18 search hits
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Direkte Messung und Bewertung des nebelgebundenen Eintrags von Wasser und Spurenstoffen in ein montanes Waldökosystem
(2004)
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Thomas Wrzesinsky
- Der nebelgebundene Eintrag von Wasser und Spurenstoffen kann in den Bergwäldern Mitteleuropas eine wichtige Rolle spielen. Die Quantifizierung dieses Eintrags stieß jedoch in der Vergangenheit auf messtechnische Grenzen. Nach der Entwicklung und Erprobung eines Systems aus einem Tropfenspektrometer zur schnellen Messung der Größenverteilung (40 Tropfengrößenklassen zwischen ø1,5 und 50 µm) im Nebel und einem Ultraschallanemometer zur Bestimmung des vertikalen Windes konnten an der Ökosystemmessstation „Waldstein“ von April 2001 bis März 2002 Messungen zur Nebelwasserdeposition durchgeführt werden. Zusätzlich wurden die Sichtweite und die chemische Zusammensetzung (pH, elektrische Leitfähigkeit, Na+, K+, NH4+, Mg2+, Ca2+, Cl–, NO3–, SO42– und PO43–) des Nebelwassers gemessen. Zur Sammlung von Nebelwasser wurde ein aktiver beheizbarer Nebelsammler entwickelt und parallel zu den Wasserflussmessungen eingesetzt. Die Proben wurden automatisch alle acht Stunden genommen. Die Sammelmengen betrugen im Median 249 ml und erlaubten die gewünschten chemischen Analysen. Im Untersuchungszeitraum waren 223 Nebeltage zu verzeichnen. Der Nebelanteil betrug 25,7 %. Für die Qualitätskontrolle der gemessenen Flüsse wurden die Daten auf Stationarität und Turbulenz überprüft und der Datensatz entsprechend angepasst. Die Messung der Nebeldeposition im Untersuchungszeitraum ergab einen Eintrag von 108 kg ha–1 a–1 Wasser für die turbulente Deposition und 17 kg ha–1 a–1 für den Eintrag über Sedimentation. Der turbulente Eintrag dominiert also mit ca. 86 % die Nebeldeposition. Die Summe aus beiden Eintragsarten entspricht einem Nebelniederschlag von 125 mm p. a. Eine klare Saisonalität der Nebelwasserflüsse ist erkennbar. Die höchsten Nebelniederschläge sind im Spätherbst und im Winter zu verzeichnen, monatlich bis zu 24 mm (Januar) wurden gemessen. Die geringste Nebeldeposition wurde im August mit ca. 1 mm gemessen. Die ermittelten Tropfenspektren zeigen bei der Anzahlverteilung Maxima bei 2, 6 und 9 µm sowie ein Maximum von 12 µm in der Massenverteilung. Für die Massengrößenverteilung sind Verteilungen mit Maxima bei 9, 12 und 15 µm häufig. Die gemessenen Flüssigwassergehalte lagen bei einem Median von 156 mg m–3 und erreichten Maxima von 2639 mg m–3 (5-min-Mittel). Den größten Anteil am Fluss hatte die Größenklasse von 14,5 bis 15,5 µm Tropfendurchmesser. Tropfen kleiner 7 µm wurden effektiv emittiert, die größeren deponiert. Die im Untersuchungszeitraum gefallene Menge an Regen und Schnee beträgt 1414 mm. Der Anteil des Nebels am atmosphärischen Eintrag von Wasser beträgt demzufolge etwa 8 %. Für insgesamt 253 Nebelereignisse wurden im Untersuchungszeitraum Proben gewonnen. Außerdem wurden zum Vergleich auf wöchentlicher Basis wet-only-Proben genommen. Die Konzentrationen in Nebel- und Regenwasser sind hoch variabel. Die Mediane liegen im Nebelwasser bei pH 4,14, 621 µeq l–1 für NH4+, 487 µeq l–1 für NO3– und 321 µeq l–1 für SO42–. Diese 3 Hauptionen machen ca. 87 % der Gesamtkonzentration aus. Die Konzentrationen im Nebelwasser sind deutlich gegenüber dem wet-only-Niederschlag erhöht. Die Anreicherungsfaktoren sind 18,1 (NH4+), 13,1 (NO3–) bzw. 11,5 (SO42–). Der nebelgebundene Eintrag der wichtigsten Ionen wurde aus der Konzentration und dem Nebelwasserfluss errechnet. Die eingetragenen Mengen sind 9,8 kg ha–1 für NH4+ (7,9 kg ha–1 für wet-only), 27,9 kg ha–1 für NO3– (25,1) bzw. 14,0 kg ha–1 für SO42– (15,0). Die durch feuchte oder okkulte Deposition eingetragene Menge ist für diese Ionen also im gleichen Größenbereich wie die Menge aus Regen und Schnee. Der Stickstoffeintrag beträgt insgesamt 13,9 kg N ha–1 a–1 (11,8 für wet-only). Der im Unterschungszeitraum durch den Bestandesniederschlag gemessene Eintrag von Stickstoff liegt bei 23,3 kg N ha–1 a–1. Die Differenz aus Bestandesniederschlag einerseits und wet-only und Nebel andererseits liegt mit –0,9 kg N ha–1 a–1 nahe Null. Zusätzliche Einträge sind durch die trockene Deposition (z. B. durch partikuläres Nitrat und Salpetersäure) zu erwarten. Der Umsatz von Stoffen im Kronenraum spielt dann eine wichtige Rolle in der Schließung der Ökosystembilanz für die verschiedenen Stoffe.
