16 search hits
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Pedogenic carbonates in loess formation rates, formation conditions and source apportionment assessed by isotopes and molecular proxies
(2011)
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Martina Gocke
- Interest in secondary (pedogenic) carbonates as an archive for paleoclimatic reconstructions in arid and semiarid regions has increased during recent decades. Their carbon (C) isotope composition represents the conditions prevailing during their formation because they are formed by precipitation of Ca2+ from soil solution with dissolved CO2 from soil air originating from root and rhizomicrobial respiration. Thus, pedogenic carbonates are an important tool for estimation of age of pedogenesis and for reconstruction of the local paleovegetation. Potential reequilibration of pedogenic carbonates with younger soil CO2 can entail loss of chronological and paleoenvironmental information. Although methodological resolution of these studies depends on the time scale of pedogenic carbonate formation and recrystallization, its rates and periods remain unknown. The first objective therefore was the first-time assessment of the time frame of pedogenic CaCO3 formation and recrystallization under controlled conditions. The other aim was to reveal the potential of rhizoliths, a special form of pedogenic carbonates (calcified roots), from a loess-paleosol sequence for paleoenvironmental studies. In loess as a common soil parent material, initial CaCO3 recrystallization rates were successfully determined with the 14C isotopic exchange approach by exposing loess to artificially labeled 14CO2 and subsequent quantification of 14C incorporated in secondary (recrystallized) CaCO3. Within the range of natural soil CO2 concentrations, recrystallization rates increased strongly with CO2 concentration. In further studies, loess was exposed to 14CO2 respired by roots and rhizomicrobial organisms of plants labeled in 14CO2 atmosphere, to estimate the effects of several factors (root vicinity, temperature, accumulation depth) on the recrystallization rate. Rates from planted loess were two orders of magnitude higher than those from unplanted loess, mostly in the range of 10-5 day-1. Significantly higher CaCO3 recrystallization rates in rhizosphere than in loess distant from roots were attributed to three factors: high CO2 concentration from root and rhizomicrobial respiration, low pH caused by release of CO2 and root exudates, and high Ca2+ and HCO3- concentration caused by water uptake by roots. Considerable influence of the latter was demonstrated by low CaCO3 recrystallization rates at low temperatures and vice versa, reflecting the increasing transpirational pull with increasing temperatures. Assuming repeated recrystallization of both primary and secondary CaCO3, extrapolation of initial CaCO3 recrystallization rates showed that at least 102 – 103 years are necessary for complete recrystallization of CaCO3 in ‘root-free’ loess by formation of secondary CaCO3, depending on length of the growing season. Increasing temperature promoted CaCO3 recrystallization rates, but the contrast was compensated for recrystallization periods because of the negative effect of increasing temperature on length of the growing season. In contrast, pedogenic carbonates can form much faster close to roots (101 – 102 years) because of mass flow to the roots leading to rhizolith formation. As a consequence of this wide temporal spectrum of pedogenic carbonate formation, variable methodological resolution has to be considered in paleoenvironmental studies based on stable isotope composition of pedogenic carbonates, depending on climatic factors and formation of carbonate concretions. Rhizoliths, formed by encrustation of roots with secondary CaCO3, yield high potential for paleoenvironmental studies. At the late Pleistocene loess-paleosol sequence of Nussloch, SW Germany, rhizolith CaCO3 was completely secondary and not contaminated by postsegregational alteration. Radiocarbon dating of one rhizolith sample reinforced the assumption of potential postsedimentary formation of rhizoliths. In the investigated profile, stable C isotope composition indicated C3 source vegetation for organic matter (OM) of both loess and rhizoliths, but lipid molecular proxies revealed grass biomass as origin of loess OM, and shrub or tree roots as source of rhizoliths. Moreover, OM in loess adjacent to rhizoliths was considerably contaminated by rhizomicrobial and root remains at least up to a distance of 5 cm. Alteration of loess OM and its isotope composition by postsedimentary penetration of deep-rooting plants might entail uncertainties for paleoenvironmental studies based on loess OM. In summary, the important role of vegetation on pedogenic CaCO3 formation and recrystallization was shown under controlled and field conditions. Plant roots and associated microorganisms have direct influence on these processes, while further factors of pedogenesis like climate exert an indirect effect, but on the long term probably are of greater importance than effects on the rhizosphere scale.
