Institut für Bodenkunde und Standortslehre

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Ecosystemic Effect Indicators to assess Effects of agricultural Landuse on Ecosystems
(2000) Merkle, Andrea Hildegard; Kaupenjohann, Martin
Agricultural production and its material and non-material emissions may cause side-effects in ecosystems. These effects have to be assessed and evaluated. The aim of the present study is to provide a tool that relates emissions of agricultural production and affected ecosystems. This tool represents an indicator approach. The needed indicators are defined as ecosystemic effect indicators (EEI). Within the study a multistage procedure is developed which should be pursued in identifying indicators. To assess which emissions must be regarded in detail an estimation of relevance by means of an emission and input classification preceeds the indicator development. Subsequently, EEI are developed for the relevant emissions and inputs. The derivation of EEI is carried out by the following steps: 1) One starts top-down at the target 'maintaining the ecosystem functioning' in this work depicted by the utility functions. A list with characteristics that are dependent on hierarchical levels is compiled for each relevant function. 2) Starting at one specific input a list of potential receptors is compiled bottom-up. These represent potential effect indicators. 3) By overlapping the lists of the steps 1 and 2 one yields EEI specific for the utility function and the input under consideration. The step 3 is performed by means of expert knowledge. The advantage of the indicator approach is its operativeness which is site-independent. The results of the study show that EEI may be deemed to be promising tools to picture human influences in particular of agricultural production on ecosystems. The results of the case study provide the basis to assess effects on ecosystems for some major stressors. In cases where critical values are available site specific quantitative statements concerning ecological effects within the frame of sustainable agriculture are enabled by the present method for the derivation of indicators. Subsequently, necessary measures can be deduced
Models for the representation of ecological systems? The validity of experimental model systems and of dynamical simulation models as to the interaction with ecological systems
(2001) Haag, Daniel; Kaupenjohann, Martin
Models guide the investigation of ecological phenomena and the managemant of man-environment interactions. Based on six papers, this thesis critically examines characteristic features, limitations and the scientific and societal role of experimental model systems (as well-tried instruments of knowledge production) and of dynamical simulation models (as representatives of relatively recent computer models). Experimental model systems are described as materially and conceptually closed systems with a limited number of parameters. They consist of a material component which is encoded into a formal (numerical) system through the measurement of defined parameters. The transfer of statements derived from model systems to natural systems is critically discussed. Dynamical systems - the paradigm for the representation of ecosystems - permit the simultaneous handling of a large number of parameters. Dynamical systems are conceptually closed systems and are based on the notion of an abstract state (focussing on 'being'). I contrast this view with an image of ecosystems as conceptually open systems ('becoming') which emphasizes the evolutionary openness of ecological systems, the internal production of novelty, and the emergence of system level properties. Taking the nitrogen cycle and its human alterations as an example, model concepts and limitations to the derivation of cause-effect-relationships in ecological systems are illustrated. Acknowledging the limited predictive capacity of simulation models and the intrinsic perspectivity of the identification of 'relevant' phenomena and parameters and drawing on new forms of knowledge production (as described by science studies), a modified role for model building and for simulation models - particularly with respect to science for policy - is sketched.
Function and composition of the soil microbial community in calcareous grassland exposed to elevated atmospheric carbon dioxide
(2003) Ebersberger, Diana; Kandeler, Ellen
Terrestrial ecosystems generally respond to rising atmospheric carbon dioxide (CO2) concentrations with increased net primary productivity and increased water use efficiency. This may change the amount and quality of organic substances entering the soil and fuelling microbial metabolism. Soil microorganisms and their activity might also be affected by increased soil moisture at elevated CO2. This thesis was designed to analyse the response of the soil microbial community in a species-rich calcareous grassland in the Swiss Jura Mountains, which had been exposed to ambient and elevated CO2 concentrations (365 and 600 ppm) for six growing seasons. In the first study, laboratory incubation experiments were conducted to explore the relationship between litter quality under elevated carbon dioxide and enzymes involved in carbon cycling. Naturally senescent, mixed litter from the long-term field experiment was incubated with soil material for 10, 30 and 60 days. Soil samples were then obtained close to the litter layer using a microtome cutting device. Litter and soil samples were analysed for invertase and xylanase activity. The lower litter quality produced under elevated CO2, i.e. wider C/N ratio, yielded lower invertase and xylanase activities of litter. Litter addition stimulated activities in adjacent soil. Invertase activities of adjacent soil were not affected by litter quality, while soil xylanase activity was higher in soil compartments adjacent to litter from elevated CO2 plots. The reduced enzyme activities of litter produced under elevated CO2 can slow decomposition, at least during the initial stages. Since the effects of litter quality on enzyme activities in adjacent soil were small, we conclude that CO2-induced belowground C-inputs (e.g. increased root mass) and altered moisture conditions are more important controls of enzyme activities than altered litter quality. In the second study, functional diversity of the soil microbial community was assessed by analysing N-mineralisation and activities of enzymes of the C-, N-, P- and S-cycle of soil samples taken in spring and summer 1999, in the 6th season of CO2 exposure. In spring, N-mineralisation increased significantly by 30% at elevated CO2, while there was no significant difference between treatments in summer. The response of soil enzymes to CO2 enrichment was also more pronounced in spring, when alkaline phosphatase and urease activities were increased most strongly, by 32% and 21%, respectively. In summer, activity differences between CO2 treatments were greatest in the case of urease and protease (+21% and +17% at elevated CO2). The significant stimulation of N-mineralisation and enzyme activities at elevated CO2 was probably caused by higher soil moisture and/or increased root biomass. In the third study, soil microbial community structure of soil samples taken in spring and summer 1999 was analysed by means of PLFA profiles and 16S rDNA fingerprints obtained by PCR-DGGE. PLFA profiles were not affected by elevated CO2. Ordination analysis of DNA fingerprints revealed a significant relation between CO2 enrichment and variation in DNA fingerprints. This variation must be attributed to low intensity bands because dominant bands did not differ between treatments. Diversity of the bacterial community (number of bands in DNA fingerprints and Shannon indices) was not affected. The observed minute, but significant changes in the structure of the soil bacterial community might be caused by changes in the quality of rhizodeposits at elevated CO2. These could either result from altered rhizodeposition of individual plants or from altered species composition of the calcareous grassland.The 4th part of the thesis compiles data on soil microorganisms, soil fauna, soil structure and nitrogen cycle of calcareous grassland after CO2 exposure for six growing seasons. Microbial biomass, soil basal respiration and the metabolic quotient were not altered significantly. PLFA analysis revealed no significant shift in the ratio of fungi to bacteria. Protozoans, bacterivorous and fungivorous nematodes, acarians, collembolans, and root-feeding nematodes were not affected by elevated CO2. Total nematode numbers averaged slightly lower (-16%) and nematode mass was significantly reduced (by 43%) due to fewer large-diameter nematodes classified as omnivorous and predacious. CO2 exposure resulted in a shift towards smaller aggregate sizes; this was caused by higher soil moisture. Reduced aggregate sizes result in reduced pore neck diameters. This can confine the locomotion of large-diameter nematodes and possibly accounts for their decrease. The CO2 enrichment also affected the nitrogen cycle. N stocks in living plants and surface litter increased, but N in soil organic matter and microorganisms remained unaltered. N mineralisation increased considerably, but microbial N did not differ between treatments, indicating that net N immobilization rates were unaltered.
