Browsing by Subject "Soil carbon decomposition"

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    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.