Faculté des sciences

Spatio-temporal variability and restoration effects on below-ground biodiversity and soil ecosystem functioning at the Thur floodplain, Switzerland

Samaritani, Emanuela ; Mitchell, Edward A. D. (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2013.

Biodiversity and its link to ecosystem functioning are the main subjects of a large variety of studies nowadays, because of the tremendous rapidity at which they are altered as a consequence of direct human impact or as a delayed effect of global changes. Floodplains are acknowledged to be among the most diverse ecosystems on the planet, in terms of species diversity, but also genetic, habitat... More

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    Summary
    Biodiversity and its link to ecosystem functioning are the main subjects of a large variety of studies nowadays, because of the tremendous rapidity at which they are altered as a consequence of direct human impact or as a delayed effect of global changes. Floodplains are acknowledged to be among the most diverse ecosystems on the planet, in terms of species diversity, but also genetic, habitat and functional diversity. Restoration projects aiming at recovering the biodiversity and associated functions and services provided by riverine floodplains are increasing and there is a pressing need for understanding their spatio-temporal complexity. In this work, I investigate the impact of floods and soil habitat diversity on the spatial and seasonal heterogeneity of soil carbon pools and fluxes, ecosystem functioning proxies, bacterial and eukariotic diversity and community structure. The study site is a restored floodplain on the Thur river in northeast Switzerland. In seven functional processes zones, over a six seasons period, soil samples were collected and soil texture and nutrient, carbon pools and fluxes, temperature, soil moisture and flooding regime, enzymatic activity, respiration and microbial biomass were measured. Soil DNA was extracted and bacterial and eukaryotic communities were analysed using respectively terminal restriction fragment length polymorphism (t-rflp) profiles and Illumina high-throughput sequencing.
    Our results showed gradual changes in texture along the gradient of distance from the river, which together with water retention capacity and inundation regime, explained the spatial variability of carbon fluxes. Also increasing along this gradient was total eukaryotic microorganism diversity. Bacterial community structure was tightly linked to ecosystem functioning proxies, but what significantly influenced its variability was soil texture and nutrients. Soil diversity, nutrients and moisture also determined decomposition rates heterogeneity. Temperature and inundation regime mostly appear as indirect drivers of soil diversity and ecosystem functioning. But effects of soil conditions and climate result in patterns that vary extremely among taxa. Although the bacterial community was spatially different, temporal effect of climatic conditions dominated and resulted in strong temporal variation in the community structure. On the other hand micro eukaryotic communities were spatially much more differentiated than temporally. But different eukaryotic taxa showed contrasting patterns. Taxa that were more abundant were also more evenly distributed across FPZ and seasons, while less abundant taxa, and especially those related to aquatic environments show higher variability.
    Soil texture and nutrient content, both maintained by natural pulsing flooding regime, proved to be important determinants of bacterial community structure heterogeneity and eukaryotic diversity. Causal relationships between habitat, processes and biodiversity are highly complex and direct connections are difficult to establish, unless specific organism ecology is considered, but all biotic and abiotic factors analysed in this study showed a strong dependence on the soil structure and the natural flooding regime.