Land Preparation Methods and Soil Quality of a Vertisol Area in the Central Highlands of Ethiopia
(2005) Jijo, Teklu Erkossa; Stahr, Karl
The industrialization of agriculture led to societal concerns for environmental protection and food quality in developed countries. On the other hand, the need for increased agricultural productivity to address the persistent poverty and food insecurity in developing countries is intensified. Thus, improved management systems to meet the double objectives of increased productivity and sustained environmental quality are increasingly required. The assessment of soil quality and productivity are among the means of monitoring the various management systems to achieve the goals. Among the interrelated definitions formulated for soil quality, a committee established by Soil Science Society of America for the same purpose defined it as the capacity of soil to function within natural and managed ecosystem boundaries to sustain plant and animal productivity, maintain or enhance soil, water and air quality and support human health and habitation. The central idea in most of the definitions is the capacity of the soil to function. The capacity of a soil to function depends on its inherent properties derived from its genesis and the dynamic properties resulting from the prevailing management systems. Most of the hitherto soil quality assessments considered agricultural production as the major management goal. As this study was conducted in the highlands of Ethiopia where food security remains a basic challenge, the primary management goal could not be different. Shortage and fragmentation of land driven by population pressure have become issues of concern in the area. With a continually dwindling national land-holding average of only one ha per household, farmers struggle to produce enough to feed their families. Since the possibility of expanding agricultural land is limited, increased production is realistic only from higher productivity per unit land per unit time. Covering about 8 million ha, Vertisols are among the high potential soils, where significant increase in productivity is likely. However, their productivity is constrained by their physical and hydrological properties, manifested by their hardness when dry and their stickiness when wet, impeding land preparation. The traditional management systems led neither to increased productivity nor to enhanced soil quality. Thus, the need for alternative technologies is paramount. Despite a concerted effort during the last two decades to develop improved technologies for the soils, land preparation for agricultural productivity and sustaiability remains a major challenge. In addition to technical difficulties associated with their nature and deep-rooted poverty and illiteracy, lack of farmers? participation is believed to have hampered the development and adoption of robust technologies. The challenge facing the soil management research in Ethiopia is thus double fold: development of technologies that swiftly increase agricultural production and ensure judicious use of the land resources. Farmers are the ultimate decision makers on their plots, at least in Ethiopia, often irrespective of the consequences of their decisions. Simple technologies are required to manipulate their decisions in favour of the desired goals. This requires development of technologies that fit into their aspiration, tradition and socio-cultural values with their participation in the generation and evaluation of the technologies. This study was to identify alternative land preparation methods for increased productivity and economic profitability, while maintaining or enhancing the soil quality of the Vertisols. The hypothesis tested was that the alternative land preparation methods improve soil productivity and maintain or enhance soil quality. Three alternatives, Broad Bed and Furrow (BBF), Green Manure (GM) and Reduced Tillage (RT) with the traditional method, Ridge and Furrow (RF) were compared for 6 years, setting crop yield, economic profitability, and soil erosion and soil quality as performance indicators. This on station experiment was complemented by a participatory assessment at a small watershed scale. The objectives of the latter were identification of local soil functions, definition of soil quality concepts, and identification of soil quality indicators and evaluation of the soils for the major functions. Land preparation methods influence soil functions through their effects on soils qualities. Among the soil physical quality indicators considered, GM increased aggregate stability and reduced surface crust strength due to its increased OM content and microbial activities. While RT led to least penetration resistance, infiltration, water-holding capacity, and moisture content were less sensitive to the treatments. The chemical characteristics and plant nutrients response was not consistent indicating the need of longer time for the effects to show a clear trend. Organic carbon and MBC content of the soil increased due to RT and GM, but the increment was not proportional leading to lower microbial quotient. This indicates SOM build up with a long-term soil quality improvement. The effect on runoff was inconsistent during the first three years (1998-2000), but BBF and RT slightly increased. In 2001 and 2002, BBF drained 67% and 54 %, respectively, of the seasonal rainfall as runoff while RT routed 61% and 53%. There is a non significant tendency of increased soil and nutrient losses from BBF and RT due to the increased runoff. BBF significantly increased the grain yield of lentils by 59% (1.03 t ha-1 to 1.63 t ha-1) compared to the control. Similarly, RT resulted in the highest grain yield of wheat (1.86 t ha-1) and tef (1.34 t ha-1). Economically, BBF is the most profitable option for lentils with 65% increase in total gross margin while RT resulted in 11% and 8% increase in gross margin of wheat and tef, respectively, as compared to the control. The soil quality index was not significantly affected by the land preparation methods. Nevertheless, GM has shown a slight enhancement with the highest SQI, followed by BBF and RT. Thus, the land preparation methods are favoured in order of GM> BBF> RT> RF, for soil quality. The relative enhancement of soil quality by GM was linked mainly to its increased Corg content. The performance indicators (productivity, economic profitability, soil conservation and soil quality) are also affected differently. A matrix ranking of the effects on the indicators showed that none of treatments is superior for all the indicators. The average of the ranks (no weight attached) showed that BBF was the most favourable followed by RT. Therefore, the methods are preferred in the order of BBF> RT>GM=RF considering the overall indicators. The superiority of BBF and RT corresponds to their productivity and economic benefits. For soil quality and erosion control, GM is a favourable option. However, as its economic benefit was low, further improvement is required. In addition, lack of fast growing legumes tolerant to both shortage and excess water, failure of the short rain for planting, cost of chopping and incorporating the cover crops and the possible need of special equipment for incorporating may hinder its wider application and hence need further investigation. The success of the alternatives depends on the farmers? capacity and willingness to invest. As the issues of soil quality and land degradation are more of societal concerns than of the individual farmers, external technical and financial incentives are desirable to enhance their capacity and to initiate their interest. Institutional and policy issues influencing agriculture and natural resource management and uncertainties like variation in weather deserve judicious consideration.
Community Structure and Activity of Nitrate-Reducing Microorganisms in Soils under Global Climate Change
(2006) Deiglmayr, Kathrin; Kandeler, Ellen
Since the beginning of the Industrial Revolution, atmospheric carbon dioxide concentrations have been steadily increasing and, thus, contributed to a warming of the climate and altered biogeochemical cycles. To study the response of soil microorganisms to altered environmental conditions under global climate change, the nitrate-reducing community was regarded as a model community in the present thesis. This functional group, which performs the first step in the denitrification pathway, was selected because it is phylogenetically very diverse. In particular rising levels of atmospheric carbon dioxide as the most important catalyst of temperature rise and the retreat of glaciers in the Alps as one of the most evident consequences of climate change were investigated. The behaviour of nitrate reducers was investigated in a biphasic approach: (i) at the level of its enzyme activity of the nitrate reductase and (ii) at the level of community structure, which was characterised by RFLP (Restriction Fragment Length Polymorphism)-fingerprints using the functional gene narG. The effect of elevated atmospheric carbon dioxide concentrations on nitrate-reducing micro-organisms was studied in the Swiss FACE (Free Air Carbon dioxide Enrichment) experiment including the rhizosphere of two functional plant types (Lolium perenne and Trifolium repens), two N fertilisation levels and two sampling dates (June and October 2002). Whereas in June no significant treatment effect was observed, the nitrate reductase activity proved to be significantly reduced under elevated atmospheric carbon dioxide at the autumn sampling date. Simultaneously, elevated enzyme activities were recorded under Trifolium repens and high N fertilisation pointing to a control of nitrate reductase activity by nitrate availability at the time of sampling. The community structure of nitrate reducers, however, showed a different response pattern with sampling date and the strongly varying pH of the different experimental plots constituting the main driving factors. With respect to the three experimental factors atmospheric carbon dioxide, plant type and N fertilisation the composition of the nitrate reducers revealed a high stability. The microbial succession of nitrate-reducing microorganisms was studied in the rhizosphere of Poa alpina across the glacier foreland of the Rotmoosferner/Oetz valley. Sampling was performed in August and at the end of the short period of vegetation in September. The nitrate reductase activity increased significantly with progressing successional age, whereas organic carbon together with nitrate concentrations in the soils explained the major part of this effect. The microbial community of nitrate reducers revealed a significant shift across the glacier foreland, with pH and organic carbon representing the most important environmental factors inducing this shift. A detailed analysis of the clone libraries that were constructed for the youngest and the oldest site in the glacier foreland pointed to the tendency of lower diversity in the late succession compared to the young succession. Possibly an increasing selective pressure due to higher densities of microorganisms and, hence, a higher competition for limited resources contributed to the decline in diversity. In conclusion, the functional group of nitrate reducers responded to changing environmental conditions under global climate change particularly through altered enzyme activities. The amount and the direction of this response depended strongly on the nitrate availability and the organic carbon content in soils. The community structure of nitrate-reducing microorganisms, however, proved to be resilient towards short-term substrate fluctuations. This indicates that the genetic pool of this group of soil microorganisms possesses a high functional stability characterized by a relatively persistent composition and an independent modulation of enzyme activity.