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Biomass and Nutrient Studies of Selected Tree Species of Natural and Plantation Forests: Implications for a Sustainable Management of the Munessa-Shashemene Forest, Ethiopia
(2004)
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Asferachew Abate
- Plantation forests with exotic tree species have been introduced to alleviate the problems of deforestation in Ethiopia. In the future, more plantation forests with fast growing species should be grown for coping with the ever-increasing demands for fuelwood and other forest products. However, it is not known whether plantation forests are sustainable or not. For the sustainability of plantation forests with exotic tree species, it is of paramount importance to thoroughly understand their ecological and social attributes through a holistic approach. For this reason, a multidisciplinary project was initiated in the Munessa-Shashemene Forest. Such an approach gives valuable information about the sustainability of plantation forests when the basic ecological features of the natural forests are compared with plantation forests. As an integral part of the multidisciplinary project, the objectives of this study are to: 1) quantify the fine roots and aboveground biomass of selected tree species in both natural and plantation forests; 2) quantify the macronutrient stocks of the fine roots and aboveground components of selected trees species in both natural and plantation forests; and 3) evaluate the implication of the changes in the biomass and macronutrient stocks for a sustainable management of forests. The study focused on four tree species, Podocarpus falcatus (Thunb.) Mirb., Podocarpaceae and Croton macrostachys Hochst. ex Del. Euphorbiaceae, were selected from a natural forest. Cupressus lusitanica Miller, Cupressaceae and Eucalyptus globulus Labill. Myrtaceae were selected from plantation forests. Root architectures of the study trees were studied by excavation. The live fine root biomass (<2 mm in diameter) of the dry and wet seasons was determined from samples collected at the distances of 1, 2 and 3 meters from the bole of the study trees. At each of the distances, root cores were taken at the depth intervals 0-10, 10-35, 35-60, 60-85 and 85-100 cm using a hand auger. Linear regression equations were used to estimate the aboveground biomass on the basis of the relation between DBH and dry weights of the aboveground plant components. Macronutrient concentrations were determined following a standard laboratory procedure. Studies on the root architecture revealed that C. lusitanica has a shallow root and is more susceptible to windthrow compared to E. globulus. With the exception of E. globulus, the dry season live fine root (LFR) biomass was higher for all trees studied. The seasonal variation in the fine root biomass was attributed to the changes in soil moisture of the study area. For all trees investigated, the mean annual LFR biomass was highest at the depth interval 0-10 cm at all distances. The favorable soil texture, pH and organic matter content at the depth interval 0-10 cm might be responsible for higher LFR biomass. The significantly higher LFR biomass of P. falcatus (1.34 kg m-2) coupled with its higher macronutrient stocks compared to C. macrostachys (0.32 kg m-2) suggest the importance of P. falcatus in the sustainability of the natural forest by transferring more macronutrients to the soil through its fine roots. Similarly, the significantly higher total LFR biomass of C. lusitanica (0.88 kg m-2) coupled with its higher macronutrient stock compared to E. globulus (0.27 kg m-2) indicated less depletion of soil nutrients by the former. The stand structure of the natural and plantation forests differed largely. In the natural forest, the density of C. macrostachys was much higher (143 ± 72 trees ha-1) than the density of P. falcatus (73 ± 39 trees ha-1). Generally, the structural change of the natural forest due to selective cutting of P. falcatus was found to have negative implications on the sustainability of the natural forest. The differences in the structure of C. lusitanica and E. globulus, despite their similar densities, resulted in a significantly lower understory ground cover by herbaceous and shrub species in the former. The effect of a poor understory growth on the floor litter thickness and thereby on nutrient capital of the soil may negatively affect the sustainability of C. lusitanica plantation. The harvesting of the stemwood of C. lusitanica and E. globulus removes a substantial amount of nutrients from the plantation sites. Furthermore, the current practice of collecting foliage, twigs and branches for firewood by the local people results in a higher depletion of nutrients. In order to make the plantation forests sustainable, the silvicultural practice in the future should consider on site conservation of foliage and bark. It is recommended that more studies on aboveground and belowground biomass, fine root turnover, and nutrient concentrations of the plantation forests should be carried out in a chronosequence in order to gain more insight on their sustainability.
