5 Naturwissenschaften und Mathematik
Soil erosion and conservation potential of row crop farming in mountainous landscapes of South Korea
- Soils play an essential role for mankind because they provide fundamental ecosystem services required for human life, primarily for the production of food by providing the environment for plant growth. However, soils worldwide became highly threatened by human induced degradation, especially as a consequence of accelerated erosion by water during recent decades. In consideration of climate change and an increasing food demand of a rising population, there is an urgent need to conserve the soil resources by implementing effective erosion control measures for agricultural production. The effective implementation of those measures strongly depends on the specific conditions of particular regions and requires the analysis of the existing farming systems and their capability for erosion control.
Objective of this thesis is the analysis of the major agricultural practices applied for row crop cultivation in mountainous watersheds of South Korea with respect to water erosion and the identification of their conservation potential. Our first two studies analyze the subsurface flow processes, the runoff patterns, and the associated erosion rates of the widely applied plastic covered ridge-furrow system (plastic mulch), and our third study investigates the impact of herbicide applications on erosion associated with conventional and organic farming. To analyze the flow processes induced by the plastic mulch cultivation, we conducted four irrigation experiments on potato fields that represent a smooth surface, uncovered ridges, and plastic covered ridges with and without a developed crop canopy. With an automatic sprinkler, we irrigated small plots with a dye tracer solution of Brilliant Blue and potassium iodide, collected surface runoff, and excavated soil profiles to visualize the subsurface flow patterns, which were subsequently analyzed by image index functions. We found that the ridge-furrow system, especially when ridges are covered with plastic, decreased infiltration and generated high amounts of surface runoff, whereas a developed crop canopy increased infiltration due to interception and stem flow. The analyses of the subsurface flow patterns show that the plastic covered ridge-furrow system induces preferential infiltration in furrows and planting holes due to its topography and the impermeable covers, but that the impact on flow processes in the soils is relatively small compared to the impact on runoff generation. To identify the patterns of overland flow and the erosion rates associated with the plastic mulch system, we installed runoff collectors to monitor runoff and sediment transport of two potato fields with concave and convex topographies, and we applied the EROSION 3D model to compare the plastic covered ridge-furrow system to uncovered ridges and a smooth surface. We found that plastic mulch cultivation considerably increases soil erosion compared to uncovered ridges as a consequence of high amounts of surface runoff. Our results show that the ridge-furrow system concentrated overland flow on the concave field, resulting in severe gully erosion, but prevented flow accumulation and reduced erosion on the convex field, which demonstrates that the effect of this cultivation strategy is primarily controlled by the field topography and its orientation. To analyze the effects of conventional and organic farming on water erosion, we measured multiple vegetation parameters of crops and weeds of conventional and organic farms cultivating bean, potato, radish, and cabbage, and we simulated long-term soil loss rates with the Revised Universal Soil Loss Equation (RUSLE). We found that organic farming reduced erosion for radish, as a result of an increased weed biomass due to the absence of herbicides, but that it increased erosion for potato due to lower crop coverage, presumably as a consequence of crop-weed competition or herbivory associated with the absence of agricultural chemicals. Although we demonstrated that a developed weed cover in the furrows can potentially decrease the erosion risk for row crops, our results show that the average annual erosion rates of both farming systems exceed by far any tolerable soil loss.
In consideration of the generally high soil loss found in our studies, we conclude that the applied farming practices are not capable for effective erosion control and soil conservation in this region. However, based on our findings, we could identify possible modifications of those practices that can help to reduce the risk of erosion in the future. We recommend perforated plastic covers for ridges to reduce runoff generation, and the orientation of the ridge-furrow system along the contours or towards field edges to prevent flow accumulation and gully formation. Additionally, we suggest residue mulching of furrows to protect the soil surface from overland flow, and the cultivation of winter cover crops after harvest to maintain a better soil cover throughout the year.
Site-specific modelling of turbulent fluxes on the Tibetan Plateau
- The Tibetan Plateau attracts attention in recent decades due to its influence on the East-Asian Monsoon and regional hydrology. Therefore estimates of the regional energy and water balance have come into the focus, utilising remote sensing and regional model approaches, but such attempts require surface-specific flux data of high quality for validation. Eddy-covariance measurements are qualified for this task, but these are scarce on the Tibetan Plateau, incomplete due to quality filtering and potentially biased due to the well-known closure gap of the observed energy balance as well as small-scale heterogeneity. This thesis is related to the infrastructural EU project CEOP-AEGIS, aiming at a standardised processing of eddy-covariance data – including correction of the energy balance closure and gap-filling – on the Tibetan Plateau.