Die Rolle des Porenraums im Kohlenstoffhaushalt anthropogen beeinflusster Niedermoore des Donaurieds
(2007) Höll, Bettina; Stahr, Karl
The use of peatlands in Central Europe for hundreds of years has led to their degradation (loss of organic matter) due to intensive mineralisation. Re-wetting of formerly drained peat aereas has been a popular method of retaining existing peatlands. The effect of re-wetting of degraded fens on their C-pools and C-fluxes is unknown. The protection of these natural resources combined with the creation of biological C-sinks might render the protection and conservation of peatland ecosystems more attractive. Water-logging leads to the accumulation of water in previously air-filled soil pores, something that might increase the C-pool of the soil. It is unknown whether the pore space, which possibly accounts for up to 90% of peatlands, contains carbon components that are similar to those found in the solid soil substance. It is also unknown how much the utilisation of peatlands affects the composition of C-components of the pore space. The major objectives of the present study were (1) to assess the temporal and spatial variability of the C-components in the pore space in fens undergoing different anthropogenic use (drainage, re-wetting) and (2) to assess the role of the pore space in the C-budget. In a Southern German area known as the Schwäbisches Donaumoos, carbon components of the gaseous phase (CO2, CH4) and the liquid phase (CO2/DIC, CH4, DOC, POC) were collected at different depths (5, 10, 20, 40, 60, 80 cm) from different drained (deep, moderately) fen sites and from a long-term re-wetted fen site. Sampling was done at weekly intervals between April 2004 and April 2006. The samples of the water phase and gas phase were collected at the respective sites using slotted PVC tubes and soil-air probes. Gas was analysed using a gas chromatograph and dissolved organic carbon was analysed using a TC water analyser. The fen sites were characterised by selected static parameters of the solid substance and dynamic parameters such as redox potentials, temperature, water level, soil-moisture tension and pH value. The specific use of the fens, which is closely related to the water budget of the area, was a decisive determinant of the amounts of carbon in pore space. Although the solid soil substance in fen sites accounted for less of 10% of the total substance (solid + pores), it still contained a higher amount of carbon (60 -152 kg C m-3) than the pore space. Furthermore the amount of time that the carbon remains is eventually longer in the solid soil substance than in the pore water. Assuming the pore water works only as a short time reservoir. Filling of the pore space with either air or water had a decisive effect on the amount of C. The investigations showed that the amount of C in the air-filled pore space contained an annual average of 15 g C m-3 (deep-drained area), whereas the water-filled pore space contained on average 263 g C m-3 (re-wetted area). The variable anthropogenic effects on fens led to area-specific situations (e.g. groundwater level) that not only affected the amount of C but also had a significant effect on the composition of C components. Dissolved inorganic carbon (DIC), with an average proportion of 55-72%, accounted for the largest proportion of dissolved carbon. Particulate organic carbon (POC) had similar concentrations to dissolved organic carbon (DOC), whereas dissolved methane (CH4) only accounted for a minor proportion (< 0.1%) of the entire carbon of the liquid phase. The DIC concentration was highest in the water from the pores of re-wetted fen. Independent from the use of the fens, different DIC isotope signatures of the ground, karst and spring waters (-11.7‰ to -14.3‰) in comparison to the pore waters (-16.7‰ to -18.4‰) were observed. The further differentiation into the 13C ratios of CO2 contained in the gaseous phase (-23.0‰ to -26.6‰) suggests that DIC ‘accumulated’ in the pore water by way of biogenic CO2. DOC concentrations were lowest in the re-wetted fen. The temporal variability of DOC was related to changes in the bioavailability of DOC. This was also observed in the moderately drained area. The low degree of aromatisation (= higher bioavailability) associated with higher DOC concentrations led to significantly lower values in the re-wetted area compared to the moderately drained area. The microbially easily available DOC proportion was not only temporally but also spatially limited and had a significant effect on the CO2 and CH4 concentrations. At similar depths, CO2 values 10- to 1000-fold higher than CH4 levels could be measured in the gaseous phase (2.7-67 mg CO2-C l-1 vs. < 5.3 mg CH4-C l-1). The highest concentrations were measured in the re-wetted fen. The CO2-C/CH4-C ratios rarely achieved ratios of below 100:1. Due to the higher concentrations of CO2, it can be assumed that the carbon dioxide could compensate for the effect of methane on the climate, on the condition that comparable CO2-C/CH4-C ratios are found in the emissions. The protection of fens as natural resources could be related to carbon uptake (results of the gas exchange to the atmosphere) and higher carbon amounts in the pore space. The amount of time that the carbon remains in the pore waters is correlated to carbon turnover and hydrological conditions. The latter are also important when assessing the indirect emissions, playing an important role in drained fens and rounding out carbon balances.
Towards regionalisation of soils in Northern Thailand and consequences for mapping approaches and upscaling procedures
(2008) Schuler, Ulrich
The purpose of this study was to identify the key factors of soil formation and distribution in North-western Thailand in order to enable predictive mapping. Further objectives were to investigate different soil mapping approaches, regarding their suitability for specific areas and scales and to assess the potential of local (soil) knowledge for soil mapping and land use planning. In order to cover the essential petrographic inventory of North-western Thailand three different petrographic areas were selected. The investigated topics were petrographic diversity, spatial organisation of soils and variability of their physical and chemical properties, as well as local soil knowledge of farmers in the respective areas. In addition thematic maps were generated comprising geology, WRB soil groups, and different soil properties. Soil mapping was carried out by using four different methods, the intensive mapping approach, maximum likelihood method, randomised grid cell approach, as well as elicitation of local knowledge. The intensive mapping approach comprised transect investigations, investigation points of the randomised grid cell approach, and additional investigation points, which were selected for areas with increased soil and petrographic variability. All maps based on the intensive mapping approach contained the highest sampling point density and maximum amount of information. These maps were used as a reference for the other mapping approaches applied here.