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Soil organic matter dynamics in a temperate forest influenced by extreme weather events
(2011)
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Andrea Schmitt
- Climate models predict an increase in surface temperature and a change in intensity and kind of precipitation in the future for Europe depending on the region with effects on C cycling and soil organic matter (SOM). We investigated the influence of extreme weather events (frost/drought) on the quality and quantity of SOM in a Haplic Podzol under a 140 years old Norway spruce forest in the Fichtelgebirge mountains (Bavaria, German) within two laboratory and two field studies. In one laboratory study, we investigated the effect of frost intensity and repeated freeze/thaw cycles. Undisturbed soil columns comprising organic layer and top mineral soil were treated as followed: Control (+5 °C), frost at –3 °C, –8 °C and –13 °C. After a two-week freezing period, frozen soils were thawed at +5 °C and irrigated with 80 mm water at a rate of 4 mm per day. After the third cycle, SOM pools of the treatments were compared with those of non-dried control columns. Under field conditions from late December 2005 until middle of February 2006 we removed the natural snow cover during winter on three replicate plots. Hence we induced soil frost to 15 cm depth (in a depth of 5 cm below surface up to -5°C) from January to April 2006, while the snow-covered control plots never reached temperatures below 0 °C. In the second laboratory experiment after air-drying for five weeks, undisturbed soil columns were re-wetted at different intensities (8, 20 and 50 mm per day) and time intervals, so that all treatments received the same amount of water per cycle (100 mm). After the third cycle, SOM pools of the treatments were compared with those of non-dried control columns. Under field conditions, a throughfall exclusion (TE) experiment was conducted in the summers 2006 and 2007 using a roof installation followed by re-wetting compared to non-manipulated control plots. On 18th January 2007, the heavy low pressure system Kyrill caused large damages at our control plots whereas the TE sites were less influenced. Therefore, for this study, only data were used from the control plots before Kyrill and from the soil structure undisturbed TE plots. SOM quantity and quality was followed by biomarker analysis: lignin, neutral sugars and phospholipid fatty acids (PLFA) as measure for microbial biomass. Amounts of lignin contents were not significantly affected by repeated freeze/thaw cycles. However, intensive frost slightly enhanced lignin mobilization in the O layer and the translocation into the B horizon. While soil frost did not influence lignin concentrations, the decomposition rate of vanillyl monomers (Ac/Ad)v decreased at the end of the frost period, these results confirm reduced mineralisation under frost. In contrast, lignin phenols were not systematically affected by the drying/rewetting-experiment and the moisture regime. The sum of PLFA (soil microbial biomass) was not affected by the frost respectively drying event, suggesting that most soil microorganisms were well adapted or recovered more quickly than the accumulation of microbial residues such as microbial sugars directly after the experiment. However, PLFA patterns indicate that fungi are more susceptible to soil frost than bacteria. The ratio of fungi to bacteria were generally not altered through drying, however, at least in the L horizon, warmer and drier weather led to a dominance of fungi while a cooler and moister regime favoured bacteria. Increasing water stress was indicated by a higher PLFA (cy17:0+cy19:0)/ (16:1w7c+18:1w7c) ratio suggesting that the microbes suffered from water stress in the organic layer and uppermost mineral soil. While soil microbial biomass was not affected by the moisture regime, the structure of soil microbial community changed. Gram-positive bacteria and actinomycetes were reduced whereas gram-negative bacteria, fungi and protozoa were stimulated by the reduced moisture regime. In the subsequent summer after the freezing experience, soil microbial biomass was significantly higher at the snow-removal plots (SM) compared to the control despite lower CO2 respiration and increasing water stress indicator. These results suggest that soil microbial respiration and therefore the activity was not closely related to soil microbial biomass but more strongly controlled by substrate availability and quality. Both freezing/thawing and drying/re-wetting reduced the amount of microbial sugars due to reduced mineralisation. However, also the hydrolysable plant sugars decreased in all soil horizons. We postulated that the only possible explanation for the disappearance of plant and microbial sugars upon soil freezing or drying are chemical alterations of sugar molecules leading to SOM stabilization, also known as SOM aging. Further studies are required to quantify the effect of temperature or moisture regime to the observed changes in soil sugar concentrations.