In a pre-analysis step, particular issues about data quality of turbulent fluxes (sensible heat flux and latent heat flux/evapotranspiration) at Tibetan Plateau sites have been addressed. One of them is the degradation of data quality due to the frequent occurrence of near-ground free convective conditions. Another issue arises from coordinate rotation for non-omnidirectional sonic anemometer, which requires a careful handling. In consequence, a sector-wise planar-fit is recommended, disregarding the sector influenced by the anemometer's mounting structure. This can reduce occurrences of invalid momentum flux data, whilst no effect on scalar fluxes can be seen.
As a main topic, this thesis investigates the application of process-based modelling to estimate turbulent flux exchange between the surface and the atmosphere for typical surface types on the Tibetan Plateau. Therefore a case study has been carried out at Nam Co, Tibetan Plateau. Turbulent flux measurements over dry and wet grassland as well as over a shallow lake have been conducted during the summer monsoon season of 2009, and modelled with the land surface scheme SEWAB and a hydrodynamic multilayer model for the lake. Adaptations were implemented to the land surface scheme with regard to the special conditions on the Tibetan Plateau, such as extreme diurnal variation of surface temperature and variation in soil moisture, further called TP version. The analysis includes a consequent model comparison with eddy-covariance data, using model parameters derived independently rather than applying optimisation strategies. Specific attention has been devoted to the impact of observed energy balance closure and its correction, establishing a new correction method according to the Buoyancy flux.
The land surface model reasonably represented the dry and the wet grassland site by only setting the site-specific model parameters, and the TP version performed overall better than the original version, while laboratory measurements of soil parameters failed to improve model performance in comparison to standard parameter values. Soil temperature and moisture measurements as well as field based knowledge of the soil type have been identified as minimum requirements for model parameter acquisition. Lake surface fluxes have been modelled reliably, the lake depth has been taken into account. These results can be transferred to any lake on the Tibetan Plateau given the required forcing data including a representative lake surface temperature.
The choice of the surface model and the selection of the energy balance closure correction method are inter-related problems. The correction partitions the balance residual to the sensible and latent heat flux. This can be typically done according to the Bowen ratio, or according to the presented new method which attributes a larger fraction to the sensible heat flux. Testing both methods leads to partly ambiguous model performance, especially with respect to the used parameter sets. It clearly leads to shifts in model bias, while the R² metric suggests higher model compatibility to the Bowen ratio correction. The latter agrees with previous findings with respect to SEWAB modelling, but is in contradiction with recent experimental findings, attributing the closure gap to secondary circulations, driven by buoyancy. Future research on model structure should account for such processes.
As expected, the flux measurements showed distinct differences between the investigated land use types in magnitude and dynamics. The used models were able to resolve these differences in general with contrasts between surface types exceeding model errors. This must be considered when validating regional flux estimates with eddy-covariance data from the dry Nam Co station. The findings from this thesis provide the basis to process eddy-covariance data on the required level as described above.
Mineral sequestration of CO2 by reaction with alkaline residues
- With the onset of industrialization within the last 150 years, a significant increase in the concentration of the greenhouse gas carbon dioxide (CO2) is recorded in the atmosphere. According to current scientific understanding the rising atmospheric CO2 levels can be linked with high probability to the observed phenomenon of global warming. Consequently, the reduction of anthropogenic greenhouse gas emissions has become a global challenge of environmental research and policy.
In this thesis, a novel approach to achieve a long-term mineral sequestration of CO2 was studied, using alkaline residue materials. This study examined for the first time a process that allows for rapid removal of CO2 from flue gas through reaction with lignite fly ashes in aqueous solution.
In this process the basicity of the residues is utilized for mineral trapping of CO2 by precipitating stable calcite. Lignite fly ashes are a cheap, inexpensive, highly reactive byproduct of coal combustion. Due to the exposure to heat, these waste streams generally contain high amounts of reactive Ca/Mg (hydr)oxides and thus offer a high alkalinity. The alkaline residues were therefore not considered as an environmental problem, but rather as useful reactants for technical CO2 neutralization in the context of combustion processes.
Carbonates are end-products of weathering processes at the earth surface and mineral carbonation is thus assessed to be a permanent and safe storage option of CO2. Compared to alternative forms of carbon storage (e.g. the injection into gas reservoirs) cost-intensive monitoring programs for safety reasons can be omitted. Also, the carbonation generally leads to heavy metal fixation in the residues, allowing for an environmentally less problematic disposal of the products or even their industrial re-use (e.g. road construction, cement industry). Due to the common high reactivity of alkaline fly ashes no pre-treatment (e.g. grinding, using chemical additives) is needed compared to the use of natural silicate minerals as feedstock material. For these reasons mineral carbonation of alkaline residues can be considered as a process with low costs and low energy consumption, thus making it an interesting CO2 reduction pathway from an economical point of view.
In Chapter 1, the mechanisms and rates of reactions between alkaline lignite fly ash and CO2 in aqueous suspensions were evaluated. Aqueous laboratory experiments showed that CO2 from flue gas can be bound directly as carbonate. Additionally, solutions with high dissolved inorganic carbon content are formed, which can be injected into aquifers for mineral CO2 sequestration. As the dissolution rates of the alkaline mineral phases are high, gas phase CO2 transfer into the aqueous phase is mostly the limiting factor for the overall carbonation process. CO2 dissolution is controlled by the solution pH, by the available surface area of the gas/water interface and by the gradient at that interface.