Substrate availability affects abundance and function of soil microorganisms in the detritusphere
(2008) Poll, Christian; Kandeler, Ellen
Plant litter is the major source of soil organic carbon (SOC). Its decomposition plays a pivotal role in nutrient recycling and influences ecosystem functioning and structure. Soil microorganisms are the main protagonists of litter decomposition. Among other factors, their activity is controlled by the physicochemical conditions of the soil. This interaction is strongly influenced by the soil structure, resulting in a heterogeneous distribution of microorganisms, substrates and physicochemical conditions at the small-scale. Due to this heterogeneity, microhabitats differ in their decomposition rate of organic C. Considering microhabitat diversity is therefore important for understanding C turnover. In the detritusphere, plant litter closely interacts with the soil by releasing soluble C into the adjacent soil and providing new sites for microorganisms. The abundant readily available substrates characterise the detritusphere as a hot spot of microbial activity and C turnover. Despite the important role of this microhabitat, the interaction of physicochemical conditions with soil microorganisms remains unclear. This thesis was designed to clarify the effect of litter C transport on the spatial and temporal availability of substrates and therefore on microbial abundance and activity in the detritusphere. This goal was addressed in three studies. The first study focused on the influence of solute transport conditions on microbial activity and substrate utilisation by the microbial community. In two 2-week microcosm experiments, diffusion and convection were considered as transport mechanisms; both mechanisms were studied at two different water contents. The second study aimed to identify temporal patterns of diffusive solute transport and microbial activity at two water contents during an 84-day incubation. Both studies emphasised the important role of fungi in the detritusphere. The third study therefore identified fungi that benefit from freshly added litter. The three studies combined classical soil biological methods and modern techniques. Analysis of microbial biomass, ergosterol content, CO2 production, and enzyme activities provided general information on the mineralisation of litter C as well as on microbial activity and abundance. A convective-diffusive solute transport model with a first-order decay was used to interpret enzyme activity profiles. This allowed the underlying factors determining the spatial dimension of the detritusphere to be identified. By adding plant residues with a different 13C signature than the SOC, it was possible to quantify the transport of litter C into different C pools. The incorporation litter C into different microbial groups, for example, was traced by coupling of phospholipid fatty acid (PLFA) extraction with 13C analysis. Fungal species were identified by constructing clone libraries based on 18S rDNA and subsequent sequencing. The results of the first study indicated that the transport rate of soluble substrates determines the spatial dimension of the detritusphere, with an enlarged detritusphere after convective versus diffusive transport. The isotopic ratios of bacterial and fungal PLFAs differed under both transport mechanisms, indicating different substrate utilisation strategies: bacteria relied on the small-scale transport of substrates, whereas fungi assimilated new C directly in the litter layer. Water content affected only diffusive C transport and modified the temporal pattern of microbial activity by enhancing transport at higher soil water content. The expected chronological order of C transport, microbial growth and enzyme release was verified in the second and third study. During the first two weeks, mainly easily available and soluble litter compounds were mineralised and transported into the adjacent soil. After this initial phase, depolymerisation of complex litter compounds started. During the initial phase, enhanced C transport induced greater microbial biomass and activity, and increased fungal diversity. During the later phase, however, substrate availability and microbial activity were reduced. Measurements of microbial biomass C and ergosterol indicated that the initial phase was dominated by bacterial r strategists, whereas fungal K strategists dominated the later phase. Sequencing of fungal 18S rDNA detected a shift in the fungal community during the initial phase, pointing to growth of pioneer colonisers, especially Mortierellaceae. These fungi do not produce ergosterol and therefore were not detected by the ergosterol measurements. Accordingly, the r strategists consist of both bacteria and fungi. During the later phase, the fungal community was dominated by the cellulose-degrading fungus Trichocladium asperum. Based on these results, the original concept was modified and a two-phase conceptual model of litter C turnover and microbial response in the detritusphere was developed. In conclusion, this thesis yields new insight into litter decomposition at the small-scale. Combining classical methods with modern techniques enabled the development of a conceptual model of litter C turnover and microbial response in the detritusphere. This provides a useful basis for future studies addressing, for example, the impact of global change on the interaction of decomposition and soil microorganisms.
Rhizodeposition and biotic interactions in the rhizosphere of Phaseolus vulgaris L. and Hordeum vulgare L.
(2008) Haase, Susan; Kandeler, Ellen
Biochemical processes at the soil-plant interface are largely regulated by organic and inorganic compounds released by roots and microorganisms. Several abiotic and biotic factors are suspected to stimulate rhizodeposition and, thus, contribute to enriching of the rhizosphere with plant-derived compounds. This thesis focused on the effects of two factors, (i) the elevation of atmospheric CO2 concentration accompanied by nutrient limitation in the soil and (ii) low-level root infestation by plant-parasitic nematodes, on the quantity and quality of rhizodeposits with consequences for plant-nutrient acquisition and plant-microbial interactions in the rhizosphere. Experiments were largely conducted in mini-rhizotrones, which allowed a localized collection of rhizodeposits and rhizosphere soil along single roots. Since the beginning of the industrial revolution atmospheric CO2 concentrations have been steadily increasing. This probably impacts terrestrial ecosystems by stimulating plant photosynthesis and belowground allocation of the additional fixed C. Increased root exudation, promoting rhizosphere microbes, has been hypothesized as a possible explanation for the lower plant N nutritional status under elevated CO2, due to enhanced plant-microbial N competition. Legumes may counterbalance the enhanced N requirement by increased symbiotic N2 fixation. The effects of elevated CO2 on factors determining this symbiotic interaction were assessed in Phaseolus vulgaris L. grown under limited or sufficient N supply and ambient or elevated CO2 concentration. Elevated CO2 reduced N tissue concentrations but did not affect plant biomass production. 14CO2 pulse-labelling revealed no indication for a general increase in root exudation by the whole root system, which might have forced N-competition in the rhizosphere under elevated CO2. However, a CO2-induced stimulation in the exudation of sugars and malate, a chemoattractant for rhizobia, was detected in apical root zones, as potential infection sites. In nodules, elevated CO2 increased the accumulation of malate as a major C source for the microsymbiont and of malonate, with functions in nodule development. Nodule biomass was also enhanced. Moreover, the release of nod-gene-inducing flavonoids was stimulated under elevated CO2, suggesting a selective stimulation of factors involved in establishing the Rhizobium symbiosis. Since elevated-CO2-mediated effects on exudation by Phaseolus vulgaris L. are restricted to root apices, the abundance and function of the soil microbial community were investigated at two levels of spatial resolution to assess the response of microorganisms in the rhizosphere of the whole root system and in apical root zones to elevated CO2 and different N supply. At the coarser resolution, the microbial community did not respond to CO2 elevation because the C flux from the whole root system into soil did not change. At the higher spatial resolution, the CO2-mediated enhanced root exudation from root apices led to higher enzyme activities of the C and N cycle in the adhering soil at an early stage of plant growth. At later stages, however, enzyme activities decreased under elevated CO2. This might reflect a shift in microbial C usage from the decay of polymers towards soluble carbohydrates derived from increased root exudation. CO2 elevation or N supply did not affect the abundance of total and denitrifying bacteria in rhizosphere soil of apical root zones. Thus, the microbial community in the rhizosphere of bean plants responded to elevated CO2 by altered enzyme regulation and not by enhanced growth. Beyond N, plants and microorganisms may also compete for micronutrients such as Fe in the rhizosphere. Hordeum vulgare L., a model plant with high secretion of phytosiderophores (PS) under Fe limitation, was investigated to assess the effects of elevated CO2 on PS release, Fe acquisition and potential impacts on rhizosphere microbial communities. Experiments were conducted in hydroponics and soil culture with or without Fe-fertilization and ambient or elevated CO2 concentration. Elevated CO2 stimulated biomass production of Fe-sufficient and Fe-deficient plants in both culture systems. Secretion of PS in apical root zones of N deficient plants increased strongly under elevated CO2 in hydroponics, but no PS were detectable in root exudates from soil-grown plants. However, higher Fe shoot-contents of plants grown in soil culture without Fe supply suggest an increased efficiency for Fe acquisition under elevated CO2. Despite the evidence for altered PS secretion under elevated CO2, no significant influence on rhizosphere-bacterial communities was detected. Low-level herbivory by parasitic nematodes is thought to induce leakage of plant metabolites from damaged roots, which can foster microorganisms. Other factors such as alterations in root exudation or morphology in undamaged roots, caused by nematode-host interactions were almost not considered yet. Hordeum vulgare L. was inoculated with 0, 2000, 4000 or 8000 root-knot nematodes (Meloidogyne incognita) for 4 weeks. In treatments with 4000 nematodes, shoot biomass, total N and P content increased by the end of the experiment. One week after inoculation, greater release of sugars, carboxylates and amino acids from apical root zones indicates leakage from this main nematode penetration site. Low levels of root herbivory stimulated root hair elongation in both infected and uninfected roots. This probably contributed to the increased sugar exudation in uninfected roots in all nematode treatments at three weeks after inoculation. Root-knots formed a separate microhabitat within the root system. They were characterised by decreased rhizodeposition and an increased fungal to bacterial ratio in the surrounding soil. This study provides evidence that, beside leakage, low-level root herbivory induces local and systemic effects on root morphology and exudation, which in turn may affect plant performance and competition. In conclusion, this thesis extends our knowledge about the potential impact of two different plant-growth-affecting factors on rhizosphere processes, particularly at the small scale and is, thus, interesting for future assessment of management strategies in agriculture under global climate change.
