19 search hits
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Turnover and fluxes of carbon and nitrogen in a spruce forest under natural and extreme meteorological conditions
(2010)
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Kerstin Schulze
- Climate models predict an increase in the intensity and frequency of extreme meteorological climate events like extended summer droughts, heavy rainfall or intensive frost periods with largely unknown effects on microbial activity and pysico-chemical soil properties and their impact on availability of soil organic matter. The influence of drying/rewetting (A/W) and freezing/thawing (G/A) events on solution chemistry and leaching losses of soils is barely known. This thesis aimed to study the effects of A/W and G/A events on soil solution chemistry and solute fluxes, in particular, of dissolved organic carbon (DOC) and inorganic nitrogen (NH4+, NO3-) in a podzol soil under a Norway spruce forest. A field experiment was designed to study the effects of (i) summer drought by exclusion of natural throughfall and subsequent rewetting and of (ii) soil frost by removal of natural snow cover. In complementary laboratory experiments with undisturbed soil columns, (i) drying/rewetting cycles were simulated with different rewetting intensities and (ii) freezing/thawing cycles were induced using different freezing temperatures. In the second part of this work, total C and N stocks as well as radiocarbon signatures of soil organic carbon (SOC) from different soil horizons and density fractions were investigated. A/W increased the DOC concentrations in the organic layer and upper mineral soil. More DOC was released from the organic layer to the mineral soil. However, the effects on total DOC leaching were smaller due to reduced water fluxes. Specific UV absorbance and emission fluorescence detected a switch in the release of easily decomposable DOC to hardly decomposable DOC during the wetting phase. Prolonged summer drought and incomplete rewetting due to hydrophobicity of SOM in the organic layer and upper mineral horizon reduced net N mineralisation as well as concentrations and fluxes of the NH4+ and NO3-. The net nitrification rate in the organic layer was more negatively influenced than net ammonification, indicating that nitrifiers are more sensitive to drought stress than ammonifiers. The effect of soil frost strongly depended on soil freezing temperature. Only soil frost at temperature below -8°C led to short periods of additional DOC production in the organic layer. Spectroscopic properties and ∆14C signatures of DOC implied a disruption of soil aggregates and desorption of older DOC from the mineral associated organic matter fraction of the Oa horizons by G/A events. Severe soil frost below -8°C inhibited the activity of nitrifiers and ammonifiers with decreased NH4+ and NO3- concentrations and fluxes in the mesocosm experiment. A delayed (by 4 months) increase in NO3- concentration in the upper soil horizon by moderate soil frost (-5°C) was attributed to reduced Immobilisation by heterotrophic microorganisms. Summarised, drying and the effect of hydrophobicity led to long-term, severe soil frost to short-term reduction in N mineralisation and N leaching. The effect of increased NO3- concentrations as delayed response to G/A needs further research in case of potentially changes in the N balance. Drying as well as freezing induced changes in the soil structure and properties and led to increased DOC concentrations. Moderate soil temperature had much less effects on C and N in this temperate forest soil. The results of this thesis demonstrated the potential of extreme meteorological events on the quality and availability of dissolved C and N. Both, A/W and G/A cycles decreased C and N mineralisation, increased the sink strength of the soil by the accumulation of SOC and N, considering constant C and N litter input. However, optimal temperature and moisture conditions in other seasons could compensate the sink strength of soils. This work underpins the need for holistic and long-term investigations to understand and model the impact of extreme meteorological conditions on the dynamics of dissolved C and N.
