Browsing by Subject "Carbon"

Now showing 1 - 3 of 3
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    Binder‐free N‐functionalized carbon electrodes for oxygen evolution reaction
    (2023) Song, Feihong; Straten, Jan W.; Lin, Yang‐Ming; Ding, Yuxiao; Schlögl, Robert; Heumann, Saskia; Mechler, Anna K.
    The oxygen evolution reaction (OER) is one of the bottlenecks of electrochemical water splitting. Metal‐free carbons from biomass are highly abundant and can be easily synthesized. Their low price, high conductivity and functionalization makes them promising materials. Herein, we report about free‐standing carbon electrodes as electrocatalysts for the OER. In contrast to powder‐based catalysts, free‐standing electrodes not only avoid additives, but also facilitate post analysis and better reflect industrial conditions. Here, the performance of pure carbon electrodes is compared to those of N‐functionalized ones. Utilizing several analytical techniques, the difference in performance can be rationalized by physical properties. Especially, the analysis of the gaseous products is shown to be of crucial importance. It reveals that N‐doped carbons generate more oxygen and are more robust against carbon corrosion. This illustrates the importance of measuring selectivity especially for carbon electrocatalysts, as higher currents do not necessarily result in higher catalytic activity.
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    Biological regulation of subsoil C-cycling
    (2019) Preußer, Sebastian; Kandeler, Ellen
    Soils are the largest terrestrial reservoir of organic carbon (OC). Substantial proportions of the stored OC are found in stabilized form in deeper soil layers. Beside the quality and quantity of C input from plant biomass, C storage in soil is primarily controlled by the microbial decomposition capacity. Various physical, chemical and biological factors (e.g., substrate availability, temperature, water content, pH, texture) vary within soil profiles and directly or indirectly influence the abundance, composition and activity of microbial communities and thus the microbial C turnover. While soil microbiological research has so far focused mainly on processes in topsoil, the mechanisms of C storage and turnover in subsoil are largely unknown. The objective of the present thesis was therefore to investigate the specific influence of substrate availability and different environmental factors as well as their interactions on microbial communities and their regulatory function in subsoil C-cycling. This objective was addressed in three studies. In the first and second study, one-year field experiments were established in which microbial communities from different soil depths were exposed to altered habitat conditions to identify crucial factors influencing the spatial and temporal development of microbial abundance and substrate utilization within soil profiles. This was achieved by reciprocal translocation of soils between subsoil horizons (first study) and topsoil and subsoil horizons (second study) in combination with addition of 13C-labelled substrates and different sampling dates. In the third study, a flow cascade experiment with soil columns from topsoil and subsoil horizons and soil minerals (goethite) coated with 13C-labelled organic matter (OM) was established. This laboratory experiment investigated the importance of exchange processes of OM with reactive soil minerals for the quality and quantity of dissolved OM and the influence of these soil micro-habitats on microbial abundance and community composition with increasing soil depth. In the first study, the reciprocal translocation of subsoils from different soil depths revealed that due to comparable micro-climatic conditions and soil textures within the subsoil profile, no changes in microbial biomass, community composition and activity occurred. Moreover, increasing microbial substrate utilization in relation to the quantity of added substrate indicated that deep soil layers exhibit high potential for microbial C turnover. However, this potential was constrained by low soil moisture in interplay with the coarse soil texture and the resulting micro-scale fragmentation of the subsoil environment. The bacterial substrate utilization was more affected by this spatial separation between microorganisms and potentially available substrate than that of fungi, which was further confirmed by the translocation experiment with topsoil and subsoil in the second study. While the absolute substrate utilization capacity of bacteria decreased from the more moist topsoil to the drier subsoil, fungi were able to increase their substrate utilization and thus to partially compensate the decrease in C input from other sources. Furthermore, the addition of root litter as a preferential C source of fungal decomposer communities led to a pronounced fungal growth in subsoil. The third study demonstrated the high importance of reactive soil minerals both in topsoil and in subsoil for microbial growth due to extensive exchange processes of OM and the associated high availability of labile C. In particular copiotrophic bacteria such as Betaproteobacteria benefited from the increased C availability under non-limiting water conditions leading to a pronounced increase in bacterial dominance in the microbial communities of these soil micro-habitats. In conclusion, this thesis showed that subsoil exhibits great potential for both bacterial and fungal C turnover, albeit this potential is limited by various factors. This thesis, however, allowed to determine the specific effects of these factors on bacteria and fungi and their function in subsoil C-cycling and thus to identify those factors of critical importance. The micro-climate in subsoil, in particular soil moisture, was the primary factor limiting bacterial growth and activity, whereas fungi were more strongly restricted by substrate limitations.