Soil (chrono-) sequences on marine terraces : pedogenesis in two coastal areas of Basilicata and Agrigent, Southern Italy
(2009) Wagner, Stephen; Stahr, Karl
This work contributes to the understanding of the Quaternary history in Mediterranean landscapes in Southern Italy. The time frame of soil formation processes was therefore investigated on two sequences of marine terraces. A central question of this study was whether soil formation reflects progressive terrace ages. Chronofunctions were used to apply relative indices of soil development in different pedostratigraphic levels. The newly developed indices generally show incessant weathering on continuously older terraces and therefore true soil chronosequences. A few terraces may however once have formed a single terrace body which was later separated and uplifted by tectonic activity.
Ein Vergleich zwischen Barometrischer Prozessseparation (BaPS) und 15N-Verdünnungsmethode zur Bestimmung der Bruttonitrifikationsrate im Boden
(2010) Schwarz, Ulrich; Streck, Thilo
Besides the carbon cycle, the nitrogen cycle plays a central role in soil. A key process of this cycle is nitrification. In practice, nitrification is measured as gross or net nitrification. Net nitrification rates are measured by determining the net change in the nitrate or ammonium pool over a period of time. Net rates are difficult to interpret, because the net nitrification rate is the sum of nitrate producing and consuming processes. In contrast, gross nitrification quantifies the total production of nitrate via nitrification. There are two methods for measuring gross nitrification: the 15N-Pool dilution technique and Barometric Process Separation (BaPS). In the 15N-Pool dilution technique, nitrate en-riched with the heavier isotope 15N is added to soil, and the dilution of the 15N pool and the change in the nitrate pool are measured over time. The BaPS method measures changes in pressure and the oxygen- and carbon dioxide concentration of the atmosphere in a closed chamber. The gross nitrification rate can then be computed by a step-by-step solution of the gas balance equations. In the present study, 15N enriched nitrate was added to soil and then put into the BaPS-incubation chamber. By this procedure gross nitrification rates were measured simultaneously with both the 15N-Pool dilution technique and the BaPS method. The aim of the present study was to find out under which conditions the two methods yield similar results and under which conditions different results. In the latter case, the thesis aimed at elucidating the cause for the disagreement between both methods. For this purpose extensive research on two agricultural soils from North China and three soils from Southwest Germany was undertaken. The two methods were compared under the following conditions: 1) application of ammonium fertilizer, 2) addition of nitrification inhibitors, 3) varying soil wa-ter contents, and 4) different soil temperatures. Moreover, a new methodological approach was tested: the 13CO2-Pool dilution technique. Combining this method with the 15N-Pool dilu-tion technique and the Barometric Process Separation made it possible to exactly determine the pH and respiration coefficient in situ. Both techniques corresponded well in soil with pH<6. In soil with higher pH, both methods led to very different results. The reason is that pH has a strong impact on the calculation of the nitrification rate in the BaPS method. In nearly all experiments with neutral to alkaline soils, the BaPS technique yielded higher nitrification rates than the 15N-Pool dilution technique if pH was determined in 0.01 M CaCl2. With pH determined in water, there was good agreement or nitrification rates were too low. Fertilization with ammonium did not in-duce an increase of nitrification in a sandy soil with pH<6. A decrease in nitrification to less than 60% was achieved by the application of the nitrification inhibitor DCD. For both techniques a positive correlation between temperature and nitrification rates was found. There was no correlation between water filled pore space and nitrification rate.
Effect of reduced nitrogen deposition on microbial activity, abundance and diversity in forest soils
(2012) Enowashu, Esther Eneckeh; Kandeler, Ellen
The deposition of nitrogen has increased many-fold due to anthropogenic activities. Since forest ecosystems are often limited by N availability, elevated N inputs from the atmosphere can have a fertilization effect but in the long-term, excess N can influence above- and below-ground production. One of the consequences of N deposition and increased N inputs is a shift in microbial community structure and function as ecosystems move towards N saturation. Soil microorganisms through the action of enzymes play an important role in N dynamics. Thus, the availability and turnover of N depends strongly on microbial abundance, diversity and activity which are in turn influenced by soil properties. Studies on the effects of high nitrogen inputs and the response of forest ecosystems to nitrogen saturation are many and well understood. However, the reversibility of N-induced shifts in forest ecosystem processes is largely unknown. This thesis was therefore designed to study the response of soil microorganisms to reduced N deposition. A biphasic approach was employed to look into (i) the general microbial functional status of the Solling forest site as well as (ii) the microbial community structure which may be a key regulator of two important processes of N transformation: denitrification and proteolysis. The goal of the present thesis was addressed in three studies. Denitrification is considered sensitive to environmental changes and the response of nitrate-reducers and denitrifiers to reduced N deposition was determined in the first study. The goal of the second study was to investigate the overall microbial activity of the Solling forest profiles especially focussing on enzymes involved in the N cycle. This revealed a pronounced activity of peptidases whereby a set of novel pepN primers encoding alanine aminopeptidase enzyme was designed in the third study to determine the group of bacteria involved in proteolysis in forest as well as agricultural and grassland soils. The Solling experimental station was established more than two decades ago and it gave the opportunity to study the N cycle in a natural forest ecosystem at different sampling dates and depths. A combination of classical biological methods and modern molecular techniques were used in the studies. Soil physico-chemical parameters (OC, Nt, NO3-, NH4+, pH, % Water content) were analysed to gain more information on mineralization and immobilization of N in the soil profiles. The analysis of microbial biomass, ergosterol content and the activity of several enzymes of the N, C and P cycles as well as enzyme activity of nitrate reducers was determined in order to interpret microbial functions. The abundance of nitrate reducers and denitrifiers were determined by quantitative PCR of 16S rRNA, nitrate reductase (narG and napA) and denitrification (nirK, nirS and nosZ) genes. The diversity of peptide degrading bacteria was analysed by PCR, cloning and sequencing and the construction of pepN gene libraries. The results of the first study indicated that time and space were the main drivers influencing the abundance and activity of the nitrate reducers and denitrifier communities in the forest soil profiles. Reduced N deposition had a of minimal effect. Interestingly, the ratios of nosZ to16S rRNA gene and nosZ to nirK increased with soil depth thereby tempting to conclude that the size of denitriﬁers capable of reducing N2O into N2 might be bigger in the mineral horizons. In the second study, a stronger response of N cycling enzymes to reduced N deposition could be seen. However, these responses especially that of specific peptidases differed in magnitude which could be indicative of a modification of the reaction rates of the different N cycling enzymes. Correlation of nutrients (N, C, P) with microbial biomass and enzyme activities in the soil profiles revealed that substrate availability was the main factor influencing microbial activity. In the third study, analyses of gene libraries from extracted DNA from forest, agricultural and glacier soil samples revealed a high diversity of pepN sequences related to mainly α-Proteobacteria. A majority of the sequences showed similarity to published data revealing that the amplified region of pepN might be conserved. Linking diversity and enzymatic data, lowest diversity was observed in the agricultural soil where activity levels of alanine aminopeptidase were lowest indicating the importance of diversity studies for ecosystem functioning. In conclusion, this thesis offers valuable contributions to understanding the impact of N deposition. The approach used was suitable to assess the response of the different microbial communities to reduced N deposition. The magnitude of the response depended strongly on space, time and substrate availability in soils as well as their interactions.
Assessment of hydrology and dynamics of pesticides in a tropical headwater catchment in Northern Thailand
(2013) Hugenschmidt, Cindy; Streck, Thilo
The dissertation deals with assessment of hydrology and the dynamics of pesticides in a tropical headwater catchment in northern Thailand. Rainfall and runoff characteristics are recorded and investigated, pesticide dynamics during single events are monitored and studied. Finally, a hydrological model is applied.