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Soil-Landscape Modelling in an Andean Mountain Forest Region in Southern Ecuador
(2010)
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Mareike Ließ
- Soil-landscapes are diverse and complex due to the interaction of pedogenetic, geo-morphological and hydrological processes. The resulting soil profile reflects the balance of these processes in its properties. Early conceptual models have by now resulted into quantitative soil-landscape models including soil variation and its unpredictability as a key soil attribute. Soils in the Andean mountain rainforest area of southern Ecuador are influenced by hillslope processes and landslides in particular. The lack of knowledge on the distribution of soils and especially physical soil properties to understand slope failure, resulted in the study of this particular soil-landscape by means of statistical models relating soil to terrain attributes, i.e. predictive soil mapping. A 24 terrain classes comprising sampling design for soil investigation in mountainous areas was developed to obtain a representative dataset for statistical modelling. The soils were investigated by 56 profiles and 315 auger points. The Reference Soil Groups (RSGs) Histosol, Stagnosol, Umbrisol, Cambisol, Leptosol and Regosol were identified according to the World Reference Base for Soil Resources (WRB). While soil profiles and auger points were described in their horizon composition, thickness, soil cohesion, bulk density and texture were analysed in soil profiles only. The prediction of soil parameters was carried out with Classification and regression tree (CART) and Random Forest (RF) method. At this, prediction uncertainty was addressed with hundredfold model runs based on different random Jackknife partitions. Problems with the prediction of the RSGs, likely caused by inconsequence within the WRB (absolute and relative values as decision criteria), resulted in the proposal of “incomplete soil classification”, which relates the thickness of the diagnostic WRB horizons to the upper 100 soil centimetres. Histosol and Stagnosol have been distinguished as dominant RSGs within the inves-tigation area. While Histosol probability depended on hydrological parameters; highest Stagnosol probability was predicted on slopes < 40° and above 2146 m a.s.l. Whether the first mineral soil horizon displays stagnic properties or not, likely depends on physical soil properties in addition to terrain parameters. Incomplete soil classification resulted in histic and stagnic soil parts dominating the first 100 cm of the soil volume for most of the research area. Comparing CART and Random Forest (RF) in their model performance to predict topsoil texture and bulk density as well as mineral soil thickness by hundredfold model runs with random Jackknife partitions, RF predictions resulted more powerful. Altitude a.s.l. was the most important predictor for all three soil parameters. Increasing sand/ clay ratios with increasing altitude, on steep slopes and with overland flow distance to the channel network are caused by shallow subsurface flow removing clay particles downslope. Deeper soil layers are not influenced by the same process and therefore showed different texture properties. Terrain parameters could only explain the spatial distribution of topsoil properties to a limited extent, subsoil properties could not be predicted at all. Other parameters that likely influence soil properties within the investigation area are parent material and landslides. Strong evidence was found that topsoil horizons did not form from the bedrock underlying the soil profile. Parent material changes within short distance and often within one soil profile. Landslides have a strong influence on soil-landscape formation in shifting soil and rock material. Soil mechanical and hydrological properties in addition to terrain steepness were hypothesized to be the major factors in causing soil slides. Thus, the factor of safety (FS) was calculated as the soil shear ratio that is necessary to maintain the critical state equilibrium on a potential sliding surface. The depth of the failure plane was assumed at the lower boundary of the stagnic soil layer or complete soil depth, depending on soils being stagnic or non-stagnic. The FS was determined in dependence of soil wetness referring to 0.001, 0.01, 0.1 and 3 mm/h net rainfall rate. Sites with a FS ≥ 1 at 3 mm/h (complete saturation) were classified as unconditionally stable, sites with a FS < 1 at 0.001 mm/h as unconditionally unstable. The latter coincided quite well with landslide scars from a recent aerial photograph.