The maximum conversion of 5.2 moles of CO2 per kg fly ash (≈ 0.23 kg kg-1) obtained at 75 °C demonstrates the high potential of alkaline fly ashes to sequester CO2. This value accounts for a CO2 sequestration capacity of nearly 3.5 million t of CO2 in Germany alone based on the available lignite fly ash, which corresponds to 2 percents of the CO2 emissions from lignite power combustion (168 million t a-1 in 2009).
In Chapter 2, laboratory carbonation experiments are described, which were carried out with the individual mineral phases CaO and MgO in aqueous solution. The process showed parallels with the reactions observed during carbonation of lignite fly ashes, suggesting that Ca and Mg (hydr)oxides can be used as proxies to estimate alkaline waste reaction with CO2 in general.
The carbonation of CaO happens fast, occurs at high pH values > 12 and is controlled at the mineral surface by the dissolution of Ca(OH)2. As long as Ca(OH)2 is available CO2 uptake by the system is high and leads to the simultaneous precipitation of calcite (CaCO3). Under similar conditions MgO carbonation is a slower and much more complex process. In the presence of MgO an initial pH of ~ 10.8, indicating solubility equilibrium, was reached. Subsequently, TDIC concentrations and EC increased almost linearly. The pool of MgO based alkalinity can be made available for mineral trapping if the kinetic restrictions for precipitation of Mg-carbonate can be overcome, e. g. by running the processes at higher temperature (> 50 °C) and higher s/l-ratio. Corresponding to related work the precipitation of hydromagnesite (Mg5(CO3)4 (OH)2 ∙ 4H2O) is found for temperatures above 50 ° C already at a suspended amount of 4 g L-1. Precipitation of nesquehonite (MgCO3 ∙ 3H2O)) starts upon a suspended amount of MgO of more than 10 g L-1 at 25 ° C.
In Chapter 3 the setup and the results of a model are shown, which was used to simulate and evaluate the process of alkaline material carbonation over time. Experimentally derived specific dissolution rates for CaO/MgO and CO2 are used for the development of a kinetic geochemical model based on the freely available PHREEQC algorithm. The software offers the access to databases, which containing thermodynamic constants of all common dissolved species in natural and industrial processes.
Experimental assays conducted in an aqueous carbonation reactor (see Chapter 1 and 2) were used as reference to test the model and evaluate its robustness and sensitivity.
The reaction course of the experiments based on the use of the pure phases (CaO and MgO) was successfully reproduced by our simulations. The developed model may thus be used as a valuable tool for the optimization of technical scenarios/facilities for CO2 sequestration. In order to study different mineral sequestration scenarios for calcite precipitation, we used the simulation to test the variation of process parameters and the addition of chemical additives (CaCl2, CaSO4). Finally, the simulation of the carbonation of lignite fly ash was tested using our simplified model based on CaO, MgO, calcite, anhydrite as kinetic reactants. It was shown that advanced techniques to determine the exact mineralogy of combustion residues and the extension of the availability of thermodynamic data of specific mineral phases are necessary to improve geochemical modelling in future work.
In Chapter 4, the potential contribution of lignite fly ash to mineral CO2 trapping in a high anhydrite (CaSO4) containing aquifer were analyzed. The study examined the possibility of combining underground CO2 storage and geothermal heat/energy production from an anhydrite rich aquifer. In such a scenario Ca2+ for the precipitation of calcite could be provided from the dissolution of the calcium sulfate. The dissolution of anhydrite concurrently releases acid, being counterproductive with respect to the formation of carbonates. The possibility of pH buffering by the addition of alkaline lignite fly ash is therefore appraised to optimize the conditions of carbonate precipitation.
The performed laboratory experiments, as basis for thermodynamic simulations with PHREEC, confirmed that the buffering capacity derived from the fly ashes is essential for calcite precipitation in such a system. Already with an addition of 0.1 weight percent of fly ash per volume of the injection solution the amount of precipitated calcite was maximized. The dissolution of anhydrite is associated with a concurrent increase in pore space and can balance the porespace reduction by precipitation of carbonates and secondary silicates in the geothermal reservoir.
Dynamic Self-Assembly of Magnetic Colloidal Particles
Dynamic self-assembly represent one of the most powerful tools in Nature to spontaneously organize a system on a hierarchy of different scales.
Most of the processes at the nano/micro scale occur at very low Reynold’s number where inertia can be neglected. Creeping flow magnetic systems can be characterized by the Mason number.
The Mason number measures the ratio between the viscous and the magnetic torque and is the main parameter governing the behavior of paramagnetic colloids investigated in this thesis.