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Water in the Earth’s Interior: Thermodynamics and kinetics of hydrogen incorporation in olivine and wadsleyite
(2004)
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Sylvie Demouchy
- (1) Hydrogen diffusion in olivine The kinetics of hydration of dry single crystals of San Carlos olivine was determined by performing experiments under water-saturated conditions. The experiments were performed at 1.5 GPa, 1000°C for 5 hours in a piston cylinder apparatus, or at 0.2 GPa, 900°C, for 1 and 20 hours in TZM cold-seal vessels. Polarized Fourier-transform infrared spectrometry (FTIR) was employed to quantify the hydroxyl distributions in the samples after the experiments. The new data obtained show a strong anisotropy of diffusion, with the diffusion coefficient D[100]>D[010]> D[001] at 900°C for short duration experiments. This initial mechanism of diffusion possibly involved a redox-exchange between proton and polaron. After longer duration experiment, the anisotropy of diffusion is different with D[001]>D[010]» D[100]. For this second stage of diffusion a model of hydrogen-metal vacancy associated defects is proposed, where the vacancies are the slower diffusing species with the diffusion laws: [100], [010] = 10-(5.6±3.2) exp [-(175 ± 76)/RT] [001] = 10-(1.4±0.5) exp [-(258 ± 31)/RT] (2) Hydrogen diffusion in forsterite The kinetics of hydration linked to magnesium-vacancy diffusion within dry synthetic forsterite single crystals was determined by performing similar experiments and analyses as in the previous section. The experiments were performed at 1.5 GPa, 1000°C for 3 hours in piston cylinder apparatus, or at 0.2 GPa, 900-1110°C, for 3-20 hours in TZM cold-seal vessels. The chemical diffusion coefficients are marginally slower than in iron-bearing olivine for the same diffusion process, but the anisotropy of diffusion is the same, with the [001] axis the fastest direction of diffusion and [100] the slowest. Fits of the diffusion data to an Arrhenius law yield similar activation energies for each of the crystallographic axes; a global fit to all the diffusion data gave an activation energy around 211 ± 18 kJmol-1. Thus hydration likely occurs by coupled diffusion of protons and octahedrally coordinated metal vacancies. The diffusion rates are fast enough to modify water contents within xenoliths ascending from the mantle but they are probably too slow to permit a total equilibration in a new dry or wet environment. (3)Dehydration profiles in natural mantle-derived olivine within basalt First evidence for water diffusion in a natural mantle-derived olivine are presented from peridotite samples. The samples are olivine crystals within lherzolite xenoliths from the Quaternary alkali basalts of the Pali-Aike volcanic field in Patagonia. Water content and distribution was studied using unpolarized and polarized FTIR and analyses shows that olivine, Cr-diopside and orthopyroxene contain a significant amount of water, with up to 13 wt ppm H2O for olivine and up to 250 wt ppm H2O in the pyroxenes. In contrast, analysis of optically clear-parts of small garnet crystals indicates that they are dry. Oriented Infrared profiles show that olivine grains larger than 0.5 mm have hydroxyl-depleted rims. These water concentration profiles suggest that partial dehydration occurred during the ascent of the xenolith-bearing magma to the Earth’s surface, confirming that dehydration is occurring in the nature. From a combination of analyses of natural xenoliths with experimental diffusion works, ascent duration of the host magma is estimated to several hours, suggesting a fast rise up to the surface. (4)Temperature and pressure dependence of water solubility in iron-free wadsleyite Previous experimental studies indicate that the maximum solubility of water in wadsleyite may vary as a function of pressure and temperature. Therefore wadsleyite samples were synthesized using a multi-anvil press. One series of experiments were performed at a fixed pressure of 15 GPa and at various temperatures and in a second series the temperature was fixed at 1200°C and pressure was varied from 13 to 18 GPa. The starting material corresponds to a composition of Mg2SiO4 + 5wt% H2O. The water content was quantified by ion probe (SIMS). Results show that at 15 GPa, the water concentration decreases significantly with increasing temperature from 2.5 wt% H2O at 900oC down to 0.93 wt% H2O at 1400oC; the corresponding wadsleyite Mg/Si ratios increase from 1.79 to 1.93 over this temperature range. Up to 17 GPa, no significant effect of pressure on the water content was observed. Moreover, together with previous results on ringwoodite, these data imply a strong decrease of the water partition coefficient between wadsleyite and ringwoodite with temperature. (5) Computer simulation on hydrous point defect in iron-free wadsleyite The general utility lattice program (GULP), a semi-empirical method, was used to simulate the formation of point defects (Mott-Littleton method) in wadsleyite and especially hydrogen incorporation and their corresponding infrared frequencies.
