Résultat de la recherche
Spatio-temporal variability and restoration effects on below-ground biodiversity and soil ecosystem functioning at the Thur floodplain, Switzerland
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.
Seasonal Net Ecosystem Carbon Exchange of a Regenerating Cutaway Bog: How Long Does it Take to Restore the C-Sequestration Function?
We measured the net ecosystem exchange (NEE) and respiration rates and modeled the photosynthesis and respiration dynamics in a cutover bog in the Swiss Jura Mountains during one growing season at three stages of regeneration (29, 42, and 51 years after peat cutting; coded sites A, B, and C) to determine if reestablishment of Sphagnum suffices to restore the C-sequestration function. From the younger to the older stage Sphagnum cover increased, while net primary Sphagnum production over the growing season decreased (139, 82, and, 67 g m−2 y−1 for A, B, and C respectively), and fen plant species were replaced by bog species. According to our NEE estimations, over the vegetation period site A was a net CO2-C source emitting 40 g CO2-C/m2 while sites B and C were accumulating CO2-C, on average 222 and 209 g CO2-C/m2, respectively. These differences are due to the higher respiration in site A during the summer, suggesting that early regeneration stages may be more sensitive to a warmer climate. Methane fluxes increased from site A to C in parallel with Eriophorum vaginatum cover and vascular plant leaf area. Our results show that reestablishing a Sphagnum cover is not sufficient to restore a CO2-sequestrating function but that after circa 50 years the ecosystem may naturally regain this function over the growing season.
Patterns of earthworm communities and species traits in relation to the perturbation gradient of a restored floodplain
Little is known about the diversity and ecology of earthworms in floodplains, as well as their response to natural and anthropic perturbations (e.g. floods, river channelisation, floodplain restoration). We characterised the patterns of earthworm communities and species traits in the different habitats of a lowland restored floodplain in Switzerland. In addition to classical species-based metrics, such as species richness and Shannon diversity, species traits were used to calculate the community weighted means (CWMs) of traits and functional dispersion (FDis). We hypothesised that trait-based metrics would reveal clearer patterns than classical approaches. The distribution of earthworm traits varied among habitats in relation to changes in flooding frequency: poorly developed gravel bar soils most exposed to flooding were characterised by high abundance of small epigeic species and low abundance of large anecic species. Differences in anecic and endogeic earthworm community structure matched flood frequency. In agreement with our hypothesis, CWMs were more strongly correlated to environmental variables than species composition, diversity, or functional diversity. Based on these results, the ratio of the relative abundances of epigeic and anecic species, and the differences in species composition within anecic and endogeic ecological types of earthworms were identified as indicators of soil development in floodplains.
Patterns of earthworm communities and species traits in relation to the perturbation gradient of a restored floodplain
Little is known about the diversity and ecology of earthworms in floodplains, as well as their response to natural and anthropic perturbations (e.g. floods, river channelisation, floodplain restoration). We characterised the patterns of earthworm communities and species traits in the different habitats of a lowland restored floodplain in Switzerland. In addition to classical species-based metrics, such as species richness and Shannon diversity, species traits were used to calculate the community weighted means (CWMs) of traits and functional dispersion (FDis). We hypothesised that trait-based metrics would reveal clearer patterns than classical approaches. The distribution of earthworm traits varied among habitats in relation to changes in flooding frequency: poorly developed gravel bar soils most exposed to flooding were characterised by high abundance of small epigeic species and low abundance of large anecic species. Differences in anecic and endogeic earthworm community structure matched flood frequency. In agreement with our hypothesis, CWMs were more strongly correlated to environmental variables than species composition, diversity, or functional diversity. Based on these results, the ratio of the relative abundances of epigeic and anecic species, and the differences in species composition within anecic and endogeic ecological types of earthworms were identified as indicators of soil development in floodplains.
Heterogeneity of soil carbon pools and fluxes in a channelized and a restored floodplain section (Thur River, Switzerland)
Due to their spatial complexity and dynamic nature, floodplains provide a wide range of ecosystem functions. However, because of flow regulation, many riverine floodplains have lost their characteristic heterogeneity. Restoration of floodplain habitats and the rehabilitation of key ecosystem functions, many of them linked to organic carbon (C) dynamics in riparian soils, has therefore become a major goal of environmental policy. The fundamental understanding of the factors that drive the processes involved in C cycling in heterogeneous and dynamic systems such as floodplains is however only fragmentary.
We quantified soil organic C pools (microbial C and water extractable organic C) and fluxes (soil respiration and net methane production) in functional process zones of adjacent channelized and widened sections of the Thur River, NE Switzerland, on a seasonal basis. The objective was to assess how spatial heterogeneity and temporal variability of these pools and fluxes relate to physicochemical soil properties on one hand, and to soil environmental conditions and flood disturbance on the other hand.
Overall, factors related to seasonality and flooding (temperature, water content, organic matter input) affected soil C dynamics more than soil properties did. Coarse-textured soils on gravel bars in the restored section were characterized by low base-levels of organic C pools due to low TOC contents. However, frequent disturbance by flood pulses led to high heterogeneity with temporarily and locally increased C pools and soil respiration. By contrast, in stable riparian forests, the finer texture of the soils and corresponding higher TOC contents and water retention capacity led to high base-levels of C pools. Spatial heterogeneity was low, but major floods and seasonal differences in temperature had additional impacts on both pools and fluxes. Soil properties and base levels of C pools in the dam foreland of the channelized section were similar to the gravel bars of the restored section. By contrast, spatial heterogeneity, seasonal effects and flood disturbance were similar to the forests, except for indications of high CH4 production that are explained by long travel times of infiltrating water favoring reducing conditions. Overall, the restored section exhibited both a larger range and a higher heterogeneity of organic C pools and fluxes as well as a higher plant biodiversity than the channelized section. This suggests that restoration has indeed led to an increase in functional diversity.