The work presented in this thesis explores new dynamic regimes of colloidal dynamics which occur when suddenly switching to high Mason numbers.
In a static magnetic field the equilibrium structure of paramagnetic colloids are chains. At high Mason number in a rotating magnetic field the time averaged equilibrium conformation is a two dimensional cluster.
By switching from a static to a rotating magnetic external field, we cause a transient dynamics from a static to the dynamic equilibrium state.
The first question addressed in this thesis is: what is the physics that determines the transient folding pathway from one to the other equilibrium state?
Dynamic magnetic fields were used by others to propel top down DNA-linked chains of paramagnetic colloids in a liquid.
The second question asked is whether we can dynamically self-assemble swimmers taking a fully bottom up approach?
The third question is: is it possible to assemble more complex dynamic patterns that lead to motion of the swimmers governed by more collective coupled hydrodynamics that goes beyond slender body theory of the linked chains?
This thesis answers the three questions and contributes to the understanding of colloidal dynamics and self-assembly in dynamic magnetic fields in the regime of high Mason numbers.
We explore two aspects of the dynamic self-assembly i.e. the transient kinetics between two dynamic self-assembled equilibria and the dynamically self-assembled propulsion of magnetic swimmers beyond slender body hydrodynamics.
The thesis therefore aims at achieving magnetic control over the assembly of complex dynamic colloidal structures.
Impact of time and spatial averages on the energy balance closure
- Secondary circulations are large and relatively stationary eddies, which are caused by the surface heterogeneity and normally reside away from the ground. They are believed to be the cause of the energy balance closure problem at the earth's surface, because their contribution to the turbulent fluxes is missed by a fixed eddy-covariance tower measurement that has a typical averaging time of 30 minutes. In this thesis, data from the LITFASS-2003 experiment was used to investigate the impact of time and spatial averages on the energy balance closure. This data consisted of many observations over a large heterogeneous landscape that could generate secondary circulations; some of which might be still near the earth's surface.
For the time average analysis, the averaging time was extended to increase the possibility that secondary circulations were picked up by the sensor. Two approaches, which were the modified ogive analysis and the block ensemble average, were applied to analyze the data from ground-based measurements. The modified ogive analysis requiring a steady state condition, could extend the averaging time up to a few hours and suggested that an averaging time of 30 minutes was still overall sufficient for the eddy-covariance measurement over low vegetation. The block ensemble average, on the contrary, did not require a steady state condition, but could extend the averaging time to several days. However, this approach could only improve the energy balance closure for some sites during specific periods, when secondary circulations existed in the vicinity of the sensor. Based on this approach, it was found that the near-surface secondary circulations mainly transported sensible heat, which led to an alternative energy balance correction by the buoyancy flux ratio approach, in which the attribution of the residual depended on the relative contribution of the sensible heat flux to the buoyancy flux. The fraction of the residual attributed to the sensible heat flux by this energy balance correction was larger than in the energy balance correction that preserved the Bowen ratio.
In the spatial average analysis, two energy balance correction approaches, the buoyancy flux ratio and the Bowen ratio approaches, were applied to the area-averaged fluxes (composite fluxes) in order to include contribution from secondary circulations. These composite fluxes were aggregated from multiple ground-based measurements. The energy balance corrected fluxes were validated against the spatial average fluxes, which were measured by an aircraft and a large aperture scintillometer (LAS). In this validation, the backward Lagrangian footprint model was used to estimate the source area of the measurement. It was found that both energy balance correction approaches did improve the agreement between time and spatial averages fluxes. This suggested that the contribution from secondary circulations could be properly accounted by the energy balance correction.
All findings in this thesis, therefore, depict that secondary circulations significantly transport energy in the atmospheric surface layer. The energy balance correction, accomplished by using either the Bowen ratio approach or the buoyancy flux ratio approach, is necessary to estimate the actual vertical transport of energy at the earth's surface.
Drivers of forest dynamics: Joint effects of climate and competition
- The present dissertation thesis addresses different aspects of forest dynamics and possible changes due to climate change. Various modelling approaches are used to explore joint effects of climate and competition on forest dynamics with a focus on temperate forests. Motivated by scientific interest, this thesis is aimed at contributing to the establishment of fundamental knowledge for proper ecosystem management. Each of the research projects of this thesis explores a facet of forest dynamics. It appears that for projections of forest dynamics under climate change particularly, it is critical to consider competition among trees.
In the first study, the joint effect of climate and competition on forest dynamics in a mountain forest of New Zealand was investigated. The landscape simulation model LandClim was calibrated based on empirical data and applied to reproduce a 1700 years forest succession under stationary climate at the slope of Mt. Hauhungatahi, North Island. Although designed for European temperate forests, LandClim was capable of simulating NZ´s forest dynamics.