TUSEC - Bilingual-Edition : eine Methode zur Bewertung natürlicher und anthropogener Böden (Deutsche Fassung)
(2013) Stahr, Karl; Lehmann, Andreas; David, Susanne
The ‘TUSEC-book’ is a manual for soil evaluation in the temperate zone, comprising English and German versions in one volume. New and innovative are the explicit consideration of anthropogenic soils with the TUSEC evaluation and the differentiation of the evaluation system into two evaluation methods for different levels of detail. Both the part describing the detailed method and the part showing the overview method are again divided into chapters with step-by-step descriptions of methods concerning the evaluation of the widely known soil functions. Every one of these chapters is introduced with principal explanations on the respective soil functions and descriptions of the specificities of the method are provided. Thereby, details on the methodological extensions, which are necessary for the evaluation of anthropogenic soils, are highlighted. Whereas the chapters concerning the detailed evaluation method reflect well-tested procedures, only a beta version is available for the overview method.
Transport of pesticides in a river of a tropical mountainous watershed in northern Thailand
(2013) Sangchan, Walaya; Streck, Thilo
In the northern region of Thailand, in the upland areas population growth and migration of people from the lowlands have rapidly driven land use changes. The expansion of cultivation to increasingly vulnerable areas such as the slopes of mountainous watersheds has led to increasingly adverse impacts on the environment. In particular, intensive application of pesticides poses a contamination risk for stream water and the aquatic ecosystem. This thesis identified the transport patterns of pesticides with different physico-chemical properties during single runoff events under farmer?s practice conditions on the catchment scale. Moreover, the exposure concentrations of frequently used pesticides in surface water and sediment in the watershed were measured in the frame of long-term monitoring. The data were used to calculate pesticide loads in the Mae Sa watershed (Chiang Mai, Thailand) and to assess the ecological risk of pesticides for the aquatic ecosystems. Prior to start of the monitoring program, methods to extract and analyze pesticides in the surface water and sediment samples were established. The pesticides in water samples were extracted by solid phase extraction with a graphitized carbon black sorbent. The recoveries of pesticides in a simultaneous analysis ranged from 58 % to 117 % for the seven pesticides (dichlorvos, atrazine, dimethoate, chlorothalonil, chlorpyrifos, (α, β) endosulfan, cypermethrin) with a high repeatability of the method (Relative Standard Deviation, (RSD)<20 %), except for chlorothalonil (RSD=27 %). For analysis of sediments, the QuEChERS method was adapted. Extraction conditions such as solvent, partitioning of pesticide due to salt effect and clean up step with dispersive solid phase extraction were optimized. Except for dichlorvos in the bed sediment sample and for dimethoate in bed and suspended sediments, recoveries were between 81 % and 116 %. The results show that the QuEChERS method is a valuable method for extracting pesticides from sediment samples. To identify the transport pathways contributing to pesticide losses from soil to the Mae Sa River, automatic gauging stations were installed at the headwater (HW) and outlet (OL) of the watershed to measure discharge and to collect water samples for pesticide analysis. During three runoff events in May, August and September 2008, water samples were collected in a high temporal resolution (1 hour). The potential transport pathways of pesticides were elucidated by time series analysis. Three different input patterns of pesticides were observed: (a) pesticide peaks during the rainfall events as discharge increased, (b) sporadic high concentrations of pesticides during the falling limb of the runoff peak, and (c) low concentrations but more or less continuous values on a baseline level. A chromatographic effect was observed for many pesticides, for example between dimethoate and chlorpyrifos. Highly mobile pesticides such as atrazine and dimethoate were likely to suffer loss at the beginning of the runoff event, while strongly sorbing pesticides such as chlorpyrifos were slightly delayed. This indicates an interaction with the soil matrix, during transport along a sub-surface pathway. The results obtained in the middle of the rainy season in August and September events showed that antecedent rainfall plays an important role in triggering pesticide transport by preferential interflow. In both events the sporadic appearances of strongly sorbing pesticides such as chlorothalonil and chlorpyrifos after peak flow suggest this transport type. For ecotoxicological risk assessment, the highly dynamic nature of pesticide input to surface waters must be considered in the design of representative monitoring schemes. Not only the periods during rain event and peak runoff, but also the following recession phase, during which short and pulsed concentration peaks might show up, must be captured by a representative sampling scheme. Therefore, a high temporal resolution is advisable. To study the long-term dynamics of seven selected pesticides in the Mae Sa River and to evaluate their environmental impacts to aquatic organisms, the exposure concentrations of the pesticides in water and sediment samples were monitored at three stations (HW, Mae Sa Noi flume (MSN), and OL) in the watershed over a period of one and half year (from July 2007 to November 2008). Aquatic risk assessment concerning the observed pesticide concentrations was performed by using the risk characterization ratio (RCR). Chlorpyrifos was the most frequently detected pesticide in surface water at the HW and OL. Cypermethrin was the most frequently detected pesticide in bed and suspended sediment samples along the Mae Sa Noi tributary and at the HW. Regarding the change of pesticide use in the area (compared with data recored in 2002), the measurements suggest that the use of endosulfan has been reduced in recent years, while the observed concentrations of chlorothalonil and chlorpyrifos were in the same concentration ranges as in 2002. The temporal distribution of pesticides shows that the concentrations are highest during the rainy season. Outstandingly high losses of dichlorvos and atrazine were found at Mae Sa Noi flume. Loads of chlorothalonil and chlorpyrifos in stream water were extremely high in the headwater area. Based on interview data of pesticide use in the Mae Sa watershed, in both years the losses of single pesticides to surface water ranged from 0.004 % (chlorothalonil) to 4.7 % (dimethoate) of the applied pesticide mass. The loss of atrazine could not be included because the data did not contain information on the application rate of atrazine. The risk assessment shows that particularly dichlorvos and endosulfan have a high potential to cause adverse effects to the aquatic ecosystem. The RCRs of endosulfan and cypermethrin show that they are the main stressors in the sediment phase. This reveals that aquatic ecosystem of the Mae Sa watershed is facing adverse effects by the contamination of surface water and sediment with pesticides. Hence, measures are urgently needed to reduce the loss of pesticides from soil to surface waters.
Effects of resource availability and quality on soil microorganisms and their carbon assimilation
(2014) Kramer, Susanne; Kandeler, Ellen
Soil microorganisms play a pivotal role in decomposition processes and therefore influence nutrient cycling and ecosystem function. Availability and quality of resources determines activity, growth and identity of substrate users. In agricultural systems, availability of resources is dependent on, for example, crop type, management, season, and depth. At depth substrate availability and microbial biomass decrease. However, there remain gaps in our understanding of C turnover in subsoil and how processes in the topsoil may influence abundance, activity, and function of microorganisms in deeper soil layers. With respect to substrate quality it is thought that bacteria are the dominant users of high quality substrates and more labile components whereas fungi are more important for the degradation of low quality and more recalcitrant substrates (i.e. cellulose, lignin). Therefore, this thesis was designed to increase our understanding of C turnover and the influence of both availability and quality of substrates on microorganisms in an agricultural soil. In the first and second studies, a recently established C3-C4 plant exchange field experiment was used to investigate the C flow from belowground (root) and aboveground (shoot litter) resources into the belowground food web. Maize plants were cultivated to introduce a C4 signal into the soil both by plant growth (belowground / root channel) and also by applying shoot litter (aboveground litter channel). To separate C flow from the shoot litter versus the root channel, maize litter was applied on wheat cultivated plots, while on half of the maize planted plots no maize litter was returned. Wheat cultivated plots without additional maize litter application served as a reference for the calculation of incorporated maize-C into different soil pools. Soil samplings took place in two consecutive years in summer, autumn and winter. Three depths were considered (0-10 cm: topsoil, 40-50 cm: rooted zone beneath the plough layer, 60-70 cm: unrooted zone). In the third study a microcosm experiment with substrates of different recalcitrance and complexity was carried out to identify primary decomposers of different plant litter materials (leaves and roots) during early stages of decomposition (duration of 32 days) and to follow the C flow into the next higher trophic level (protozoa).
