Browsing by Subject "Temperature sensitivity"

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Carbon and nitrogen transformations in alpine ecosystems of the Eastern Alps, Austria
(2007) Koch, Oliver; Kandeler, Ellen
This thesis investigated net CH4 and net CO2 emissions from sites in the alpine region of the Eastern Alps, Austria. Four mature alpine sites (one dry meadow and three fen sites) were selected and the influence of abiotic (radiation, temperature, soil water conditions) and biotic (above-ground standing plant biomass) environmental controls on diurnal and seasonal emission patterns were studied. For a better understanding of the response of soil C- and N pools to global warming, the temperature sensitivity of activities involved in C- and N cycling were determined. The first part of the thesis dealt with net methane fluxes measured over a period of 24 months. During snow-free periods, average methane emissions of the fen sites ranged between 19 and 116 mg CH4 m-2 d-1. Mean emissions during snow periods were much lower, being 18 to 59% of annual fluxes. The alpine dry meadow functioned as a small methane sink during snow-free periods (-2.1 mg CH4 m-2 d-1 (2003); -1.0 mg CH4 m-2 d-1 (2004)). The diurnal and seasonal methane uptake of the dry meadow was positively related to soil temperature and negatively related to water-filled pore space (wfps). In the fen, the seasonal methane fluxes were related to soil temperature and groundwater table. The live above-ground standing plant biomass contributed to net methane fluxes only at those sites with higher water table positions. This study provided evidence that alpine fens acted as methane sources throughout the year, whereas an alpine meadow site acted as a net methane sink during snow-free periods. In the second part of the thesis the CO2 balance was estimated based on diurnal flux measurements and on the influence of photosynthetic active radiation (PAR), plant green area index (GAI), soil temperature and wfps. The daylight net ecosystem CO2 emission rate was influenced by PAR and GAI throughout snow-free seasons. The seasonal net CO2 emission rate at night was positively related to soil temperature, while low wfps reduced flux rates at the meadow and at the driest fen study site but reinforced carbon loss at the wetter fen sites. The daily average ecosystem net CO2 gain during snow-free periods at the meadow was 3.5 g CO2 m-2 d-1 and at the fen sites between 1.5 and 3.4 g CO2 m-2 d-1. The mean average daily CO2 emission during snow periods was low, being -0.9 g CO2 m-2 d-1 for the meadow and between -0.2 and -0.7 g CO2 m-2 d-1 for all fen sites. All sites function as significant annual net carbon sinks, with a net carbon gain from 50 to 121 g C m-2 a-1 (averaged over both years), irrespective of water balance. The results indicate that alpine fen sites, that have built up a large carbon stock in the past, are not expected to gain a further carbon surplus compared with meadows under the current climate. Temperature is important for regulating biological activities. The third part of the thesis focused on temperature sensitivity of soil C mineralization, N mineralization and potential enzyme activities involved in the C- and N cycle (ß-glucosidase, ß-xylosidase, N-acetyl-ß-glucosaminidase, tyrosine aminopeptidase, leucine aminopeptidase) over a temperature range of 0-30°C. The objective was to calculate Q10 values and relative temperature sensitivities (RTS) and to quantify seasonal (summer, autumn, winter) and site-specific factors. The Q10 values of C mineralization were significantly higher (average 2.0) than for N mineralization (average 1.7). The Q10 values of both activities were significantly negatively related to soil organic matter quality. In contrast, the chemical soil properties, microbial biomass and sampling date did not influence Q10 values. Analysis of RTS showed that the temperature sensitivity increased with decreasing temperature. The C- and N mineralization and potential aminopeptidase activities (tyrosine, leucine) showed an almost constant temperature dependence over 0-30°C. In contrast, ß-glucosidase, ß-xylosidase and N-acetyl-ß-glucosaminidase showed a distinctive increase in temperature sensitivity with decreasing temperature. Low temperature at the winter sampling date caused a greater increase in the RTS of all activities than in autumn and summer. Our results indicate a disproportion of the RTS for potential enzyme activities of the C- and N cycle and a disproportion of the RTS for easily degradable C compounds (ß-glucose, ß-xylose) compared with the C mineralization of soil organic matter. Thus, temperature may play an important role in regulating the decay of different soil organic matter fractions.
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.