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The reactivity of ferric (hydr)oxides towards dissolved sulphide
(2010)
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Katrin Hellige
- Ferric (hydr)oxides are ubiquitous with different characteristics such as stability, reactivity and surface properties and play an important role in redox reactions in many environments such as soils, marine sediments, lakes, and ground water. Under anoxic conditions, ferric (hydr)oxides are reduced by dissolved sulphide or by microorganisms. This reaction generates Fe(II) which may precipitate as iron hydroxide, adsorb to the ferric (hydr)oxide surfaces and transform the ferric (hydr)oxides into more stable minerals, or precipitate as iron sulphide. During the reductive dissolution adsorbed species like arsenic may be released from the oxide surfaces to solution. Furthermore, the generation of ferrous iron in ground water systems, their transport through the groundwater-surface water interface, and subsequent iron oxidation and precipitation contribute to the acidification of lakes or sediments as a result of both mining activities and natural processes. Hence, the redox reactions between dissolved sulphide and ferric (hydr)oxides are of fundamental importance for the elemental cycles of sulphur and iron and in particular for the carbon and electron flow in groundwater, soil, and lake systems. The overall chemical pathway of the reactions and their kinetics are reasonably understood. There is less knowledge on the transient stages and the electron transfer processes during the reactions which involve the formation of amorphous or disordered, as well as, nucleation of (metastable) crystalline phases at the reacting interface as a function of time. Furthermore, the interaction between dissolved sulphide and ferric (hydr)oxides can be regard as a key reaction ultimately leading to pyrite formation in both marine and freshwater sediments. However, the knowledge on the pathways and on the controlling factors of pyrite formation is still limited. Therefore this work focused on anoxic abiotic kinetic batch and flow-through experiments with various ferric (hydr)oxides and dissolved sulphide at pH 4 and pH 7. TEM, X-ray diffraction, Mössbauer spectroscopy, and wet chemistry were used to explore the nanocrystalline products which formed over time during the reaction. Furthermore, these experiments should be contribute to the elucidation of the role of Fe2+ regarding the iron sulphide formation and the transformation of Fe(III) oxides. The electron transfer reaction between dissolved sulphide and ferric (hydr)oxides and the deeper insight into the processes occurring at the ferric (hydr)oxides surfaces were investigated in chapter 2 and 3. Batch experiments with dissolved sulphide and ferrihydrite, lepidocrocite, and goethite were performed under well-defined conditions at pH 7 and at room temperature in a glove box with a special emphasis on the characterization of nanocrystalline products forming at different time steps over a reaction time of 14 days. The temporal evolution of the chemical species and the solid phases indicate that the reaction progress was highly dynamic. After two weeks we observed the formation of secondary minerals and pyrite in all experiments as a result of excess-Fe(II) formation. Ferrihydrite was transformed completely via dissolution-precipitation processes into more stable minerals such as magnetite, hematite, pyrite, and into minor amounts of goethite. In the experimental solution with lepidocrocite and goethite the host mineral remained and we detected only pyrite as new mineral. Small amounts of goethite were transformed to hematite while the pyrite formation in the experimental solution with lepidocrocite was accompanied by traces of magnetite. In chapter 4, the reaction kinetic of dissolved sulphide and ferric (hydr)oxides were studied under abiotic, anoxic, and flow-through conditions at pH 4 and 7 and at room temperature over a time period of 6 hours. Various synthetic Fe(III) (hydr)oxides with a broad range of crystallinity and different surface and bulk properties were used in order to assess how variations in these properties influence the kinetics of chemical Fe(III) (hydr)oxide reduction. These experiments showed, as well as, the batch experiments, that the formation of Fe(II) and S(0) was decoupled. In the presence of ferrihydrite and lepidocrocite the generated Fe(II) due to the reaction with dissolved sulphide adsorbed to their surfaces and was accompanied by an electron transfer which led to the formation of excess-Fe(II). These processes seem to be accelerating the reductive dissolution of ferrihydrite and lepidocrocite by dissolved sulphide. Goethite behaved differ: the adsorption of Fe(II) onto the goethite surface occurred without an electron transfer. Thus, the generated Fe(II) controls the reductive dissolution of various ferric (hydr)oxides by dissolved sulphide.
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Der Markenwert touristischer Destinationen und seine Implikationen für das Destinationsmanagement aus tourismusgeographischer Perspektive. Herleitung und Überprüfung eines empirischen Messinstruments am Beispiel des UNESCO Weltkulturerbe Bamberg.