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Biogeochemistry of Organotin and Organolead compounds in a Forested Catchment in NE-Bavaria, Germany
(2004)
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Jen-How Huang
- Organotin-compunds (OTC) and Trimethyllead (TML) have a higher toxicity than their corresponding inorganic forms and may affect the functioning of ecosystems. Little is known about their behaviour and fate in the terrestrial environment. The goal of this thesis was to investigate the biogeochemistry of OTC (methyltin, butyltin and octyltin compounds) and TML in a forested catchment, especially their input and output budget. The occurrence of OTC, TML, Sntotal and Pbtotal in the atmosphere, soils, precipitation, and runoff in a forested ecosystem in NE-Bavaria, Germany were investigatedand the inputs and outputs in the solute phase determined. In addition, their ad-desorption and transformation (degradation) in forest soils was studied using batch experiments and long term incubations, respectively. OTC and TML concentrations in the gas phase during April to June 2003 was on average 110 pg Sn m–3 and 0,34 pg Sn m–3. Tri-, di-substituted and octyl species were the dominant OTC in the gas phase. In aerosols, only butyltin compounds, dimethyltin and monomethyltin (<500 pg Sn m–3) were found and mono-substituted OTC predominated. For OTC, the washout factors were in the order: mono- >= di- > tri-substituted OTC, and the gas / particle partition coefficients were mono- >> di- >> tri-substituted OTC. Aerosol particles serve as a sink for OTC in the atmosphere, especially for monomethyltin and monobutyltin. From August 2001 to August 2002, the concentrations of tin and lead compounds in precipitation were in the order: fog > throughfall > bulk precipitation. Average concentrations of OTCtotal ranged from 57 ng Sn l–1 in fog to 5.8 ng Sn l–1 in bulk precipitation. The concentrations of Sntotal were between 490 ng Sn l–1 in fog and 140 ng Sn l–1 in bulk precipitation, on average. Average concentrations of TML were 1 ng Pb l–1 in fog and 0.1 ng Pb l–1 in bulk precipitation and 9.6 and 0.76 micro-g Pb l–1 in case of Pbtotal. The annual total deposition from the atmosphere, estimated as throughfall + litterfall fluxes, amounted to 3.7 mg Pb ha-1 yr-1 for TML and 52 g Pb ha-1 yr-1 for Pbtotal. The contribution of litterfall was 1.5% and 32%, respectively. The annual total deposition of OTCtotal was 172 mg Sn ha–1 yr–1, with 26% represented by litterfall. The total deposition of Sntotal was 4.9 g Sn ha–1 yr–1, of which 4.1% was litterfall. The annual flux with runoff from the catchment was 0.5 mg Pb ha-1 yr-1 for TML and 2.8 g Pb ha-1 yr-1 for Pbtotal. The annual runoff of OTCtotal and Sntotal amounted to 25 mg Sn ha–1 yr–1 and 2.4 g Sn ha–1 yr–1, respectively. The mass balance showed high retention of tin and lead compounds in the catchment. The total soil storage of OTCtotal and Sntotal in the catchment were 6.7 g Sn ha–1 and 1.9 kg Sn ha–1. Total soil storage in the catchment was 11.6 mg Pb ha-1 for TML and 222 kg Pb ha-1 for Pbtotal. The dominance of mono-substituted compounds in precipitation is well reflected in the concentrations and storages of OTC in both upland and wetland soils. More than 90% of the soil storage of TML was found in the wetland soils of the catchment representing only 30 % of the area. Most Pbtotal (>90%) was found in the upland soils. In upland soils, TML was only detectable in the forest floor. OTC degraded slowly in soils with half-lives (t½) estimated from 0.5 to 15 years. The degradation rates in soils were generally in the order mono- >= di- > tri-substituted OTC. Decomposition rates of OTC in the forest floor were higher than in wetland and mineral soils. TML degraded rapidly in the forest floor (Oa) with a t½ of 0.09 years. The degradation of TML in Fen (t½ = 1.2 years) and in the mineral soil (Bw-C, t½ = 1.7 years) was much slower. Emission of tetramethyltin and tetramethyllead from wetland soils was not observed, suggesting little Sn and Pb methylation in the wetland soils. The adsorption and desorption isotherms for all species and soils were linear over the concentration range of 10–100 ng Sn, Pb ml–1. The strength of OTC adsorption correlated well with the carbon content and cation exchange capacity of the soil and was in the order mono- > di- > tri-substituted OTC and butyltin > methyltin compounds. The adsorption and desorption showed a pronounced hysteresis. The ratio of total soil storages in the catchment to the present annual input was 3.6 years for TML, reflecting the rapid degradation of TML in forest soils. The ratios of OTC soil storages to their present annual deposition ranged from 4.3 to 400 years. These high ratios reflect probably the high stability, low mobility of OTC in soils and the variation of the annual total deposition.
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Austausch von Silizium durch die dreiwertigen Kationen von Eisen und Aluminium in CaSiO3-Perowskiten: Mechanismen und Auswirkungen auf die Mineralogie der Übergangszone
