G/A events on soil solution chemistry and solute fluxes, in particular, of dissolved organic carbon (DOC) and inorganic nitrogen (NH4+, NO3-) in a podzol soil under a Norway spruce forest. A field experiment was designed to study the effects of (i) summer drought by exclusion of natural throughfall and subsequent rewetting and of (ii) soil frost by removal of natural snow cover. In complementary laboratory experiments with undisturbed soil columns, (i) drying/rewetting cycles were simulated with different rewetting intensities and (ii) freezing/thawing cycles were induced using different freezing temperatures. In the second part of this work, total C and N stocks as well as radiocarbon signatures of soil organic carbon (SOC) from different soil horizons and density fractions were investigated. A/W increased the DOC concentrations in the organic layer and upper mineral soil. More DOC was released from the organic layer to the mineral soil. However, the effects on total DOC leaching were smaller due to reduced water fluxes. Specific UV absorbance and emission fluorescence detected a switch in the release of easily decomposable DOC to hardly decomposable DOC during the wetting phase. Prolonged summer drought and incomplete rewetting due to hydrophobicity of SOM in the organic layer and upper mineral horizon reduced net N mineralisation as well as concentrations and fluxes of the NH4+ and NO3-. The net nitrification rate in the organic layer was more negatively influenced than net ammonification, indicating that nitrifiers are more sensitive to drought stress than ammonifiers. The effect of soil frost strongly depended on soil freezing temperature. Only soil frost at temperature below -8°C led to short periods of additional DOC production in the organic layer. Spectroscopic properties and ∆14C signatures of DOC implied a disruption of soil aggregates and desorption of older DOC from the mineral associated organic matter fraction of the Oa horizons by G/A events. Severe soil frost below -8°C inhibited the activity of nitrifiers and ammonifiers with decreased NH4+ and NO3- concentrations and fluxes in the mesocosm experiment. A delayed (by 4 months) increase in NO3- concentration in the upper soil horizon by moderate soil frost (-5°C) was attributed to reduced Immobilisation by heterotrophic microorganisms. Summarised, drying and the effect of hydrophobicity led to long-term, severe soil frost to short-term reduction in N mineralisation and N leaching. The effect of increased NO3- concentrations as delayed response to G/A needs further research in case of potentially changes in the N balance. Drying as well as freezing induced changes in the soil structure and properties and led to increased DOC concentrations. Moderate soil temperature had much less effects on C and N in this temperate forest soil. The results of this thesis demonstrated the potential of extreme meteorological events on the quality and availability of dissolved C and N. Both, A/W and G/A cycles decreased C and N mineralisation, increased the sink strength of the soil by the accumulation of SOC and N, considering constant C and N litter input. However, optimal temperature and moisture conditions in other seasons could compensate the sink strength of soils. This work underpins the need for holistic and long-term investigations to understand and model the impact of extreme meteorological conditions on the dynamics of dissolved C and N.

Gross N turnover and soil solution chemistry as influenced by fluctuations of soil water potential and water table in a Podzol and a fen soil