Under non-stationary climate, forests likely remain in disequilibrium with climate for some time due to the longevity of trees and competitive prevention of establishment. This aspect was investigated in a mixed beech-oak forest in Germany, using LandClim and the forest gap model SILVA in a cooperation study. Furthermore, a possible ‘climatic turning point’ was investigated, the point at which species dominances change due to changes in competitiveness caused by climate change. Both models projected a potential climatic turning point at a mean annual temperature of 11-12 °C and precipitation sum of 500‑530 mm. However, the change of species composition in existing mixed stands was much slower since the turning point also depended on inherited stand structure. Based on these projections the promotion of oak at dry sites seems advisable due to its superior resistance and resilience to drought.
The applied simulation models consider joint effects of climate and competition but no changes in species sensitivity to competition along climatic gradients. The Spanish National Forest Inventories provided a solid basis to develop a statistical model for the influence of climate and competition on tree growth. The results indicated that in Mediterranean forests the effect of competition increases with aridity potentially resulting in an additional disadvantage for drought sensitive oaks compared to pines under climate change.
Under the prerequisite that forest dynamics will be affected by climate change, different forest management strategies on adaptation are currently discussed. One option is the promotion of tree species that are better adapted to anticipated future climates (such as oak instead of beech in Germany) and also the introduction of non-local ecotypes of local species. This increase in biodiversity intuitively appears promising because it is in line with the insurance hypothesis and the portfolio effect theory. Within this thesis the effect of ecotype mixing as an increase of within stand diversity under consideration of self-thinning was assessed. It was shown that ecotype mixing in forest stands might lower the risk of yield losses and at the same time might exempt the portfolio effect from its drawback of lower chances for high yields.
Climate not only affects demographic rates of tree species but also all other ecosystem components. Disturbances are an important component of forest dynamics because they initiate successions and thereby influence species coexistence. Climate will alter disturbance regimes not only directly but also due to interactions among disturbances, for example an increased risk of insect outbreaks due to weak tree defence caused by severe drought stress. Most disturbance interactions have been observed to be positive, implying that increases of disturbances in quality and quantity due to climate change will be amplified. Furthermore, systems containing positive feedback loops are considered to be mostly unstable, which would result in forest collapse. A theoretical study on disturbance interactions showed why positive feedback loops of disturbances do not necessarily lead to a forest collapse. Disturbance interactions might cause only a minor part of disturbances, whereas direct changes due to climate change are of much higher importance.
The described studies reflect the diversity of the research field forest dynamics and innovative ecological methodology. Nevertheless, the present thesis is not an exhaustive discussion of drivers of forest dynamics under climate change. Forest dynamics and its drivers provide a range of open research questions posing a challenge for fundamental an applied research of high relevance for society.
Impact of extreme hydrological conditions on belowground carbon cycling and redox dynamics in peat soils from a northern temperate fen
Cristian Estop Aragonés
- Peatlands have an important role in the global carbon cycle and constitute the largest pool of carbon stored in terrestrial ecosystems due to their disproportionally high areal soil carbon density. This globally relevant carbon stock is the result of a process mostly initiated after the last glaciation period. A key factor for this long term carbon accumulation is the relative low decomposition of organic matter in these predominantly water logged ecosystems. Hydrological conditions play thus a fundamental role in peatlands and the feedback of carbon cycling in these ecosystems in response to climate change is under debate. Peatlands are important CO2 sinks but also constitute global sources of CH4. The atmospheric exchange and production rates of these greenhouse gases are strongly influenced by the hydrological regime. An increased frequency of extreme meteorological conditions resulting in drying and flooding events is predicted to occur in the future.
The major issue regarding the climate change debate at the global scale is how rapid these greenhouse gases are being released to the atmosphere. Despite the general consensus regarding the broad effects of drying and flooding on CO2 and CH4 exchange, belowground processes producing such greenhouse gases and their response to water table dynamics is underrepresented and usually simplified or overgeneralized. Temperature, moisture, oxygen content and nutrient content are among the major environmental controls for organic matter decomposition rates in peat soils. Another important and intrinsic control is peat quality or humification degree of organic matter. The interrelation and relevance of all these factors vary among sites and with hydrological condition in a temporal and spatial scale.
This work presents investigations focusing on belowground redox processes aiming to witness the dynamic interrelation of soil physical and chemical (soil gas and pore water chemistry) variables, and evaluates the relevance of some controls of organic matter decomposition during a wide range of hydrological conditions. Most of this work shows information under in situ conditions and complementary laboratory experiments were performed minding the in situ observations. The findings contribute to general knowledge by providing raw data in fen peats under fluctuating and contrasting water table conditions in a relatively high spatiotemporal resolved scale. Dryings led to increased air filled porosity, O2 intrusion, CO2 degassing, inhibition of methanogenesis and renewal of electron acceptors. The opposite trend occurred upon rewetting with pulses of iron and sulphate reduction and delayed methane production to a variable extent. Upon flooding, continued anaerobic conditions stimulated the accumulation of reduced products, methanogenic precursors (acetate and hydrogen) and CH4.