(2004)
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Ulrich Bläß
- Um den Einfluss dreiwertiger Kationen sowohl auf die strukturellen Eigenschaften von CaSiO3-Perowskit als auch die daraus resultierenden Auswirkungen auf den Erdmantel abschätzen zu können, sind Hochdruck- Hochtemperaturexperimente in den beiden Systemen CaSiO3 – CaFeO2,5 und CaSiO3 – CaAlO2,5 bei Drücken zwischen 3 und 25 GPa und Temperaturen zwischen 800° und 1900°C durchgeführt worden. Die Ergebnisse im eisenhaltigen System zeigen bei den P-T-Bedingungen der Übergangszone eine nur geringe Eisen-Löslichkeit über den Defektmechanismus (4 mol% bei 16 GPa). Diese vermutlich ungeordnete Phase ist durch eine breite Mischungslücke von einem neuartigen Defektperowskit mit Ca(Fe0,4Si0,6)O2,8 Stöchiometrie getrennt, in welchem die Ordnung von Sauerstoffdefekten innerhalb diskreter Lagen zu einer 10-fachen Überstruktur entlang der pseudokubischen [111]-Richtung führt. Bei einer Sauerstofffugazität von ca. Re-ReO2 ist dieser Defektperowskit zwischen ca. 11 bis 22 GPa und 1000° bis >1800°C stabil, zerfällt jedoch bei einer niedrigeren Sauerstofffugazität von Mo-MoO. Experimente oberhalb der Stabilität des geordneten Defektperowskits geben Hinweise auf eine erheblich ansteigende Eisen-Löslichkeit in reinem CaSiO3-Perowskit. Auf eisenreicheren Zusammensetzungen oder bei tieferen Drücken treten abgesehen von dem Endglied Ca2Fe2O5 mit Brownmillerit-Struktur keine weiteren Defektperowskite auf. Dessen Hochdruckstabilitätsgrenze ist experimentell bestimmt worden und liegt bei ca. 4,5 GPa. Die Gleichgewichts-Phasenbeziehungen aller Zusammensetzungen sind für ausgewählte Drücke beschrieben. Neben den beschriebenen Perowskit-Phasen treten als weitere Phasen im wesentlichen Dicalciumsilikat, eine calciumarme Ferritphase und teilweise Andradit auf. Vergleichbare Experimente im aluminiumhaltigen System haben die Existenz zweier Defektperowskite mit Ca(Al0,4Si0,6)O2,8 und Ca(Al0,5Si0,5)O2,8 Stöchiometrie aufgedeckt. Dabei weist erstere Phase eine analoge 10-fache Überstruktur wie der eisenhaltige Defektperowskit auf. Ihr Stabilitätsfeld ist mit ca. 9 GPa bis 18 GPa zu etwas niedrigeren Drücken verschoben bei einer gleichzeitig erhöhten Liquidus-Temperatur. Aufgrund ähnlicher Gitterkonstanten ist eine ausgedehnte Mischkristallbildung mit dem eisenhaltigen Defektperowskit zu vermuten. Der andere Defektperowskit mit Ca(Al0,5Si0,5)O2,8 Stöchiometrie besitzt dagegen entlang der pseudokubischen [111]-Richtung eine 8-fache Überstruktur mit vermutlich gleicher Ordnung von Sauerstoffleerstellen innerhalb der Defektlagen. Sein Stabilitätsfeld ist zu erheblich niedrigeren Drücken verschoben und liegt zwischen ca. 4 und 12 GPa bei ebenfalls relativ hohen Liquidus-Temperaturen. Syntheseexperimente bei P-T-x-Bedingungen unter denen keiner der geordneten Defektperowskite stabil ist, führen zu einer metastabilen Bildung von extrem feinkörnigen Defektperowskiten, welche teilweise eine partielle Ordnung der Defektlagen und eine domänenreiche Mikrostruktur aufweisen. Aufgrund dieser Resultate können in dem aluminiumhaltigen System keine Gleichgewichts-Phasenbeziehungen aufgezeigt werden. Da jedoch der Silizium-Aluminium-Austausch mit dem Einbau von Sauerstoffdefekten gekoppelt ist, ist eine mit dem eisenhaltigen System vergleichbare Löslichkeit von Aluminium in reinem CaSiO3-Perowskit zu erwarten. Alle drei Defektperowskite sind ausführlich mittels optischer Mikroskopie, Elektronenstrahlmikrosonde, Röntgen- und Elektronenbeugung, Mössbauer- und Elektronen-Energieverlust-Spektroskopie sowie bezüglich ihrer Mikrostruktur am Transmissionselektronenmikroskop untersucht worden. Die Mikrostruktur aller drei geordneter Defektperowskite ist relativ arm an Versetzungen, weist jedoch eine reiche Verzwillingung nach zwei verschiedenen Gesetzmäßigkeiten auf. Zum einen handelt es sich um ferroelastische Zwillinge, welche zusammen mit einer geringfügigen Gitterverzerrung von einer rhomboedrischen Symmetrie die Existenz einer rhomboedrischen Hochtemperaturphase andeuten. Zum anderen treten Wachstumszwillinge auf, bei welchen die Überstruktur der einzelnen Domänen entlang je einer der vier möglichen pseudokubisch dreizähligen Achsen ausgebildet ist. Mögliche Strukturmodelle dieser Defektperowskite bestehen entlang der pseudokubischen [111]-Achse abwechselnd aus Defektlagen mit Silizium in vierfacher oder eventuell auch fünffacher Koordination und Blöcken mit Perowskitstruktur, welche die dreiwertigen Kationen enthalten. Das potentielle Auftreten der Defektperowskite im Erdmantel ist diskutiert. Aufgrund der Phasenbeziehungen ist eine Bildung innerhalb peridotitischer oder MORB Zusammensetzungen nicht zu erwarten. Jedoch zeigen die Experimente dieser Arbeit, dass die Konzentration an dreiwertiger Kationen in CaSiO3-Perowskiten der Übergangszone auf wenige Prozent beschränkt ist, aber möglicherweise in den oberen Bereichen des Unteren Erdmantels in Abhängigkeit von den Verteilungskoeffizienten gegenüber Magnesiumsilikat-Perowskit erheblich ansteigt.