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Exotic Species Invasion and Biodiversity in Bangladesh Forest Ecosystems
(2011)
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Mohammad Belal Uddin
- Both, biological invasion by exotic plant species and biodiversity including spatial patterns and drivers are two major issues in tropical forest ecosystems. This dissertation deals with these two issues in a tropical forest ecosystem in Bangladesh. Considering the first issue, it comprises two manuscripts: a systematic review and a field survey in Bangladesh forest ecosystem. The review was done based on a formalized literature search in order to summarize the approaches that were hitherto applied as well as to mark gaps in tropical invasion research. A considerable number of primary research papers focused on invasion by plants in tropical forests were reviewed. The results identified ample gaps of research. Adressing these gaps may generate promising future research to understand and mitigate this great challenge in different types of tropical forests. Then a case study was conducted to examine the invasiveness and invasibility characteristics in a forest ecosystem of Bangladesh. This study seeks to find out the characteristics of exotic species and relationships between native species richness, environmental variables, disturbances and exotic plant invasion in this ecosystem. Boosted Regression Trees and Detrended Correspondence Analysis are used to determine these relationships. Most exotics are trees followed by shrubs and herbs. Fabaceae and Asteraceae contribute a large proportion of exotic species. Most of them originated from other tropical areas. Native species richness was found to be the best predictor for the number and percentage of exotic species in the study area. However, a unimodal relationship was found. Multiple other factors also influence the success of exotic species. The number and the percentage of exotic species are positively correlated with frequency of disturbances and with soil attributes (phosphorus and bulk density) but negatively correlated with topography (elevation) and conservation patterns (protection). Considering the biodiversity issue, it encompases another two manuscripts based on a case study conducting a systematic field work in the same forest ecosystem of Bangladesh. They are the first spatially explicit analysis of drivers and patterns of biodiversity in this terrestrial ecosystem based on multivariate approaches, similarity analysis and variation partitioning. One manuscript assesses the relationships between landscape and habitat characteristics, conservation patterns, and plant diversity in this tropical forest ecosystem. This study analyses the effects of soils, topographic conditions, disturbances and nature protection on plant species richness and species composition. The results reveal that biodiversity patterns in the study area are positively correlated with protection and elevation. These patterns are, however, negatively correlated with disturbances. The other manuscript focuses on the stand characteristics and spatial patterns of biodiversity as they are rarely studied in the tropics in general and in Bangladesh in particular. Data on tree species are used as they are the most conspicuous element of these ecosystems. Tree species composition was recorded in a systematic plot design and diameter was measured at breast height for each individual tree. Distance-decay approach was applied to analyze the spatial pattern of biodiversity for the whole study area and two subsamples from Satchari National Park and Satchari Reserve Forest. Analyses showed that biomass increased significantly with protection status. Plots in the Reserve Forest were associated with higher species turnover than in the National Park. This dissertation identifyies, for the first time in a systematic approach, the major drivers for invasion and biodiversity pattern in a forested area in Bangladesh. In conclusion, both, biological invasion by exotic plant species as well as biodiversity are strongly related to the disturbance regime and nature protection.