Yao-Te Chen — Thu, 19 Jan 2012 09:17:02 +0100

Given the climate scenarios, the higher frequency of drying/rewetting cycles of soils in the future can be expected. These changes of the meteorological conditions likely result in an increasing frequent and intensive drought periods in summer, causing irregular and extreme drought stress in forest soils or a drawdown of water table in wetland ecosystems, which may influence the turnover of nutrients in soils to a larger extend than previously thought. The question arises how these climate changes will influence N and C turnover in forest and fen soils. A growing number of laboratory studies on drying/rewetting of soils have been published during past decades, but many studies used either disturbed soil samples or intact soil cores in laboratory. Although soil drying is a frequent phenomenon in the field, the long-term effects of drying/rewetting and irrigation on in situ fluxes and concentrations of solutes in forest and fen soils are unclear. Several studies have investigated the influence of soil water content on net N turnover rather than gross rates. Net ammonification and nitrification include two major processes: gross ammonification and gross nitrification on the one side and microbial immobilization on the other side. To identify the response of specific processes to soil drying, gross rates need to be measured. This thesis focused on the impact of changing water potential or water table level on gross N turnover rates and soil solution chemistry in two different ecosystems in South-Eastern Germany. In a Norway spruce forest, the effects of decreasing water potential and prolonged periods of summer drought on soil gross N turnover were investigated by laboratory and field experiments. Soil solutions and throughfall were collected and the cumulative in situ fluxes of DIN, DON and DOC with forest floor percolates were calculated. In a minerotrophic fen, we studied the response of N and C mineralization and soil solution chemistry to water table fluctuations in a laboratory experiment. In the field, we collected the soil pore water in 3 depths to clarify the long-term effects of water table level on the concentrations of solutes. Homogenized soil samples of the Oi+Oe, Oa and EA horizons were taken and adjusted to 6 different water potentials in the laboratory. In the field experiment, throughfall exclusion and irrigation plots were established to simulate different precipitation patterns of a dry and wet growing season. Gross N turnover rates were determined in undisturbed soil cores from Oi+Oe and Oa+EA horizons during the drying period and after rewetting. Soil drying decreased gross ammonification rates in the O horizon. The lowest rates were found at the throughfall exclusion plots but the differences to the irrigation and control plots were not statistically significant. A substantial ammonification rate of 14 mg N kg-1 soil day-1 was observed at 3.2 MPa (pF 4.5). The laboratory study showed that gross nitrification decreased with decreasing water potential and was more sensitive to drying than ammonification in the Oa horizon; however, this was not found in the field experiment. The latter might result from the low rates and huge spatial variation, indicating the difference between disturbed samples and intact soil cores. No rewetting pulse of gross ammonification was observed, probably due to its short duration or due to the slow changes of the water potential during the natural rewetting. Although the in situ fluxes of DIN increased at the throughfall exclusion plots after rewetting, the cumulative DIN flux at the throughfall exclusion plots did not significantly exceed that at the control plots. The lowest fluxes of DON and DOC were observed at the throughfall exclusion plots because of the reduction of input with throughfall. In the studies presented here, extended drought periods caused a reduction of gross N turnover in forest soils but gross ammonification continued at considerable rates at low water potential. The hypothesis of increased N turnover and fluxes of DIN, DON and DOC as a consequence of drying/rewetting was not confirmed. In the fen site, undisturbed soil cores were taken and divided to two treatments of water table: permanently flooded and fluctuated. The later was subjected to flooding, drawdown and re-flooding. The permanently flooding enhanced gross ammonification after a lag phase of about 30 days while CO2 emissions were constantly low. The water table drawdown also increased gross ammonification, but again after a lag phase of about 30 days. The first peak of CO2 emissions appeared immediately after water table drawdown, followed by a decrease and a second peak. The ratio of CO2 emission/gross ammonification were close to 2 under anoxic condition which seems to be caused by fast N turnover in the microbial biomass-N pool and low rates of CO2 production. The changes induced by water table drawdown on the N and C turnover were found reversible after re-flooding. Drainage increases SO42- but decrease Fe, DON and DOC concentrations and vice versa when the soils were flooded. Release of DON and DOC was inhibited by increasing SO42- concentrations. Under field conditions, neither drainage nor flooding had an effect on dissolved inorganic N due to the low concentration, indicating the rapid consumption of mineralized N in the field. In the absence of plant uptake and runoff in the laboratory experiment, however, NH4+ increased during the flooding period. Soil desiccation affects the upper soil layers with largest rates of N turnover. While gross N turnover is reduced by soil desiccation, a substantial rate of ammonification was observed even at low water potentials. Nitrification was found more sensitive to desiccation than ammonification which might change the NH4/NO3 ratio of available N under dry conditions. Rewetting of dry soil does not induce a pulse of N turnover and fluxes of DIN, DON and DOC. Overall, an increasing frequency of drying/rewetting cycles seem to have only moderate effect on the N turnover and on N solute fluxes in forest soils. Fluctuations of water table play an important role for the organic matter mineralization, soil solution chemistry and inorganic N availability in minerotrophic fen soils. Acidification by oxidation of S to SO42- can be expected after water table drawdown, causing inhibition of DON and DOC release. The effect of drainage and flooding on gross mineralization and solute concentrations is reversible within a month period. The effect of changing water table regime on N and C turnover in fen soils seems to depend largely on the time scale of the fluctuations. Short term fluctuations at a daily scale will have little effect on N turnover as compared to longer term changes on a monthly scale, while short term changes seem to trigger C losses by CO2.