(2010)
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Sebastian Raum
- Vor dem Hintergrund aktueller Diskurse bezüglich subjektiv aufgeladener, imaginativer Geographien legt der Tourismus dem städtischen Raum ein Bezugssystem auf, welches nach Bildern, Geschichten und Gebäuden auf die Destination abgestimmt ist und es ermöglicht, die Destination zielgruppengerecht zu vermarkten. Als besonders zugkräftiges Argument dieser Kommodifikation beziehungsweise Touristifikation von Räumen hat sich inzwischen die Eintragung einer Destination in die Liste der UNESCO-Welterbestätten herausgestellt. Ziel der UNESCO-Welterbekonvention ist in erster Linie der Schutz des gemeinsamen Erbes aller Menschen. In den Augen vieler Kommunalpolitiker und City Manager dient das UNESCO-Prädikat jedoch vorwiegend einem möglichst effektiven Destinationsmarketing (Place Branding) und damit der Steuerung einer positiven Entwicklung des Markenwertes ihrer Destination. Ziel der vorliegenden Arbeit war es, ein modulares Messinstrument zur Bestimmung des Markenwertes eines touristischen Zielgebietes zu entwickeln. Nachdem bislang lediglich ein rein akademischer Diskurs zum Thema Markenwert für touristische Destinationen geführt wurde, kann ein solches Messinstrument durch einen interdisziplinären Ansatz unter Verwendung von Aspekten aus Ökonomie, Dienstleistungsmanagement und der Stadt- und Tourismusgeographie konkrete Handlungsempfehlungen zur touristischen Inwertsetzung und möglichst konfliktfreien Nutzung von Destinationen liefern. Nach einer Analyse und Darstellung des Standes der Wissenschaft zu den Themen Destination, Destinationsmarke und Markenwert wurde ein Modell entwickelt, welches es erlaubt, den Markenwert einer Destination zu ermitteln. Das Modell wurde dann am Beispiel der UNESCO-Welterbestadt Bamberg getestet und der Markenwert für die Destination exemplarisch ermittelt. Auf Basis der dort ermittelten Ergebnisse gelang es, strategische wie auch konkrete Handlungsempfehlungen für die betrachtete Destination abzuleiten sowie allgemeine Empfehlungen zur Vermarktung von UNESCO-Welterbestätten auszusprechen. Für Bamberg werden konkret eine strategische Destinationsentwicklung unter Beteiligung aller relevanten Gremien sowie eine stringente Zielgruppenorientierung und eine darauf ausgerichtete Umsetzung aller Maßnahmen empfohlen. Allgemein wird für UNESCO-Welterbestätten eine bessere Kooperation untereinander angemahnt. Ferner bedürfen alle Maßnahmen stets der Berücksichtigung der jeweiligen Position im Lebenszyklus der betreffenden Destination.
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Role of Dissolved Organic Nitrogen in the Soil Nitrogen Cycle of Forest Ecosystems
(2010)
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Bettina Schmidt
- In the last years, dissolved organic N (DON) has been shown to be a crucial part of the soil N cycle in forest ecosystems. Despite this, information on its dynamics, sources and fate is still lacking. Especially data from (sub)tropical forest ecosystems are scarce. Therefore, this study investigated (i) the magnitude and drivers of DON fluxes in a subtropical montane forest, (ii) the biodegradability of DON from forest floors, (iii) the abiotic formation mechanism for DON in forest floors as postulated by the Ferrous Wheel Hypothesis and (iv) the link between DON and dissolved organic C (DOC) dynamics. In a field study (2005-2008), average DON fluxes in forest floor percolates and seepage (60 cm) of a subtropical montainous cypress forest (16 and 8 kg N ha-1 yr-1, respectively) were similar to fluxes in other (sub)tropical ecosystems, and dominated total N fluxes. Dissolved organic N concentrations in the soil were independent of the water flux (meaning that no dilution effect was visible). This implies that first, the pool size of potentially soluble DON is variable and second, that this pool is hard to deplete. In contrast, the linear relationship between soil organic solute and water fluxes was positive, showing that precipitation is an important driver for DON losses in this ecosystem. Although this has also been reported from temperate ecosystems, this relationship did not hold when analyzing the combined data from various (sub)tropical and temperate forest ecosystems. The biodegradability of DON was highest in inoculated spruce-Oi water extracts in a 21-day incubation experiment, while in extracts from beech-Oi and Oa horizons, DON concentrations only slightly decreased. Dissolved organic N was recalcitrant in spruce-Oa and cypress-Oa extracts, indicating that this DON could add to the formation of stable soil N pools. As various additions of NO3- never influenced DON biodegradation, it is concluded that microbes do not necessarily prefer mineral N over DON as substrate. Mineralization was always more important than microbial uptake in samples without NO3- additions, and denitrification only played