The general assumption that the water table directly controls the oxygen content in peat was relativized. This work shows that such relation is greatly influenced by peat physical properties, which partially control the changes in moisture. Based on these findings, the mineral content and the degree of compaction in organic soils can be implemented to more accurately represent the dynamics of aeration in peats upon water table changes. Another general assumption is that drying events, i.e. temporary decline of water table below mean position, lead to increased CO2 production and emission from peat soils to the atmosphere. Such statement was also relativized and must account for the depth distribution of respiration rates in relation to the mean water table of the peat deposit. Based on these findings, the high relative contribution of upper peat layers already exposed above the water table mask the effects of increased CO2 production in deeper peat upon water table drop. Additionally, the role of moisture was shown to be little for aerobic respiration. This work also evaluates the importance of drought severity by accounting for the post drought effects on methane production. More intense and prolonged drying events led to a greater regeneration of electron acceptors in peat soil, which broadly suppressed or limited methane production upon rewetting. This relation was not simple and several factors such as water table position, post drought water table fluctuations, temperature and organic matter content contributed to the recovery of methane production after drying. The provision of substrates by fermentation processes limited peat respiration during shallow water table and drying. In contrast, accumulation of acetate and hydrogen was observed during flooding indicating a decoupling of fermentation from terminal metabolism and favouring the co-occurrence of iron reduction, sulphate reduction and methanogenesis.
High-Resolution Modelling of Surface-Atmosphere Interactions and Convection Development at Nam Co Lake, Tibetan Plateau
- The Tibetan Plateau has recently become an area of increased interest for the atmospheric and environmental sciences. Surface-atmosphere interactions and specifically the exchange of momentum, turbulent energy and water vapour as well as the development of convection are not only important for the surface energy balance and local water resources, but also have influence on the evolution of the monsoon system and climate. High-resolution, numerical atmospheric models with a fully coupled surface model are a valuable tool for the systematic investigation of surface-atmosphere interactions. Nam Co Lake, located at the northern extent of the monsoon's influence, was selected as a complex system in order to study the interaction of the land and the lake in the generation of mesoscale circulations and the development from boundary-layer clouds to moist convection.
Turbulent fluxes estimated by eddy-covariance and atmospheric profiles measured by radiosondes are used in this work in conjunction with the ATHAM (Active Tracer High-resolution Atmospheric Model) and Hybrid models. Substantial model development is undertaken for both ATHAM and Hybrid. This means a more consistent formulation of tracer and heat transport in ATHAM and improved model stability. Hybrid has been modified with an extrapolated surface temperature, to be used for the calculation of turbulent fluxes. A quadratic temperature profile based on the layer mean and surface model base temperature is assumed in each layer and extended to the surface. Compared to eddy-covariance measurements and a Surface-Vegetation-Atmosphere Transport (SVAT) Model there is an overall reasonable model performance, when tested on four days for two sites with variable environmental conditions during the 2009 summer monsoon season. At the same time, errors are reduced by 40-60% compared to the unmodified Hybrid.
Subsequently, the coupled modelling system is used for 2-dimensional cross-sections through the Nam Co Lake basin with horizontal resolutions of
200 m and at least 150 vertical layers between the surface and the model top located in the lower stratosphere. The 2-dimensional modelling approach has a
tendency to overestimate convective strength due to the underestimation of dry air entrainment and cannot reproduce fully realistic flow fields. Nevertheless,
it provides a valuable tool for systematic investigations of environmental factors, where 3D simulations are prohibitively expensive.
In simulations with several background wind speeds it is found that the model adequately simulates the mesoscale circulation system between the lake and the surrounding mountain chains. Dependent on the geostrophic wind direction there are two different mechanisms for the triggering of convection: Convective triggering, when overflowing topography, and triggering due to convergence between the lake-breeze front and the background wind. It is concluded that coupled modelling setup is capable of reproducing the system's most important dynamics, such as realistic turbulent surface fluxes, mesoscale circulations and cloud evolution.
Thereafter, the influence of the atmospheric profiles of temperature and relative humidity and the uncertainty that arises from them is discussed.
Simulations are initialised with profiles based on direct measurements (radiosondes), NCEP-I and ERA-INT reanalysis and GFS-FNL analysis data on two
days during the summer of 2012. The simulated convection from radiosondes compares reasonably well with weather observations for the first day, but
less well for the second day, when large-scale synoptic effects, which are not included in the model, gain importance. The choice of vertical profile information leads to strongly differing convection development, causing modifications of the surface energy balance and thus of the energy and water
cycle for Nam Co Lake.