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    Turbulent exchange of energy, water and carbon between crop canopies and the atmosphere : an evaluation of multi-year, multi-site eddy covariance data
    (2019) Eshonkulov, Ravshan; Streck, Thilo
    The increase of anthropogenic CO2 emissions and other greenhouse gases has raised concern about climate change. Climate change has manifold impacts on yield and yield quality, crop rotations, carbon and nitrogen cycling, water regime and agricultural production systems. To understand its consequences on environmental systems, measuring the matter and energy exchange at the land surface provides data to help validate and inform a wide range of process models. Such flux measurements at the land-surface provide an opportunity to test simulations of processes in the soil-plant-atmosphere continuum. Currently, such measurements are mainly based on the eddy covariance (EC) method, for the quality of which the energy balance closure (EBC) is a problem. The EBC significantly influences the calibration and validity of land-surface models, especially in regard to the energy and water balance at the Earth’s surface. The EBC quantifies the deviation between turbulent fluxes and available energy. It is crucial to obtain high-quality EC measurements to determine the reasons for the EBC. The research aims of this dissertation were: 1) to clarify the role of minor storage and flux terms in the energy balance, 2) to determine the possible reasons for the energy imbalance using a long-term dataset (2010-2017) from agricultural croplands, and 3) to investigate the effects of region, site, year and crop type on carbon fluxes and budgets. In the first study (Chapter 2) the contribution of minor storage terms to the EBC were investigated. I also determined the contribution of ground heat fluxes calculated by different methods. A harmonic analysis method was used to calculate ground heat fluxes from measurements of heat flux plates and soil temperature sensors. Soil heat storage and enthalpy change in the plant canopy were determined at different locations within the EC footprint. Considering minor storage terms improved the energy balance closure on average by 5.0 % in 2015 and by 6.8 % in 2016. The greatest energy balance closure improvement occurred in May of both study years. The dominant fraction of minor energy storage was energy uptake and release through photosynthesis and respiration. Additionally, the energy fluxes related to soil temperature change were also observed. The ground heat flux calculated by harmonic analysis from soil heat flux plates narrowed the EBC by 3 % compared to the calorimetric method. The results indicated that the typical correction approach to achieve energy balance closure, i.e. the Bowen-ratio method, overestimated the turbulent fluxes. The second study (Chapter 3) investigated the effects of crop type, site characteristics, wind directions, atmospheric conditions and footprint on the EBC. The long-term evaluation of EC measurements showed that, with the EC method, 25 % of the available energy could not be detected. Decreasing the flux footprint area increases the chance of a more homogeneous area. Homogeneity plays an important role in achieving a better energy balance closure. The synthesis of long-term EC data indicated that the sonic anemometer is very sensitive to orientation, not allowing accurate measurements from all wind directions. Discarding the measurements from wind directions 0° and 90° at EC4 improved the EBC from 80 to 84 %. In the third study, presented in Chapter 4, a long-term and multi-site experiment was evaluated to clarify the effects of site, year and region on the CO2 fluxes and budgets in agroecosystems. The net ecosystem exchange of CO2 fluxes – measured on six sites during eight years – was comprehensively examined. Winter rapeseed had the lowest CO2 uptake, cropping of silage maize resulted in the highest C losses. The management of harvest residues was the most effective means of controlling the C budgets. Comparing the CO2 fluxes processed with the recently developed ogive optimization method versus the conventional calculation showed that eliminating low-frequency contributions had a considerable effect. On average, the ogive optimization method delivered 6.9 % higher net ecosystem exchange rates than the conventional method. This dissertation provides new insights into how to obtain better measurements of matter and energy fluxes from EC measurements by a) considering storage terms otherwise neglected, b) using harmonic analysis for calculating ground heat fluxes, c) discarding fluxes from behind the anemometer and d) applying the ogive optimization method.