Browsing by Subject "Temperatursensitivität"

Now showing 1 - 3 of 3

Biochar amendment for C sequestration in a temperate agroecosystem : implications for microbial C- and N-cycling
(2018) Bamminger, Chris; Kandeler, Ellen
Climate warming will have great impact on terrestrial ecosystems. Different soil properties such as temperature and moisture will be altered, thereby influencing C- and N-cycles, microbial activity as well as plant growth. This may contribute to the observed increase in soil greenhouse gas (GHG) emissions under climate change. Therefore, new options are needed to mitigate theses projected consequences. Biochar is primarily suggested to be effective in long-term C sequestration in agricultural soils due to its long-term stability. In addition, it could be applied to improve various soil properties, plant growth and to reduce soil GHG emissions. To date, knowledge about such beneficial biochar effects in soil under predicted warming climate is extremely scarce. In the first study, a slow-pyrolysis biochar from Miscanthus x giganteus feedstock (600 °C, 30 Min.) was incubated for short time (37d) under controlled laboratory conditions in agricultural soil in the presence of earthworms and N-rich litter (Phacelia tanacetifolia Benth.). Biochar increased microbial abundances and the fungal-to-bacterial PLFA ratio after 37 days in arable soil applied with litter suggesting improved living conditions for microorganisms with biochar. Fungi may benefit most from newly created habitats due to colonizable biochar pores and surfaces. Additionally, fungi could have also mineralized small amounts of recalcitrant biochar-C during plant litter decomposition. Without litter, biochar led to interactions between earthworms and soil microorganisms resulting in enhanced bacterial and fungal abundances. This indicates better growth habitats for soil microbes in earthworm casts containing biochar. Soil respiration and metabolic quotients (qCO2) and N2O emissions (in litter treatments) were decreased after biochar application suggesting a more efficient microbial community and underscoring the GHG mitigation potential of the used biochar. The field experiment, investigated in the second and third study, focused on the stability and long-term soil C sequestration potential of comparable Miscanthus biochar (850 °C, 30 Min.). Related effects on soil GHG emissions, physical, chemical and microbiological soil properties as well as plant growth were determined in an agroecosystem at year-round elevated soil temperature (+2.5 °C, since 2008). The second study investigated the short-term effects of biochar on microbial abundances and growth of winter rapeseed during the first year after field application to a warmed temperate arable soil. It was found that fungal biomass and the fungal-to-bacterial ratio were increased in the warmed biochar plots only after three months in the presence of spring barley litter from the previous growing season. The disappearance of this effect points to an overall high stability of the investigated biochar. Moreover, biochar proved to be effective in mitigating negative effects of seasonal dryness on microbial abundances and early plant growth in the dry spring period in 2014. However, biochar had no effect on final aboveground biomass of winter rapeseed at harvest in the first growing season. As shown in the third study, after two vegetation periods of winter rapeseed and spring wheat, the assumption that plant productivity in already fertile temperate arable soils is unlikely to be further enhanced with biochar amendment, was confirmed. Total CO2 emissions after two years were not reduced by biochar and remained unchanged even under warming suggesting a high degradation stability of the used biochar. N2O emissions were increased in biochar-amended soil at elevated soil temperature, presumably due to enhanced water and fertilizer retention with biochar. By using the global warming potential (GWP100) of total soil GHG emissions, the storage of biochar-C in soil was estimated to compensate warming-induced elevated soil GHG emissions for 20 years. To conclude, this thesis revealed that biochar may have only minor influence on soil microorganisms and crop growth in temperate, fertile arable field soils. However, it was shown that biochar could be a valuable tool for C sequestration in temperate arable soils, thus potentially offsetting a warming-induced increase in GHG emissions. In order to face climate change impacts, more long-term studies on microbiological effects and the C sequestration potential of biochar in cultivated soil under warming are urgently needed.