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Biogeochemical Investigations in Old Growth and Disturbed Forest Sites at Mount Kilimanjaro
(2004)
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Marion Schrumpf
- Areas covered by mountain rainforests in East Africa have important functions as catchment areas for the streams in the savannah. Still, little is known about the biogeochemical processes in these often threatened ecosystems. The current study aims to present basic information on soils and different parameters of the water and nutrient cycle in intact as well as disturbed tropical mountain rainforest sites at Mt. Kilimanjaro in order to assess the long term effects of forest dissection on biogeochemical parameters. For this purpose, the chemical and physical soil properties, soil water tension, rainfall, throughfall, litter percolate and soil solution were determined on twelve study sites in mature forest, secondary forest and elder clearings between 2100 and 2300 m a.s.l. on the southwestern slopes of the mountain. The soil water tension was monitored from May 2000 to August 2002. The simultaneously collected water samples were combined to form fortnight samples and subsequently analysed for their content of K, Mg, Ca, Na, NO3-N, NH4-N, TOC, TON, TOS and TOP. The soils, which comprised of several buried horizons, were Andisols with high C and N stocks, some hydromorphic properties and comparatively low pH values. The latter led to very small ECEC values and low base saturation. Consequently, the retention of cationic nutrients in the soil was considered to be poor. Rainfall amounts were slightly less at the higher sampling sites where the mature forest was located and differed between the study years (2600 mm and 2480 mm at 2100 m, 2210 mm and 1960 mm at 2250 m). Rainfall interception was close to zero in the clearings, but reached up to 30% of incident rainfall in both studied forest types. The analyses of the soil water tension revealed the driest conditions to be under secondary forests. The reason for this was probably a combination of the topographic position of the secondary forest sites and the forest fragmentation. Since the secondary forest sites and the clearings had higher sand contents, the water holding capacity of these sites was lower than in the mature forest. With the exception of NO3-N in the forests, all nutrients in rainfall were increased upon passage through the forest canopy, but nutrient fluxes in rainfall as well as throughfall were still low compared to other mountain rainforests, especially for Mg, Ca and K. Considering the lower biomass of the clearings compared to the forest, the amount of nutrients leached from the vegetation of the clearings was relatively high. Since NO3-N was not assimilated by the vegetation of the clearings, NO3-N throughfall fluxes were higher in the clearings as were the K fluxes, which was probably attributable to different leaf properties of the vegetation at the different sites. In the mature forest, the highest concentrations of nutrients were found in the litter percolate, followed by a pronounced decline in the soil solution. This was especially the case for K. NO3-N was an exception since it exhibited the highest concentrations only in the topsoil solution. Nutrient concentrations in litter percolate and the topsoil solution were usually higher in the clearings than in both forest types. This probably resulted in higher belowground nutrient fluxes since greater water amounts reached the ground surface in the clearings. The high belowground nutrient concentrations were likely the result of the higher mineralisation rates in the clearings induced by higher temperatures and the greater nutrient contents of the litter. With increasing soil depths, nutrient concentrations in seepage water below the clearings declined so that the differences among sites were not significant at deeper soil layers. The contribution of organically bound nutrients to the total concentrations of respective nutrients was highest in throughfall water and lowest in the soil solution, in which OM concentrations were overall lower than in other mountain forests. This was most likely attributable to the adsorption of organically bound nutrients to the mineral phase of the Andisols studied. Again, the highest concentrations in seepage water were measured in the clearings. The results show that mature forests at Mt. Kilimanjaro exhibit a more closed nutrient cycle, especially for basic cations, while the nutrient cycle in the clearings is more open. This probably reflects the different nutrient usage and conservation strategies of the pioneer and the late successional vegetation. Therefore, the forest disturbance on Mt. Kilimanjaro leads to long term changes in biogeochemical cycles. The opening of the forest at lower elevations, which resulted in the formation of large clearings with impeded regeneration and highly fragmented secondary forest patches, led to a higher spatial and seasonal variability of soil moisture and nutrient contents in seepage water. The large scale effects of these forest conversions on water yields and nutrient outputs need to be tested in the future.
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Biogeochemical Consequences of Hydrologic Conditions in a Tropical Montane Rain Forest in Ecuador
(2004)
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Rainer Goller
- Tropical montane forests regulate the hydrological cycles of high-elevation areas being an important service for the local population. The objectives of my work were (1) to determine the major hydrological flow paths, (2) to quantify concentrations of dissolved organic C and organic and inorganic forms of N, P, and S, and (3) to identify the major controls of the dissolved C, N, P, and S dynamics in a tropical montane forest in south Ecuador. Three 8-13 ha large microcatchments (MCs) under montane forest at an altitude of 1900-2200 m above sea level were selected. Scientific equipment was installed on five transects, about 20 m long with an altitude range of 10 m. Three unforested sites near the microcatchments were used for rainfall gauging. Within the three monitored years between May 1999 and April 2002, on average 2448 mm of precipitation fell on the study area. The delta 18O of rainfall shows large variations (-12.6 to +2.1 per mil) related to different air-masses. The delta 18O values of throughfall and lateral flow are similar to those in rainfall. Variations in delta 18O values of the soil solution and the stream water are smaller (-9.1 to -3.0 and -5.8 to -8.7 per mil) than those of rainfall, throughfall, and lateral flow. The delta 18O values in stream water increased immediately after an intense rainstorm event to isotope values similar to those of rainfall and lateral flow. This indicated that during elevated rainfall the water flows rapidly in the organic layers to the stream channel paralleling the surface. During this time the water content of the organic layer was higher than of the upper mineral soil. Results from an isotope two-component hydrograph separation for the three microcatchments showed that new rain water ('event water') contributed 44-81 % to the total stormflow runoff during one selected peak discharge. The canopy was a small and the organic layer the major source of DON, DOP, and DOS, which were almost completely retained in the mineral soil. The organically bound forms contributed, on average in all solutions, 54 %, 78 %, and 59 % to the total concentrations of N, P, and S, respectively. The contribution of organically bound forms to total nutrient concentrations was element-specific and differed among the ecosystem fluxes. The portions of DON (23-81 %) were similar and those of DOP (5-100 %) highly variable in all solutions. There were high DOS portions in the aboveground fluxes (51-100 %) and lower portions in the mineral soil (8-47 %). The pH was positively correlated with the DOC and organic N, P, and S concentrations at the scale of pedons. Increasing DOC concentrations in stream water following rainstorms indicated that rainstorms were an important control of the dissolved organic matter dynamics on a regional scale. During the passage of the water through the forest, dissolved inorganic N (DIN) and DIP concentrations increased between the canopy and the mineral soil and strongly decreased in the mineral soil. In contrast, DIS concentrations were highest in the mineral soil and stream water. Thus, the organic layer was the major source for DIN, the canopy for DIP and the mineral soil for DIS. The mineral soil was a sink for DIN and particularly for DIP. Soil drying and rewetting promoted the release of inorganic N. High discharge levels following heavy rainstorm events were associated with pulses of NO3-N and partly also NH4-N concentrations in stream water. Nitrate-N concentrations in the stream water were positively related to runoff conditions. The DIP and DIS concentrations in throughfall and stemflow were negatively correlated to the respective water fluxes, whereas DIS concentrations in the mineral soil solutions of both studied soil depths were positively related to the rainfall volume. DIN and DIP concentrations and fluxes tended to be positively related to the pH of the organic layer. My results demonstrate that nutrient dynamics were mainly controlled by hydrological conditions in the studied steep forested catchments. Furthermore, small ecosystem inputs and outputs indicated tight cycles of dissolved N, P and S in the study forest, except for DIS.