a minor role in spruce-Oi samples (as indicated by a negative balance of all N species after 21 days). Fluorescence excitation-emission spectroscopy and subsequent parallel factor analysis identified four groups of fluorophores in the extracts. The initial concentration of two of these so-called factors was correlated with DON biodegradation, but protein-like fluorescence (which has been suggested as a proxy for dissolved organic matter biodegradation) was shown to be independent of DON biodegradation due to similar excitation-emission-maxima of recalcitrant compounds. Therefore, these factors might not always be suitable to predict DON biodegradation. The abiotic reaction of NO2- with DOC (as postulated by the last step of the Ferrous Wheel Hypothesis) was tested in a second incubation experiment in extracts with varying DOC concentrations and qualities and NO2- additions under oxic conditions. Concentrations of added NO2- never decreased within 60 min, indicating, that no DON formation from added NO2- took place. The results show, that the last step of the Ferrous Wheel Hypothesis (which has been suggested to be fast) is unlikely to occur in forest floors. Dissolved organic N and C fluxes were both highly dependent on precipitation at the cypress site, suggesting a strong link between these two classes of compounds. This assumption was supported by the first incubation experiment, where both DON and DOC biodegradation were not influenced by NO3- additions. Moreover, DOC dynamics closely resembled DON dynamics, which suggests that DON biodegradation could be driven by microbial C demand. Therefore, the often used separation of DON and DOC into functionally different compound classes is not always warranted. In conclusion, this study emphasized the need to include DON in biogeochemical N studies of both temperate and (sub)tropical ecosystems, and provided new and important insights regarding DON biodegradation, possible DON sources in forest floors and the link between DON and DOC dynamics in forest ecosystems.
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Documentation and Instruction Manual for the Krypton Hygrometer Calibration Instrument
(2010)
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Thomas Foken
Hubert Falke
- no abstract
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Jahresbericht 2009 zum Förderprojekt 01879, Untersuchung der Veränderung der Konzentration von Luftbeimengungen und Treibhausgasen im hohen Fichtelgebirge
(2010)
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Johannes Lüers
Thomas Foken
- keine Zusammenfassung
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Characterization of reactive and non reactive trace gas fluxes in and above soil
(2010)
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Anika Bargsten
- Nitrogen is one of the most important compounds on earth. All organisms need nitrogen to live and grow. Even the majority (78.08%) of the atmosphere (and so the air we breathe) is dinitrogen. Over the last century, human activities have dramatically increased emissions and removal of nitrogen to the global atmosphere by as much as three to five fold. Nitrous oxide is the fourth largest single contributor to positive radiative forcing, and serves as the only long-lived atmospheric tracer of human perturbations of the global nitrogen cycle. Nitrogen oxides belong to the so called indirect greenhouse gases. These indirect greenhouse gases control the abundances of direct greenhouse gases through atmospheric chemistry and contribute on this way to the greenhouse effect. For a better understanding of these feedback mechanisms it is necessary to know the source strength of nitric oxide and nitrous oxide. Thus, the knowledge about exchange processes of nitrogen is of interest and importance for scientist and policy makers, likewise. This thesis contributes the understanding of processes in the nitrogen cycle. The thesis is addressed on nitric and nitrous oxide emissions. Nitric oxide emissions were measured on soil samples using an automated laboratory system. Nitrous oxide emissions were measured directly on the field site using a closed chamber technique. The laboratory measurements were compared with field measurements of NO (modified Bowen ratio method) at a grass land site. The field NO fluxes were always around 1.8 ng m 2 s-1 while the laboratory derived NO fluxes were between 2.1 and 5,2 ng m-2 s-1. The agreement between the two data sets is considered to be quite good. The laboratory derived NO fluxes exceeded the field NO fluxes by a factor of 1.5 to 2.5. Most studies of nitric oxide (NO) emission potentials up to now have investigated mineral soil layers only. In this thesis soil organic matter was sampled for laboratory measurements under different understory types (moss, grass, spruce, blueberries) in a humid mountainous Norway spruce forest plantation in the Fichtelgebirge (Germany). In this thesis the response of net potential NO fluxes on physical and chemical soil conditions (water content and temperature, bulk density, particle density, pH, C/N ratio, organic C, soil ammonium, soil nitrate) was determined. Net potential NO fluxes (in terms of mass of N) from soil samples taken under the different understories ranged from 1.7 - 9.8 ng m 2 s-1 (soil sampled under grass and moss cover), 55.4 - 59.3 ng m-2 s-1 (soil sampled under spruce cover), and 43.7 - 114.6 ng m 2 s-1 (soil sampled under blueberry cover) at optimum water content and a soil temperature of 10°C. Effects of soil physical and chemical characteristics on the net potential NO flux were statistically significant (0.01 probability level) only for NH4+. Therefore, as an alternative explanation for the differences in soil biogenic NO emission we consider more biological factors like understory vegetation type, amount of roots, and degree of mycorrhization; they provide a potential explanation of the observed differences of net potential NO fluxes. Also, soil nitrous oxide (N2O) emissions in an unmanaged, old growth beech forest in the Hainich National Park, Germany, were measured at 15 plots over a one-year period (November 2005 to November 2006). The annual field N2O flux rate was 0.46±0.32 kg ha 1 yr 1. The N2O emissions showed a background emission pattern with two event based N2O peaks. A correlation analysis showed that the distance between plots (up to 380 m) was secondary for their flux correlations. Annual N2O fluxes obtained from a standard model (Forest-DNDC) parameterized with soil parameters as well as daily temperature and precipitation substantially overestimated the actual field N2O fluxes and also did not describe their actual temporal and spatial variabilities. Temporal variability was described well by the model only at plots with higher soil organic carbon and the modelled N2O fluxes increased during freezing periods only were soil organic carbon was larger than 0.06 kg-1 C kg. The results indicate that the natural background of nitrous oxide emissions may be lower than previously thought and also lower than assumed in standard modelling. This suggests a higher anthropogenic contribution to N2O emissions.
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Water flow paths in soils of an undisturbed and landslide affected mature montane rainforest in South Ecuador
(2010)
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Folkert Christian Bauer
- The number of previous hydrological studies concerning water flow paths in tropical montane rainforest is small. However, due to the increasing pressure of deforestation and land use change comprehensive knowledge of these natural ecosystems is needed if sustainable land use strategies should keep negative effects of human impacts on water flow paths as low as possible. In this context, present work addresses the identification, characterisation, and modelling of water flow paths in soils of an undisturbed and landslide affected natural Andean forest ecosystem in the south of Ecuador whose deforestation rate is one of the highest in South America. In an investigation area situated in the Andes of South Ecuador, in gentler slopes and altitudes above 2100 m ASL mainly Stagnosols and Histosols with stagnic colour pattern and low to negligible rock fragment content prevail. With increasing altitude the abundance of these soils increase, while the presence of Cambisols and Regosols is most pronounced below 2100 m ASL and clearly correlated with the slope angle. Therefore, these soils were mainly encountered in steeper, particularly landslide affected sites often resembling a melange of fine soil and high contents of rock fragments. Aside the investigation of the influence of the rock fragment content on soil hydrological- and physical parameters such as the relationship between rock fragment content and saturated hydraulic conductivity of the soil, present study aims particularly to investigate flow paths of water in soils of landslide affected and unaffected hillslopes. Therefore, we employed conventional field- and laboratory methods, dye tracer experiments including an appropriate image processing technique, as well as statistical models. Results show that both rock fragment content and bulk density control significantly, but not largely the saturated hydraulic conductivity of the mineral soils. Dye tracer experiments and soil parameters document a deeper percolation in the landslide affected hillslopes than in the landslide unaffected hillslopes, where we found preferential flow in root channels with low soil matrix interaction as dominant flow mechanism. A surface near quasi impervious layer along the interface between topsoil and subsoil limits percolation of the water giving the prerequisites of a lateral shallow subsurface flow along the interface between topsoil and organic