Influence of land use on abundance, function and spatial distribution of N-cycling microorganisms in grassland soils
(2015) Keil, Daniel; Kandeler, Ellen
This thesis focuses on the influence of land use on the abundance, function and spatial distribution of N-cycling microorganisms in grassland soils, but also on soil biogeochemical properties, as well as on enzyme activities involved in the carbon-, nitrogen-, and phosphorous cycle. The objective of this thesis was tackled in three studies. All study sites that were investigated as part of this thesis were preselected and assigned according to study region and land use within the framework of the “Exploratories for Functional Biodiversity Research – The Biodiversity Exploratories” of the Deutsche Forschungsgemeinschaft priority program 1374. The first study addressed the question whether land-use intensity influences soil biogeochemical properties, as well as the abundance and spatial distributions of ammonia-oxidizing and denitrifying microorganisms in grasslands of the Schwäbische Alb. To this end, a geostatistical approach on replicated grassland sites (10 m × 10 m), belonging to either unfertilized pastures (n = 3) or fertilized mown meadows (n = 3), representing low and high land-use intensity, was applied. Results of this study revealed that land-use intensity changed spatial patterns of both soil biogeochemical properties and N-cycling microorganisms at the plot scale. For soil biogeochemical properties, spatial heterogeneity decreased with higher land-use intensity, but increased for ammonia oxidizers and nirS-type denitrifiers. This suggests that other factors, both biotic and abiotic than those measured, are driving the spatial distribution of these microorganisms at the plot scale. Furterhmore, the geostatistical analysis indicated spatial coexistence for ammonia oxidizers (amoA ammonia-oxidizing archaea and amoA ammonia-oxidizing bacteria) and nitrate reducers (napA and narG), but niche partitioning between nirK- and nirS-type denitrifiers. The second study aimed at whether land-use intensity contributes to spatial variation in microbial abundance and function in grassland ecosystems of the Schwäbische Alb assigned to either low (unfertilized pastures, n = 3), intermediate (fertilized mown pastures, n = 3), or high (fertilized mown meadows, n = 3) land-use intensity. Plot-scale (10 m × 10 m) spatial heterogeneity and autocorrelation of soil biogeochemical properties, microbial biomass and enzymes involved in C, N, and P cycle were investigated using a geostatistical approach. Geostatistics revealed spatial autocorrelations (p-Range) of chemical soil properties within the maximum sampling distance of the investigated plots, while greater variations of p-Ranges of soil microbiological properties indicated spatial heterogeneity at multiple scales. An expected decrease in small-scale spatial heterogeneity in high land-use intensity could not be confirmed for microbiological soil properties. Finding smaller spatial autocorrelations for most of the investigated properties indicated increased habitat heterogeneity at smaller scales under high land-use intensity. In the third study, the effects of warming and drought on the abundance of denitrifier marker genes, the potential denitrification activity and the N2O emission potential from grassland ecosystems located in the Schwäbische Alb, the Hainich, and the Schorfheide region were investigated. Land use was defined individually for each grassland site by a land-use index that integrated mowing, grazing and fertilization at the sites over the last three years before sampling of the soil. It was tested if the microbial community response to warming and drought depended on more static site properties (soil organic carbon, water holding capacity, pH) in interaction with land use, the study region and the climate change treatment. It was further tested to which extent the N2O emission potential was influenced by more dynamic properties, e.g. the actual water content, the availability of organic carbon and nitrate, or the size of the denitrifier community. Warming effects in enhanced the potential denitrification of denitrifying microorganisms. While differences among the study regions were mainly related to soil chemical and physical properties, the land-use index was a stronger driver for potential denitrification, and grasslands with higher land use also had greater potentials for N2O emissions. The total bacterial community did not respond to experimental treatments, displaying resilience to minor and short-term effects of climate change. In contrast, the denitrifier community tended to be influenced by the experimental treatments and particularly the nosZ abundance was influenced by drought. The results indicate that warming and drought affected the denitrifying communities and the potential denitrification, but these effects are overruled by study region and site-specific land-use index. This thesis gives novel insights into the performance of N-cycling microorganisms in grassland ecosystems. The spatial distribution of soil biogeochemical properties is strongly dependent on land-use intensity, as in return is the spatial distribution of nitrifying and denitrifying microorganisms and the ecosystem services they perform. Yet, future work will be necessary to fully understand the interrelating factors and seasonal variability, which influence the ecosystem functioning and ecosystem services that are provided by N-cycling soil microorganisms at multiple scales.
Fate of microbial carbon derived from biogas residues applied to arable soil
(2015) Coban, Halil; Kandeler, Ellen
Soil organic matter (SOM) is the major determinant of soil fertility as it has a number of positive impacts such as improving soil physical parameters, providing nutrients for crops, and supplying energy for the microbial biomass activity in soil. Loss of organic matter is a soil threat observed worldwide. Also, bioenergy crop cultivation may accelerate SOM loss due to higher biomass harvesting compared to food crops. It is necessary to supply adequate organic matter input to arable soils in order to maintain sustainable food and biofuel production. Biogas residues (BGRs), the side-products of biogas production, are rich in microbial and plant biomass; they thus can be used as a soil conditioner and contribute to replenishing the carbon (C) pool in soil. However, our knowledge on the contribution of BGRs particularly the microbial residues present in it to SOM formation is limited, even though scientific interest on SOM formation via microbial inputs is growing. Therefore, the objective of this thesis were i) developing an approach to label microbial biomass of biogas residues, ii) tracing the fate of labelled BGRs in arable soil, iii) determining the C flux within microbial food web, and iv) determining the impacts of other soil conditioners on the mineralization of BGRs. In the first study a method was developed to label the autotrophic microorganisms in a biogas reactor using KH13CO3-amended cow manure as substrate. Analyses of phospholipid fatty acids (PLFA) and ether lipids confirmed the successful labelling of microorganisms, especially Gram-positive bacteria and methanogenic archaea. After removal of unused labelled carbonates by an acid fumigation approach, the labelled BGRs were incubated in soil for 378 days. The fate of 13C was traced in CO2 and in bulk soil with a mass balance having 93% mean recovery. Results showed that about 40% of the C derived from BGRs was rapidly mineralized within the first seven days, and mineralization reached 65% at the end of experiment. The data could be fitted to a two-pool exponential degradation model assuming two C pools each decaying exponentially. The proportions of readily degrading and stable C pools were determined to be 51% and 49%, respectively, with half-lives of 3 days and 1.9 years, respectively. The long half-life of the stable C pool in BGRs may indicate a mid-term contribution to SOM. In addition, the mineralization of SOM was enhanced by BGR-application, i.e. priming effects were detected, thus their extensive application should be avoided. A differential fatty acid approach was used in the second study for the separation of C input from BGRs to living biomass and non-living SOM. Phospholipid fatty acids (PLFA) as indicators of living biomass were compared with total fatty acids (t-FA), which are found also in necromass. Using PLFA as biomarkers of specific microbial groups, C redistribution within the microbial food web was determined. Results showed that BGRs increased the microbial biomass in soil. The sum of 13C-labelled PLFA and t-FA decreased during incubation to 60% and to 33%, respectively. The level of enrichment was different for the individual PLFA and indicated that Gram-negative bacteria were predating on Gram-positive bacteria. A contribution of ether lipids was also detected indicating C flow from decaying methanogens. This study confirmed that microbial biomass in BGRs applied to arable soil significantly contributes to SOM formation. After determining the fate of microbial C derived from BGRs in arable soil, the impacts of other soil conditioners on the mineralization of BGRs were tested in the third study. For this, labelled BGRs were incubated in soil both alone and together with compost, biochar and untreated manure. The amount of C mineralized to CO2 and the degradation rate constant of stable C pool were not affected by any of the co-amendments. However, manure resulted in a higher mineralization rate constant of the readily degrading C pool. C flow within microbial food web was from Gram-positive bacteria and methanogenic archaea to mainly Gram-negative bacteria and slightly to fungi in all treatments. This study showed that co-amending BGRs with other soil conditioners brings neither benefits nor harms in terms of the formation or the mineralization of soil organic matter. The proposed labelling approach using KH13CO3 may be useful for tracing the fate of BGRs. The enrichment in both bacteria and archaea were sufficient to be measured in an incubation experiment lasting for more than one year. However, there are disadvantages of the proposed approach such as presence of highly enriched residual carbonates. The fumigation method should be optimized for a complete removal of the highly labelled residual carbonates which will increase the precision of the overall approach.