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Experimental deformation of forsterite, wadsleyite and ringwoodite: Implications for seismic anisotropy of the Earth’s mantle
(2004)
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Hélène Couvy
- The rheological properties of the major minerals of the Earth’s mantle are still not well constrained. However, these properties are crucial for the understanding of a wide range of processes in the Earth’s interior such as mantle convection. The purpose of this work is to address the issue of the rheology of the lowermost upper mantle and of the transition zone through the mechanical properties at high pressure of olivine (with forsterite composition Mg2SiO4) and of its high-pressure polymorphs wadsleyite and ringwoodite. Indeed, the properties of the Earth’s mantle can be inferred as a first approximation from the mechanical properties of those polymorphs which volumetrically dominate the mineralogy of the region of concern. Deformation experiments have been performed on hot-pressed forsterite samples and on pre-synthesized wadsleyite and ringwoodite samples under pressure conditions of the Earth’s mantle and at 1300-1400°C. The possible influence of the phase transformation from forsterite to wadsleyite on rheology has been also investigated. Deformation has been achieved by shear using the Kawai-type multianvil apparatus. Complementary experiments on forsterite have been performed in the newly developed Deformation-DIA. Some of them have been carried out on a synchrotron beam line to perform in-situ stress and strain measurements. In order to gain a maximum of information on the deformation mechanisms and on the Crystallographic Preferred Orientation (CPO), a special attention has been devoted to the microstructural characterisation of the samples. Electron BackScattering Diffraction (ESBD) and Transmission Electron Microscope (TEM) have been mainly used. An important pressure-induced change in deformation mechanism is shown in forsterite. The deformation of forsterite at high pressure and temperature is dominated by the [001]{hk0} slip system rather than the [100](010) glide which is extensively observed at low pressure and high temperature.. Concerning the high-pressure polymorphs, their plastic behaviour has been studied with a strong emphasis on the formation of CPO. ViscoPlastic Self Consistent (VPSC) modelling is used to link the CPO with known elementary deformation mechanisms of these phases. The main features of the CPO of wadsleyite are characterized by the alignment of the [100] axes parallel to the shear direction and the alignment of the [001] axes toward the normal to the shear plane. Too many uncertainties remain on the ringwoodite CPO for them being used to interpret seismic anisotropy. Finally, we suggest that strain-induced CPO might be responsible for the seismic anisotropy observed in the lowermost upper mantle and in the upper part of the transition zone. The low seismic anisotropy of the lowermost upper mantle can be explained from the slip system change in forsterite and the CPO of wadsleyite point toward a dominant tangential flow in the upper part of the transition zone.
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A macroscopic and microscopic investigation of the magnesite – otavite solid soluition
(2004)
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Fiona Bromiley
- The magnesite (MgCO3) – calcite (CaCO3) system is the most geologically important of the carbonate minerals. At intermediate compositions a distinct ordered phase exists, dolomite [MgCa(CO3)2], which has symmetry. A complete, disordered ( c symmetry) solid solution exists, but at temperatures in excess of 1400°C. Unmixing and ordering processes that occur at high temperatures in the magnesite – calcite system make experimental work difficult. However, the use of an analogue system allows syntheses under more accessible experimental conditions. The effects of cation substitution and ordering in the magnesite (MgCO3) – otavite (CdCO3) solid solution have, therefore, been investigated using X-ray powder diffraction and Hard Mode Infrared Spectroscopy (HMIS). The results from Rietveld refinements of the X-ray powder diffraction data show that the variation of the a-axis is linear as a function of composition and is not affected by different degrees of order at intermediate compositions. However, the c-axis shows a positive deviation from linearity as a function of composition for the 800°C series. The observed deviation decreases with increasing degree of order for samples of composition Mg0.5Cd0.5CO3. Oxygen-carbon-oxygen bond angles for samples with symmetry showed a decrease in bond angle from 120° (constrained by symmetry in samples with the c structure) with increase in degree of order. Site occupancies were used to determine the long-range order parameter, Q, for samples with symmetry. The square of the order parameter varies linearly as a function of temperature, suggesting a second-order phase transition, and a critical transition temperature of Tc = 719°C. Intensity ratios between reflections present only in the low symmetry phase and reflections present in both the low and high symmetry phases can be used to determine a value proportional to the square of the long-range order parameter, Q. Intensity ratios, between the (101) and (202) reflections for samples of composition Mg0.5Cd0.5CO3, were determined from X-ray powder diffraction patterns and show a linear variation as a function of temperature, with a critical transition temperature, Tc = 716°C. The local mixing and ordering behaviour of the magnesite – otavite solid soluition has been characterised using infrared powder absorption spectroscopy. Phonon bands due to cadmium-oxygen translations, and magnesium-oxygen translations were both observed in the IR spectra for samples of intermediate composition. Band positions for cadmium-oxygen translations remained constant as a function of composition, whereas band positions for magnesium-oxygen translations showed a decrease in wavenumber as a function of composition. Vibrational bands in other spectral regions (libration and translation of carbonate groups and doubly degenetrate in-plane bending and out-of-plane bending of carbonate groups) vary linearly as a function of composition and no effect of order on band position is observed. At high frequencies the samples with c symmetry (800°C series) show a linear shift in band position as a function of composition, whereas, samples in the cadmium dolomite