layer. This is in line with previous studies performed in the same investigation area which already proved indirectly the existence of this flow. However, in none of these studies the shallow subsurface flow was assigned to certain slope inclinations or altitudes. Due to a recently published digital soil map and the results we obtained from the landslide unaffected sites, we know that particularly in hillslopes of less than 30 degrees above 2100 m ASL prerequisites are given for spatially extended shallow subsurface flow. However, even if these prerequisites are not evident for the landslide affected hillslopes, we cannot exclude the possibility of shallow subsurface flow occurrence here since soil cover of the steep terrain is relatively shallow while rainfall is high throughout the year. Therefore, and given that key parameters such as permeability of subsoil and bedrock, interception and evaporation remain unclear or were investigated exclusively such as the spatial variability of the saturated hydraulic conductivity, we conducted a series of virtual experiments in order to assess the potential occurrence of shallow subsurface flow in Cambisols below 2100 m ASL. In these experiments we also included the organic layer being highly abundant in the investigation area, whose hydraulic parameters were estimated by means of inverse numerical modelling. The virtual experiments were based on a two dimensional finite element model representing a steep forested hillslope transect of approx. 54 m length. Aside soil properties, evapotranspiration and interception, the model included the spatial variability of the saturated hydraulic conductivity, the pressure head and their spatial trends. The results of virtual experiment series show that a sound evidence of the key parameters aforementioned is obligate if process conceptualisation regarding shallow subsurface flow generation, but also landslide initiation, solute and matter transport is in the spotlight.
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Advection at a forest site – an updated approach
(2010)
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Lukas Siebicke
- The exchange of carbon dioxide (CO2) across the vegetation-atmosphere interface of a spruce forest was investigated. Horizontal and vertical advection are recognized as important terms of the Net Ecosystem Exchange (NEE) budget in addition to the commonly measured turbulent flux and storage flux. Direct advection measurements are challenging because of the instrumental accuracy required to observe small concentration gradients and small wind velocities and because of the spatio-temporal measurement resolution required to observe complex 3-D flow phenomena. This work presents an experimental multi-analyzer setup for the observation of horizontal CO2 concentration gradients with high temporal resolution and good spatial resolution with no tradeoff between the two. A statistical approach was developed to correct for inter-instrument bias by applying a conditional time dependent bias correction. This approach relies on properties of probability density distributions of concentration differences between one sample point and the spatial average of the sample point field. Sub-canopy CO2 concentration gradients observed with the above presented system showed a high spatial variability which was dependent on vegetation structure. Local concentration perturbations correlated with statistical properties of coherent structures and were explained by vertical exchange between CO2 enriched sub-canopy air and low concentration above-canopy air. The small-scale variability of CO2 concentration gradients brings into question the representativity of horizontal advection measurements for the control volume if observed with a low spatial resolution. Vertical advection estimates rely on accurate measurements of vertical wind velocity (w). Different procedures were applied during coordinate rotation to align the coordinate system of the sonic anemometer with the long-term stream lines. Spatial variability of the wind field was addressed by a sector-wise coordinate rotation. An investigation of temporal aspects of vertical wind velocity showed significant contributions from low frequencies in the spectrum of w. The impact of the data set length used for coordinate rotation on w and on vertical advection was investigated and observed to be large. A sequential coordinate rotation with controlled window length was proposed. Advection contributed significantly to NEE during the night and during transition periods at the Waldstein-Weidenbrunnen (DE-Bay) FLUXNET site. Daily NEE budgets were more realistic, compared to NEE from turbulent flux and storage change alone, if direct advection measurements from continuous and bias corrected gradient sampling were included, reducing the estimated daily carbon sequestration of the forest by almost 50 %.