Microbial regulation of soil organic matter decomposition at the regional scale
(2018) Ali, Rana Shahbaz; Kandeler, Ellen
The fate of soil organic carbon (SOC) is one of the greatest uncertainties in predicting future climate. Soil microorganisms, as primary decomposers of SOC, control C storage in terrestrial ecosystems by mediating feedbacks to climate change. Even small changes in microbial SOC decomposition rates at the regional scale have the potential to alter land-atmospheric feedbacks at the global scale. Despite their critical role, the ways in which soil microorganisms may change their abundances and functions in response to the climate change drivers of soil temperature and moisture is unclear. Additionally, most existing C models do not consider soil microorganisms explicitly as drivers of decomposition, one consequence of which is large variability in predicted SOC stock projections. This demonstrates the need for a better mechanistic understanding of microbial SOC decomposition at large scales. This thesis was designed to clarify the role of microbial SOC decomposition dynamics in response to climate change factors in two geographically distinct areas and land-use types. The hypothesis was that microbial communities would be adapted to climatic and edaphic conditions specific to each area and to the SOC organic quality in each land-use and would therefore exhibit distinct responses to soil temperature and moisture variations. Three studies were performed to address the goals of this thesis. The first study aimed to clarify temporal patterns of degradation in C pools that varied in complexity by modelling in situ potentials of microbially produced extracellular enzymes. Temperature and moisture sensitivity patterns of C cycling enzymes were followed over a period of thirteen months. The second study investigated group-specific temperature responses of bacteria and fungi to substrate quality variations through an additional incubation experiment. Here, complex environments were mimicked in order to determine the dependence of microbial responses not only on environmental conditions, but also under conditions of inter- and intra-specific community competition. Changes in microbial community composition, abundance, and function were determined at coarse (phospholipid fatty acid – PLFA, ergosterol) and relatively fine resolutions (16S rRNA, taxa-specific quantitative PCR, fungal ITS fragment). A third study investigated 1) the spatial variability of temperature sensitivity of microbial processes, and 2) the scale-specificity and relative significance of their biotic and physicochemical controls at landscape (two individual areas, each ca. 27 km2) and regional scales (pooled data of two areas). Strong seasonal dependency was observed in the temperature sensitivities (Q10) of hydrolytic and oxidative enzymes, whereas moisture sensitivity of β-glucosidase activities remained stable over the year. The range of measured enzyme Q10 values was similar irrespective of spatial scale, indicating a consistency of temperature sensitivities of these enzymes at large scales. Enzymes catalyzing the recalcitrant SOC pool exhibited higher temperature sensitivities than enzymes catalyzing the labile pool; because the recalcitrant C pool is relatively large, this could be important for understanding SOC sensitivity to predicted global warming. Response functions were used to model temperature-based and temperature and moisture-based in situ enzyme potentials to characterize seasonal variations in SOC decomposition. In situ enzyme potential explained measured soil respiration fluxes more efficiently than the commonly used temperature-respiration function, supporting the validity of our chosen modelling approach. As shown in the incubation experiment, increasing temperature stimulated respiration but decreased the total biomass of bacteria and fungi irrespective of substrate complexity, indicating strong stress responses by both over short time scales. This response did not differ between study areas and land-uses, indicating a dominant role of temperature and substrate quality in controlling microbial SOC decomposition. Temperature strongly influenced the responses of microbial groups exhibiting different life strategies under varying substrate quality availability; with soil warming, the abundance of oligotrophs (fungi and gram-positive bacteria) decreased, whereas copiotrophs (gram-negative) increased under labile C substrate conditions. Such an interactive effect of soil temperature and substrate quality was also visible at the taxon level, where copiotrophic bacteria were associated with labile C substrates and oligotrophic bacteria with recalcitrant substrates. Which physicochemical and biological factors might explain the observed alterations in microbial communities and their functions in response to climate change drivers at the regional scale was investigated in the third study. Here, it was shown that the soil C:N ratio exerted scale-dependent control over soil basal respiration, whereas microbial biomass explained soil basal respiration independent of spatial scale. Factors explaining the temperature sensitivity of soil respiration also differed by spatial scale; extractable organic C and soil pH were important only at the landscape scale, whereas soil texture as a control was independent of spatial scale. In conclusion, this thesis provides an enhanced understanding of the response of microbial C dynamics to climate change at large scales by combining field measurements with innovative laboratory assays and modelling tools. Component specific degradation rates of SOC using extracellular enzyme measurements as a proxy, group-specific temperature sensitivities of microbial key players, and the demonstrated scale-specificity of factors controlling microbial processes could potentially improve the predictive power of currently available C models at regional scale.