With respect to precipitation it is found that a large fraction of the precipitation that is generated in the simulations is deposited within the basin
and on the slopes of the surrounding mountain chains and thus locally recycled. This also means that a weather station in the centre of the basin is not representative of the system. Furthermore, Nam Co Lake may be of importance as a water supply for the region. Additionally, the choice of profile and the initial water vapour contents determine the amount of precipitation so that there are strong differences spanning one order of magnitude in the generated precipitation between the model simulations driven by different vertical profiles.
The findings from the thesis provide an example of the impacts of surface-atmosphere interactions, mesoscale circulations and convective evolution on
the Tibetan Plateau. Scaled to the entire plateau these processes are highly relevant to ecosystems, climate and the water cycle.
Quantifying water use by temperate deciduous forests in South Korea: roles of species diversity, canopy structure, and complex terrain
- About seventy percent of South Korea is covered with forests, most of which are found in the mountain regions since mountains receive more rainfall and are difficult terrains not suitable for agriculture. Because mountains are important water sources for cities and human population downstream, performing water balance for forest catchments has become a research priority. The ongoing shift from coniferous to species-rich deciduous forests due to a changing government policy and the anticipated changes in future climate, associated with increasing amount of rainfall and temperature will also impact forest water use, calling for an urgent need to understand how forests, in their current status, use water. The knowledge is vital for predicting water requirements for the future forest. The warm-deciduous temperate forests found in South Korea, however, have a high diversity of tree species, have multi-layered canopies and are mostly located on rugged mountainous terrains, which make it difficult to quantify forest water use, a basic requirement for catchment water budgeting. The main objectives of this study were to: (1) identify the roles of species diversity in tree and forest water use, (2) examine the impact of canopy structure on forest transpiration, and (3) evaluate the influence of terrain on forest water use.
Site-specific studies were carried out in three different natural deciduous forests, namely, Gyebang (GB), Gwangneung (GN) and Haean (HA) forest sites, representing the general structure of S. Korean forests. GB site is known for its high species diversity, GN site is an old forest growth at climax, with clearly defined understory and overstory canopy layers while the HA site was located with in a catchment, with strong elevation changes within short horizontal distances, rising from 400 to 1,000 m a.s.l., and in different aspects. Four locations with varying elevations and aspects were chosen in the HA site. Tree water use (TWU) and canopy transpiration (EC) were estimated from sap flux density measured with thermal dissipation probes. Understory transpiration (EU) was measured using stem heat balance while ecosystem evapotranspiration (Eeco) was determined using eddy covariance technique. Air temperatures (Ta), precipitation, solar radiation, vapor pressure deficit (VPD), wind speed were measured from weather stations and soil water content was measured from frequency domain reflectometry (FDR) sensors at the respective study sites. Vegetation surveys, including diameter at breast height (DBH), tree density, species composition, sapwood area (AS), and leaf area index were performed in all the sites. Canopy conductance (GC) and stomatal sensitivity to VPD were assessed based on transpiration and microclimate measured at each site.
A functional allometric relationship was established between AS and DBH, and also between TWU and DBH for all the study sites; first for single species and then combining all the species either in a single site or in all the sites. Irrespective of tree species, AS and maximum TWU were significantly correlated with DBH in a power function for AS (R2 = 0.77, P <0.0001) and both in power (R2 = 0.63, P <0.0001) and sigmoid functions (R2 = 0.66, P <0.0001) for TWU, for the co-occurring species as well as across the sites, suggesting that DBH can be a good predictor of stand AS and maximum TWU, based on the established allometric functions.
Early bud break and development of the understory compared to the overstory canopy resulted in an earlier onset of forest transpiration, with EU contributing 22% and 14% between April and May to the total forest transpiration. This high contribution was favored by high radiation and VPD in the understory, since the overstory was still undeveloped and open. Despite diminishing VPD and light conditions in the understory between June and August, the understory continued to transpire a substantial amount of water, contributing 10% of the total transpiration. The seasonal patterns of both EO and EU were synchronized to canopy development, while VPD and radiation determined daily trends. EO and EU accounted for 80% of Eeco in spring but only 60% during the monsoon period due to lowered radiation input, VPD, and plant area index (PAI). Thus, Eeco is largely influenced by transpiration rate and its seasonal variation and also canopy structure.
Early saturation of EC at relatively low VPD and also a rapid decrease in GC with increasing VPD were observed in the forest stand located at the highest elevation studied (950 m) in the HA site, compared to the GN and the other forest stands in HA. These differences in transpiration rates and stomatal response can be explained by greater stomatal sensitivity to VPD of 0.83 found at the 950 m site compared to 0.63–0.66 in the other study sites. However, the main controlling factor of the change in stomatal sensitivity at the 950 m stand is uncertain. Although maximum daily EC were correlated with AS of the forest stands at different sites (R2 = 0.78, P <0.01), annual EC declined with increasing elevation, i.e., 176 >175 >110 >90 mm year−1 at 340 >450 >650 >950 m, respectively. Decline in total EC was due to the decline in annual Ta, daytime VPD, and length of growing season at higher elevations. The GB site, which was located at 960 m elevation, however, did not display a same response pattern as those observed at the 950 m site. It is likely because these sites were under different environmental conditions, i.e., GB site is exposed to higher Ta and higher humidity, and is sheltered (lower wind speeds). These observations emphasize the complexity associated with estimation of transpiration in rugged terrains, since general principles do not always apply and the spatial patterns of forest transpiration are complex.