stability field, with symmetry, show a marked increase in frequency compared to their c symmetry counterparts. Cation substitution, or disordering causes broadening of IR vibrational bands for a given material. In this study an alternative method was used, which makes use of the autocorrelation function to establish average line widths for six spectral regions. The autocorrelation results, delta corr, can be interpreted in terms of local strain fields present in the structure due to cation substitution and disordering. In the low frequency region, the delta corr values show a positive deviation from linearity as a function of composition. For samples of composition Mg0.5Cd0.5CO3 the delta corr values are virtually all the same, independent of the degree of order. Over all the spectral regions, the addition of 10mol% MgCO3 (1 Mg atom every 2 unit cells) to the otavite results in a large increase in line width. It is, therefore, suggested that the strain fields surrounding the substituting magnesium atoms into the otavite structure are on the order of 30Å. In the low MIR and at a composition of Mg0.5Cd0.5CO3, an effect of order is observed as a reduction in delta corr values with respect to the sample with c symmetry. The difference in delta corr values between the ordered and disordered samples was used to determine the local-order parameter, q, which scales linearly with the long-range order parameter, Q.
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Adoption of footprint methods for the quality control of eddy-covariance measurements
(2004)
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Mathias Göckede
- Footprint models determine the spatial context of a measurement by defining a transfer function between sources or sinks of the signal and the sensor position. The resulting source area provides an important quality control tool to improve the interpretation of micrometeorological data sets. However, to date no approaches have been presented in the literature that provide a standardised footprint-based methodology that allows observers to include terrain characteristics into quality assessment and quality control strategies. One problem in this context is the small number of studies that concentrate on the validation of footprint models under the non-ideal conditions in which they are frequently being used. Therefore, for many applications, the accuracy of the source areas computed by the footprint models cannot be evaluated. To further increase the acceptance of footprint-based studies, a stronger focus on footprint validation studies for a wide variety of experimental designs is needed. This dissertation focuses on the development of a footprint-based evaluation tool for complex measurement sites that allows the combination of quality assessment results for micrometeorological measurements with characteristics of the surrounding terrain. The standardised method is easy-to-use in order to encourage its application on a large number of sites. To improve the interpretation of the obtained results, a second objective of this thesis was to develop and test approaches to validation experiments for footprint models. Göckede et al. (2004) presented an approach for the evaluation of micrometeorological measurement sites in complex terrain, which combined a method for quality assessment of eddy-covariance measurements with an analytic footprint model. Their software package provided micrometeorologists a practical tool for determining the average flux contributions from the land use type intended to observe at a specific site, or to identify footprint areas for which a high data quality could be assumed. Rebmann et al. (2005) proved the efficiency of this evaluation approach for extensive studies on a large number of sites organised in a network. Their results may serve as a tool for an improved determination of yearly sums of the net ecosystem exchange, because fluxes originating from sectors of minor quality could be excluded from the analysis. Because of these important contributions to quality control, Foken et al. (2004) integrated the site evaluation approach into a comprehensive survey on micrometeorological post-field data quality control techniques. The experiences obtained during the extensive study by Rebmann et al. (2005) allowed us identification of the major weak points of the approach, which we were able to improve in subsequent studies. Using remote sensing methods Reithmaier et al. (2005) studied the influence of the characteristics of the land use maps and different roughness length assignment schemes on the performance of the site evaluation approach. Finally, Göckede et al. (2005a) developed an updated version of the site evaluation approach, which improved the basic method by replacing the analytic footprint model with a Lagrangian stochastic footprint model that is more suitable for studies above high vegetation, and by applying a more sophisticated microscale flux aggregation method for the determination of areally-averaged roughness lengths. Although the implemented models are far more sophisticated than in the original version, the approach by Göckede et al. (2005a) still permits a practical application that allows for comparative studies of a large number of sites. With respect to the development of validation methods for footprint models using natural tracer measurements from field scale experiments, Göckede et al. (2005b) presented two different experimental approaches. Firstly, a comparison of measured flux differences and modelled land use differences for pairs of measurement positions revealed general correlations between measurement data and model results. Secondly, Göckede et al. (2005b) tested a correlation analysis between measured and modelled parameters using reference measurements and footprint results. This approach resulted in an objective quantitative evaluation of the accuracy of the footprint model. The study by Reth et al. (2005) could not be employed for footprint validation purposes because of a large systemic scatter between these measurement systems. Overall, both the paper by Göckede et al. (2005b) and by Reth et al. (2005) provided successful methods to testing the suitability of natural tracer experiments in the validation of footprint models. Although experimental deficits prevented the working out of significant differences between the results of the employed footprint models, their studies developed an improved design for natural tracer experiments that are especially designed for footprint validation purposes.