Mid-infrared spectroscopy and enzyme activity temperature sensitivities as experimental proxies to reduce parameter uncertainty of soil carbon models
(2021) Laub, Moritz; Cadisch, Georg
Models that simulate the dynamics of soil organic carbon (SOC) are crucial to understand the global carbon cycle, but current generation models are subject to major uncertainties due to two model shortcomings. Firstly, their different carbon pools are not connected to measurable SOC fractions. Secondly, there is uncertainty about the response of the different carbon pools to an increasing temperature. The aim of this thesis was thus to link the SOC model pools of the Daisy model to measurable proxies for SOC quality and pool specific temperature sensitivity. In the first study, the drying temperature for soil samples assessed by diffuse reflectance mid infrared Fourier transform spectroscopy (DRIFTS) was optimized to assure optimal representativeness of aliphatic and aromatic-carboxylate absorption bands as proxies for fast- and slow-cycling SOC pools. Their ratio was termed the DRIFTS stability index (DSI). In the second study, the DSI was used to distinguish fast- and slow-cycling SOC model pools at model initialization. In the third study, model initialization using DSI was performed to infer pool specific temperature sensitivities for the different Daisy carbon pools. Furthermore, it was tested whether the measured temperature sensitivities of different extracellular soil enzymes could be used as proxies for pool specific temperature sensitivity. Using a global collection of soil samples revealed that the absorption of all studied DRIFTS absorption bands increased significantly (p < 0.0001) with increasing drying temperature from 32°C to 105°C. This effect was disproportionally strong for the aliphatic absorption band. Due to the strong interference of the residual soil sample moisture content with the aliphatic absorption band, drying at 105°C and storage in a desiccator prior to measurement would be necessary for representative spectra for model pool initialization. In the following, a combination of medium to long-term bare fallow experiments was used, to test the utility of the DSI for SOC pool initialization. Pool partitioning by the DSI was superior to using a fixed pool partitioning under the assumption that SOC was at steady state. The DSI contained robust information on SOC quality across sites. Therefore, in the majority of cases, the application of the DSI led to significantly lower model errors than the steady state assumption. Furthermore, the application of the DSI in Bayesian calibration led to a reduced parameter uncertainty for the turnover of the slow-cycling SOC pool and the humification efficiency. The 95% credibility interval of the slow-cycling SOM pools’ half-life between 278 and 1095 years suggested faster SOC turnover than earlier studies. The DSI used for SOC model pool initialization was then combined with the lignin-to-nitrogen ratio for litter pool initialization to infer pool specific temperature sensitivities. The simulations of five field studies and laboratory incubations with fallow soil and crop-litter inputs were combined. Based on a clear pool definition, pool specific temperature sensitivities could be inferred by Bayesian calibration. However, differences in temperature sensitivities of the same pools between experiments suggested that carbon stability was not the main driver of temperature sensitivities. Instead, the main difference was found between the laboratory incubations (higher Q10 values up to 3) and the field (lower Q10 values centered around 2). In a second approach, the measured Q10 value of phenoloxidase (1.35) was used as Q10 value of the temperature function of both SOM pools and the slow crop-litter pool while ß glucosidase (1.82) was used for the fast crop litter pool. This improved field simulations by 3 to 10% compared to assuming a standard Q10 of 2 for all pools. Thus, site specific Q10 of different soil enzymes showed potential as proxy for site and pool specific temperature sensitivities. Important state variables that explain the observed Q10 value differences between experiments were identified as physical protection of SOC, substrate availability and environmental stress for microorganisms due to fluctuating state variables in the field. In conclusion, the usefulness of the DSI as an indicator of SOC stability and proxy for pool initialization was demonstrated for several soils in central Europe. In addition, it was shown that pool partitioning proxies can help to infer pool specific temperature sensitivity by Bayesian calibration. However, temperature sensitivity was not mainly a function of carbon stability.