Documentation and Instruction Manual for the Horizontal Mobile Measuring System (HMMS)

Jörg Hübner; Johannes Olesch; Hubert Falke; Franz X. Meixner; Thomas Foken — Wed, 11 Jan 2012 09:09:35 +0100

Halogens and trace elements in subduction zones

Diego Bernini — Thu, 01 Dec 2011 08:51:02 +0100

This thesis concentrates on solubilities and incorporation mechanisms of halogens and trace elements in minerals and aqueous fluids at high temperatures and pressures. The solubility of fluorine and chlorine in upper mantle minerals (forsterite, enstatite and pyrope) and halogen partitioning between aqueous fluids and these minerals were investigated by piston-cylinder experiments at 1100 °C and 2.6 GPa. Chlorine solubility in forsterite, enstatite and pyrope is below the ppm level, and it is independent of fluid salinity. The fluid-mineral partition coefficient of chlorine is 103-106, indicating extreme incompatibility of chlorine in nominally anhydrous silicates. The fluorine solubility in enstatite and pyrope is two orders of magnitude higher than for Cl, with no dependence on fluid salinity. Forsterite dissolves 246-267 ppm up to a fluid salinity of 1.6 wt. % F. At higher fluorine contents in the system, forsterite is replaced by the minerals of the humite group, which host fluorine in the hydroxyl site. The fluid-mineral partition coefficient of fluorine ranges from 101 to 103. Due to the extreme incompatibility of Cl in a peridotite mineral assemblage, fluid flow from a subducting slab through the mantle wedge will lead to more efficient sequestration of H2O (when compared to Cl) into minerals, thus inducing a gradual increase in the fluid salinity. Mass balance calculations reveal that rock-fluid ratios of (1.3-4)∙103 are required to produce the characteristic Cl/H2O signature of primitive arc magmas. This indicates that fluid flow from subducting slabs into the melting regions in the overlying mantle is not confined to narrow channels but it is sufficient to pervasively metasomatize the bulk wedge. Energetics of fluorine incorporation in forsterite and forsterite-humite chemical equilibria were explored in the system Mg2SiO4-MgF2 by first principles computations. The pressure-volume equations of state and ground-state energies were determined for orthorhombic Mg2SiO4-Mg2F4 solutions, fluorine-bearing end-members of the humite group, and sellaite (MgF2). Humite group minerals and sellaite are energetically more stable than their equivalent solid solution compounds, hence they can act as buffers of fluorine solubility in forsterite. Compressibility increases systematically with the F content for both solid solution compounds and stable minerals. Nevertheless, end member solids are systematically less compressible than the respective solid solution compounds. The pressure-volume equations of state, internal energies, configurational and excess properties were used to set up a thermodynamic model of fluorine solubility in forsterite buffered by humite-group minerals up to 1900 K and 12 GPa. Humite is the stable F buffer in the investigated pressure and temperature range. The fluorine solubility in forsterite increases with temperature, from 0.01 ppm F at 500 K up to 0.33 wt. % F at 1900 K and 0 GPa. By contrast, the effect of pressure on the fluorine solubility is small, leading to its minor decrease as pressure rises to 12 GPa. These results demonstrate that partition coefficients of fluorine between forsterite and aqueous fluid (or silicate melt) are expected to increase with increasing temperature and decreasing pressure. When fluids or melts pass through the mantle wedge, fluorine will most efficiently be stored in the high-temperature portions of the wedge, promoting mantle metasomatism beneath the arc, and it will be released when the metasomatized mantle is advected to colder regions or to higher pressures. The mobility of high field strength elements in aqueous fluids in subduction zones was addressed by in-situ zircon solubility measurements in a hydrothermal diamond anvil cell. The zircon solubilities in aqueous fluids at 865-1025 oC and 6-20 kbar buffered by quartz are very low, ranging from 1.0 to 3.3 ppm Zr, and solubilities weakly increase with temperature and pressure. Experimental results were fitted to a density model: , where c is the Zr concentration in the fluid (ppm), T is temperature (K) and rho is the fluid density (g cm-3). Additional experiments have shown that Zr solubility increases with a decrease in silica activity and with the presence of NaCl and albite due to Zr-Cl or alkali-Zr complexing but it still remains very low. Therefore, the low Zr content observed in arc magmas is due to a very low mobility of Zr in aqueous fluid.