Complexity arising from multiple tree species composition when estimating forest water use can be reduced by applying functional allometric relationship linking tree size and water use. Forest canopy structure and physical location should be taken into account since they influence the way forests use water resources by altering microclimate and plant physiology. Based on our findings, estimation of forest water use on rugged terrains require repeated measurements at relatively small spatial scales since the driving factors change rapidly over very narrow vertical distances.
Budget and fluxes of nitrogen in mountainous agroecosystems in a summer monsoonal climate under intensive land use
- A balanced nitrogen (N) cycle in intensively managed ecosystems is necessary as it underpins other ecosystem services. This study evaluated the agricultural practices in a typical mountainous catchment in South Korea in respect to N dynamics and their potential effect on water quality with the aim to develop options for a more sustainable catchment management.
In the 1st study, we used two approaches to calculate N budgets for the 5 key crops of the basin at the field scale. The gross and net N budgets for all crop types were found to be positive. Based on the small differences between the results of the two approaches we identified fertilizer N as well as soil Nmin as the dominant N input sources. As fertilizer N application was the major N input source (>50%), its reduction is the major scope of action for N savings at the field scale. A closely linked action is the synchronization of fertilizer N with soil Nmin. The large amount of fertilizer that is applied prior to planting (>60%) at the beginning of the monsoon season revealed that split applications could help reducing the fertilizer N additions and increase the low N use efficiencies (NUE). Based on the significant differences between gross and net N surplus for rice and bean fields, we identified the high amount of plant residues remaining after harvesting (>100 kg N ha-1) as a further factor for potential N savings. The 5 main crops accounted for over 80% of the total catchment N surplus (>400 Mg), even though their contribution to the area was only around 20%. A land use shift to perennial crops with lower N inputs was therefore found to be a possible but spatially limited chance to reduce N surpluses at the catchment scale. The comparison of catchment N surplus with stream N export revealed that 73-86% of the agricultural N surpluses were transported to water bodies in the catchment by either leaching or surface runoff.
In the 2nd study, we followed the fate of fertilizer N in a ridge and furrow (R/F) cultivation with polyethylene (PE) mulch by using 15N tracer. N leaching was simulated with Hydrus 2D. The comparison of 4 N fertilization levels (0, 150, 250 and 350 kg N ha-1) revealed that already 150 kg N ha-1 is sufficient to reach the maximal yield of radishes. Based on the low results of fertilizer N use efficiency (FNUE), we recommend two applications during the first 25 days of growth and a further application around day 50. These split applications adjusted to the plants’ needs increase the FNUE of the radish and decrease the fertilizer N losses during the growing season. However, split applications might be impractical in plastic covered R/F cultivations because mechanical equipment to apply fertilizer under the PE mulch is required. Based on the finding that 15N retention in soil and nitrate concentration in seepage water decreased similarly for ridges and furrows during the entire growing season, we conclude that the PE mulch had no significant effect on 15N retention in soil and on nitrate concentration in seepage water and did therefore not effectively protect the fertilizer in the ridges from percolation. Based on the simulation results, we found that the ridges and furrows contributed approximately an equal amount of leached N to the total amount. We therefore conclude that the PE mulch provided little protection for the fertilizer N in the ridges during heavy rainfall. N leaching amounts were further found to increase linearly with an increase in N addition rate as it is well known for R/F cultivations without PE mulch. The PE mulch did therefore not prevent the linear increase in leaching with an increase in fertilizer N addition. We summarize that without the use of additional measures such as split applications of fertilizer, the application of PE mulch in a summer monsoon climate with heavy rainfall events does not positively influence the N leaching rates.
In the 3rd study, we monitored soil water dynamics in the field and used this data set to simulate the influence of PE mulch on water fluxes with Hydrus 2D. We simulated soil water dynamics in 1) conventional flat tillage (CT); 2) R/F cultivation without PE mulched ridges (RT); and 3) R/F cultivation with PE mulched ridges (RTpm). The comparison of the simulated pressure heads during dry and wet periods revealed that the PE mulch induced significant soil moisture patterns only during the dry periods. During monsoon events, the effect of the PE mulch was dependent on the soil texture and the hydraulic conductivity. Summarizing the advantages and disadvantages of the R/F cultivation with PE mulch on sloped fields, the practice was observed to have the lowest amount of drainage water, the lowest evaporation rates but also the highest surface runoff rates. Hence, PE mulching might be assessed as a tool to reduce percolating water, but it concurrently increases water contribution to the river network by surface runoff.