Microbial regulation of pesticide degradation coupled to carbon turnover in the detritusphere
(2015) Pagel, Holger; Streck, Thilo
Many soil functions, such as nutrient cycling or pesticide degradation, are controlled by microorganisms. Dynamics of microbial populations and biogeochemical cycling in soil are largely determined by the availability of carbon (C). The detritusphere is a microbial “hot spot” of C turnover. It is characterized by a concentration gradient of C from litter (high) into the adjacent soil (lower). Therefore, this microhabitat is very well suited to investigate the influence of C availability on microbial turnover. My thesis aimed at the improved understanding of biochemical interactions involved in the degradation of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) coupled to C turnover. In the detritusphere gradients of organic matter turnover from litter into the adjacent soil could be identified. Increased C availability, due to the transport of dissolved organic substances from litter into soil, resulted in the boost of microbial biomass and activity as well as in the acceleration of MCPA degradation. Fungi and bacterial MCPA-degraders benefited most from litter-C input. Accelerated MCPA degradation was accompanied by increased incorporation of MCPA-C into soil organic matter. The experimental results show that the transport of dissolved organic substances from litter regulates C availability, microbial activity and finally MCPA degradation in the detritusphere. In general, litter-derived organic compounds provide energy and resources for microorganisms. The following possible regulation mechanisms were identified: i) Litter might directly supply the co-substrate alpha-ketoglutarate (or surrogates) required for enzymatic oxidation of MCPA by bacterial MCPA degraders. Alternatively it might provide additional energy and resources for production and regeneration of the needed co-substrate. ii) Additional litter-C might alleviate substrate limitation of enzyme production by bacteria and bacterial consortia resulting in an increased activity of specific enzymes attacking MCPA. iii) Litter-derived organic substances might stimulate MCPA degradation via fungal co-metabolism by unspecific extracellular enzymes, either directly by inducing enzyme production, or by supplying primary substrates that provide the energy consumed by co-metabolic MCPA transformation. A new biogeochemical model abstracts these regulation mechanisms in such a way that C availability controls physiological activity, growth, death and maintenance of microbial pools. Based on a global sensitivity analysis, 41% (n=33) of all considered parameters and input values were classified as “very important” and “important”. These mainly include biokinetic parameters and initial values. The calibration of the model allowed to validate the implemented regulation mechanisms of accelerated MCPA degradation. The Pareto-analysis showed that the model structure was adequate and the identified parameter values were reasonable to reproduce the observed dynamics of C and MCPA. The model satisfactorily matched observed abundances of gene-markers of total bacteria and specific MCPA degraders. However, it underestimated the steep increase of fungal ITS fragments, most probably because this gene-marker is only inadequately suited as a measure of fungal biomass. The model simulations indicate that soil fungi primarily benefit from low-quality C, whereas bacterial MCPA-degraders preferentially use high-quality C. According to the simulations, MCPA was predominantly transformed via co-metabolism to high-quality C. Subsequently, this C was primarily assimilated by bacterial MCPA-degraders. The highest turnover of litter-derived C occurred by substrate uptake for microbial growth. Input and microbial turnover of litter-C stimulated MCPA degradation mainly in a soil layer at 0-3 mm distance to litter. As a consequence of this, a concentration gradient of MCPA formed, which triggered the diffusive upward transport of MCPA from deeper soil layers into the detritusphere. The results of the three studies suggest: The detritusphere is a biogeochemical hot spot where microbial dynamics control matter cycling. The integrated use of experiments and mathematical modelling gives detailed insight into matter cycling and dynamics of microorganisms in soil. Microbial communities need to be explicitly considered to understand the regulation of soil functions.
Microbial community structure and function is shaped by microhabitat characteristics in soil
(2016) Ditterich, Franziska; Kandeler, Ellen
Soil microorganisms play a key role in degradation processes in soil, such as organic matter decomposition and degradation of xenobiotics. Microbial growth and activity and therefore degradation processes are influenced by different ecological factors, such as substrate availability, pH and temperature. During soil development different microhabitats are formed which differ in their physiochemical properties. There is some evidence that mineral composition is a driver for specific microbial colonization. Thereby, the heterogeneity of soils with differences in mineral composition and substrate availability can lead to a spatial distribution of soil microorganisms. At the soil-litter interface, a biogeochemical hot spot in soil, the abundance and activity of soil microorganisms increases due to high substrate availability, and degradation processes such as pesticide degradation are enhanced. This thesis aimed to clarify the influence of habitat properties on the structure and function of the microbial community in soil. In particular, focus was on mineral-microbe interactions that result from the mineral composition and substrate availability in an artificial soils system. Furthermore this thesis was designed to increase our understanding of the bacterial and fungal roles in pesticide degradation at the soil-litter interface using 4-chloro-2-methylphenoxyacetic acid (MCPA) as a model xenobiotic. These two aspects of the thesis were examined in three studies. The first study focused on the succession of microbial communities and enzyme activities in an artificial soils system with varying mineral composition and substrate availability over a period of 18 months. In the second study a microcosm experiment was used to study the bacterial pathway of MCPA degradation at the soil-litter interface. Over a period of 27 days the succession of bacterial degraders was followed. The third study focused on the degradation of MCPA in soil by nonspecific fungal enzymes, through the addition of fungal laccases as well as litter during 42 days of incubation. Both studies indicated the involvement of fungi in MCPA degradation and the importance of the ecological behavior of different degraders as a function of substrate availability. Results of the first study indicated that the microbial community was affected by mineral properties under high substrate availability and by the availability of beneficial nutrients at the end of incubation when substrate had become limited. The measured enzyme activities provided clear evidence that microbial community structure was driven by nutrient limitation during incubation. In the presence of easily available organic substrates at the beginning of the experiment, the soil microbial community was dominated by copiotrophic bacteria (e.g. Betaproteobacteria), whereas under substrate limitation at the end of incubation, more recalcitrant compounds became important to oligotrophic bacteria (e.g. Acidobacteria), which then became dominant. The results of the second study indicated that the contribution of the potential degraders to degradation of MCPA differed, and this was also seen in the succession of specific bacterial MCPA degraders. Added litter stimulated MCPA degradation due to the availability of litter-derived carbon and induced a two-phase response of fungi. This was seen in the development of pioneer and late stage fungal communities. Both fungal communities were probably involved in MCPA degradation. Therefore, the third study focused on the fungal pathway. These results indicated that the fungal laccases used had no direct influence on degradation and were as efficient as litter in providing additional nutrient sources, increasing MCPA degradation by bacteria and fungi. The observed differences between litter and enzyme addition underscored the observation that the enzyme effect was short-lived and that substrate quality is an important factor in degradation processes. In conclusion, this thesis demonstrated that soil microbial communities and therefore degradation processes are driven by mineral composition as well as substrate availability and quality. In addition, this thesis extends our understanding of degradation processes such as the degradation of xenobiotics, with MCPA as model compound, in soil. The combined insights from all three studies suggest that the use of a simple system such as the artificial soil system can increase our understanding of complex mechanisms such as degradation of pesticides.

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