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Institut für Biologie

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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.