ExchanGE processes in mountainous Regions (EGER)- Documentation of the Intensive Observation Period (IOP3) June, 13th to July, 26th 2011

Tue, 15 Nov 2011 10:19:53 +0100

Reconstruction of the Late and Mid-Pleistocene climate and landscape history in SE-Central Europe - A paleopedological and geochemical multi-proxy approach in loess-paleosol studies.

Björn Buggle — Tue, 25 Oct 2011 12:08:25 +0200

Loess-paleosol sequences (LPSS) potentially are valuable archives for past environmental conditions. In SE-Central European lowlands thick loess plateaus can be found comprising several glacial-interglacial cycles. This work focuses on key sections in the middle and lower Danube Basin to i) investigate the origin of the loess and archive genesis, ii) to set up a reliable chronostratigraphy and iii) to contribute to the reconstruction of the Mid- and Late Pleistocene climate and landscape history of the region by a paleopedological – geochemical multi-proxy approach. Furthermore, methodological investigations aim to evaluate the validity of various paleoenvironmental proxies especially geochemically based weathering indices, as well as biomarker and stable isotope approaches in LPSS research. The results from geochemical analyses reveal that alluvial material of the Danube and its tributaries represent major sources for the loess in the middle and lower Danube Basin. From the geochemical point of view the studied loess can be regarded as a representative sample of the upper continental crust altered by at least one sedimentary cycle. The chronostratigraphy of the studied sections is based on the correlation of characteristic patterns of the magnetic susceptibility to the delta 18O record of benthic foraminifera from the Ocean Drilling Program site 677, a proxy record for the global ice volume. This is supplemented by correlating magnetic susceptibility fingerprints and pedostratigraphic marker horizons to previously established chronostratigraphies from profiles in the region as well as in China. The results show that the Batajnica/Stari Slankamen LPSS (Serbia) and Mircea Voda LPSS (Romania) comprise at least the last 700.000 years of climate history i.e. the last 17 marine isotope stages. The multi-proxy approach for paleoenvironmental reconstruction involves micromorphological parameters, silicate weathering intensity as given by element composition, grain size proxies for pedogenic clay formation and wind strength, as well as determination of sedimentation rates. As most suitable proxy for silicate weathering in calcareous sediments, the molar ratio Al2O3/(Na2O + Al2O3) × 100 is introduced as Chemical Proxy of Alteration (CPA) to loess paleosol research. Moreover, diffuse reflectance spectroscopy, soil color proxies and rock magnetic proxies are applied to gain paleoenvironmental information from the concentration and assemblage of iron minerals. Focusing on the warm periods, these proxies reveal a progressive decrease of interglacial weathering and soil formation intensity over the Mid - and Late Pleistocene. Also soil forming milieu was less oxidative as reflected by the iron mineralogical composition. These findings suggest cooling and a decline of rainfall linked to a change in seasonality from a Mediterranean type of climate to a more continental steppe climate. Results from n-alkane biomarkers support that summer dryness limiting the expansion of trees was a persistent feature of interglacial climate in SE-European lowlands. In the obtained proxy dataset, increase of wind strength, gradual cooling as well as decrease of rainfall since the early Mid-Pleistocene is also evident for glacial periods. After evaluation of potential triggers, this general climatic trend is proposed to be related to Pleistocene uplift of Eurasian mountain ranges. Changes in atmospheric circulation and rain shadow effects due to mountain uplift (Himalaya, Alps, Carpathians) would provide an explanation for the westward expansion of the Eurasian steppe belt into SE-Central Europe. Future studies on LPSS may also involve highly innovative proxies such as n-alkane biomarker and their D/H isotope signature. However, the methodological investigations on modern soil profiles and samples from litterbag experiments suggest that in a LPSS these proxies might be biased by microbial reworking. Procedures for correcting n-alkane ratios based on the odd over even predominance as reworking indicator have been developed. Hence, these studies highlight the limitations but also the persisting potential of innovative approaches from organic and isotope geochemistry in paleoenvironmental investigations of loess-paleosol sequences.