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The carbon speciation in the Earth’s interior as function of pressure, temperature and oxygen fugacity
(2011)
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Vincenzo Stagno
- The redox state of the Earth’s interior will influence the speciation of volatile elements both in the mantle and in mantle derived magmas. Carbon is one of the principal elements to be affected in this way because under reducing conditions it forms graphite or diamond, and under oxidizing conditions carbonate (or CO2-bearing) minerals and melts. The cycling and residence time of carbon in the mantle can be strongly effected by the oxygen fugacity because reduced phases such as diamond and graphite are immobile and likely to remain within the mantle and potentially within subducting slabs, while at more oxidizing conditions CO2-rich fluids or melts can migrate and escape from the interior. The carbon cycle in the Earth may therefore depend on the redox state of mantle rocks. Conversely, an influx of CO2-rich fluids or melts may act to oxidize the mantle as an additional aspect of metasomatism. In the first part of this study experiments were performed to measure the oxygen fugacity at which carbon (graphite or diamond) oxidises to carbonate minerals or melts within mantle peridotite assemblages between 2.5 and 11 GPa at 1100-1600 °C. The experiments were performed up to temperatures where carbonate melts evolve towards more silicate-rich compositions. The dilution of the carbonate melt component was found to lower the relative fo2, expanding the melt stability field with respect to reduced carbon. The results allow the fo2 of the diamond formation process to be determined both as a function of pressure, temperature and melt CO2 concentration. These results also have implications for the onset of melting within up welling mantle material. Several studies have indicated that the mantle may become more reduced with depth. This means that the oxidation of elemental carbon (graphite or diamond) may occur in up welling rocks where the oxidized product is a carbonate bearing magma. When the experimental data are compared with current estimates for the fo2 of mantle rocks the implication is that peridotitic mantle will remain in the diamond stability field up to at least 100-150 km depth. Only at depths shallower than 150 km would Fe3+ in mantle silicates react with graphite to produce carbonate rich melts in a redox melting process. Redox melting should limit the depth interval over which carbonate-rich melts can form beneath ridges. Further experiments were performed to determine the fo2 at which diamond oxidises to carbonate in the transition zone and lower mantle. Experiments at 45 GPa were performed using the MADONNA D-DIA (1500 tons) apparatus with sintered diamond anvils installed at the Geodynamics Research Centre, Ehime University in Japan. The measured oxygen fugacity was found to be approximately 3 log units above the iron-wüstite oxygen buffer (deltaIW+3). As the oxygen fugacity of the transition zone and lower mantle is most likely at or below the IW buffer this confines the stability of solid carbonate to the upper mantle or to unusually oxidized regions of the deeper mantle. The oxygen fugacity at which magnesite and diamond coexist showed a slight decrease with pressure, however, implying the possibility that magnesite may become the stable host for carbon at the very base of the lower mantle. The oxygen fugacity at which mantle xenoliths equilibrated can be determined using oxy-thermobarometry equilibria. For garnet-peridotite rocks the only calibrated and tested oxy-barometer employs the equilibrium, 2Fe3Fe23+Si3O12 = 4Fe2SiO4 + 2FeSiO3 + O2 Garnet Olivine Orthopyroxene In the final section of this thesis Fe3+/ΣFe ratios of garnets produced in a peridotite assemblage in equilibrium with carbon and carbonate melts were measured between 3 and 7 GPa. The oxygen fugacity in these experiments was also constrained, which allowed a test of this widely used oxy-barometer to be made at pressures much higher than previously performed. The results indicate that the pressure dependence of this oxy-barometer may be in error and a preliminary recalibration implies that cratonic lithosphere may not be as reduced as previously considered.
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Formation of secondary organic aerosol and its processing by atmospheric halogen species – A spectroscopic study
(2011)
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Johannes Ofner
- Atmospheric aerosols play an important role in the global climate system. Through their physicochemical properties, they contribute in various ways to climate change and radiative forcing. Those properties can be considerably changed by processing the aerosols, which is especially significant for organic aerosols processed with atmospheric trace gases like halogens released through sea-salt activation or from other sources. Based on aerosol smog-chamber experiments, the formation of secondary organic aerosols (SOA) from predominantly aliphatic (α-pinene) or aromatic (catechol and guaiacol) precursors and the processing of those model SOAs with simulated molecular and naturally released halogens were studied. Different physicochemical methods were used to study the transformation of those organic aerosols. Infrared and UV/VIS spectroscopy allowed the determination of functional and structural changes during aerosol formation and processing. Using electron microscopy and measurement of the particle size distribution, the formation of the morphology and geometry of the particles was investigated. Temperature-programmed pyrolysis mass spectroscopy and ultra-high-resolution mass spectroscopy delivered detailed information on functional groups, extent of halogenation, and the macromolecular structure. Organic aerosols are significantly influenced by atmospheric halogens. Halogen species from different sources change the aerosol size distribution, the presence of functional groups, and the optical properties. Furthermore, they even form halogenated species in the solid phase of the organic aerosol.
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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.
(2011)
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Björn Buggle
- 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.
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Halogens and trace elements in subduction zones
(2011)
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Diego Bernini
- 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.
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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
(2011)
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Yao-Te Chen
- 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.
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Applying regional climate change projections for spatio-temporal risk analyses of vector-borne diseases
(2011)
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Dominik Fischer
- 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.
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The role of life history traits for coexistence and forest recovery after disturbance – a modelling perspective. Towards a better understanding of species-rich forests
(2011)
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Claudia Dislich
- 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.
