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Monthly variation in accumulation and release of phenolics in bryophytes, and their allelopathic effects are little known despite their ecological significance. We conducted a field transplant experiment to investigate Sphagnum growth and extracellular enzymes that modulate phenolic contents and release along water table level (WTL) gradient. Specifically, we assessed monthly changes in height increment, phenolic release, and allelopathic effect during the growing season, as well as extracellular enzymes at the end of the experiment, for the two Sphagnum species: Sphagnum angustifolium C.E.O. Jensen, 1896, a hollow dwelling species, and Sphagnum magellanicum Bridel, 1798, a hummock species. Both Sphagnum species showed season- and WTL-dependent height increment and released phenolics. Sphagnum angustifolium exhibited negative allelopathy regardless of WTL, while S. magellanicum demonstrated mainly positive allelopathy at high WTL and negative allelopathy at low WTL. The degree of allelopathy was not contingent on phenolic release, but phenolic production was negatively correlated with height growth of the two species at their atypical habitats. Moreover, phenoloxidase or peroxidase did not show high activity at low WTL compared to high WTL. Our findings demonstrate for the first time that the strength and direction of allelopathy in Sphagnum may vary depending on the growing precipitation and temperature, WTL, and inter- and intraspecific trait variability. Longer term experiments may further elucidate the seasonal dynamics and mechanism of Sphagnum allelopathy. Key words: allelopathy, extracellular enzymes, secondary metabolites, moss, peatland, Introduction Allelopathy, the release of chemicals by plants that affect other plants in their environment, can be a major driver in shaping natural plant communities (Wardle et al. 1997; Zackrisson [...]

Current projections suggest that climate warming will be accompanied by more frequent and severe drought events. Peatlands store ca. one third of the world's soil organic carbon. Warming and drought may cause peatlands to become carbon sources through stimulation of microbial activity increasing ecosystem respiration, with positive feedback effect on global warming. Micro-eukaryotes play a key role in the carbon cycle through food web interactions and therefore, alterations in their community structure and diversity may affect ecosystem functioning and could reflect these changes. We assessed the diversity and community composition of Sphagnum-associated eukaryotic microorganisms inhabiting peatlands and their response to experimental drought and warming using high throughput sequencing of environmental DNA. Under drier conditions, micro-eukaryotic diversity decreased, the relative abundance of autotrophs increased and that of osmotrophs (including Fungi and Peronosporomycetes) decreased. Furthermore, we identified climate change indicators that could be used as early indicators of change in peatland microbial communities and ecosystem functioning. The changes we observed indicate a shift towards a more ''terrestrial'' community in response to drought, in line with observed changes in the functioning of the ecosystem. [ABSTRACT FROM AUTHOR]

Extreme climate events are predicted to become more frequent and intense. Their ecological impacts, particularly on carbon cycling, can differ in relation to ecosystem sensitivity. Peatlands, being characterized by peat accumulation under waterlogged conditions, can be particularly sensitive to climate extremes if the climate event increases soil oxygenation. However, a mechanistic understanding of peatland responses to persistent climate extremes is still lacking, particularly in terms of aboveground-belowground feedback. Here, we present the results of a transplantation experiment of peat mesocosms from high to low altitude in order to simulate, during 3 years, a mean annual temperature c. 5 °C higher and a mean annual precipitation c. 60% lower. Specifically, we aim at understanding the intensity of changes for a set of biogeochemical processes and their feedback on carbon accumulation. In the transplanted mesocosms, plant productivity showed a species-specific response depending on plant growth forms, with a significant decrease (c. 60%) in peat moss productivity. Soil respiration almost doubled and Q10 halved in the transplanted mesocosms in combination with an increase in activity of soil enzymes. Spectroscopic characterization of peat chemistry in the transplanted mesocosms confirmed the deepening of soil oxygenation which, in turn, stimulated microbial decomposition. After 3 years, soil carbon stock increased only in the control mesocosms whereas a reduction in mean annual carbon accumulation of c. 30% was observed in the transplanted mesocosms. Based on the above information, a structural equation model was built to provide a mechanistic understanding of the causal connections between peat moisture, vegetation response, soil respiration and carbon accumulation. This study identifies, in the feedback between plant and microbial responses, the primary pathways explaining the reduction in carbon accumulation in response to recurring climate extremes in peat soils. [ABSTRACT FROM AUTHOR]

Population dynamics and feeding habits of the testate amoebae Nebela tincta and Hyalosphenia papilio were studied along a short “fen” to “bog” gradient in a Sphagnum-dominated mire (Jura, France). Samples were collected in living “top segments” (0-3cm) and early declining “bottom segments” (3-6cm) of Sphagnum fallax peat. Observations of digestive vacuole content and stable isotope analyses (13C and 15N) were used to establish the feeding behavior of both testate amoeba species. Owing to their vertical distribution, the feeding habit of H. papilio was described from top segments, and that of N. tincta from bottom segments. Among identified food sources, those most frequently ingested by N. tincta were spores and mycelia of fungi (55%), microalgae (25%) and cyanobacteria (8.5%). For H. papilio, the most frequently ingested prey were ciliates (55%) and microalgae (35%). Nonmetric Multidimensional Scaling analysis clearly demonstrated that the two species did not have the same feeding habit along the “fen-bog” gradient, and furthermore that a significant spatial split exists in the feeding behavior of H. papilio. Additionally, isotope analyses suggested that H. papilio and N. tincta did not have the same trophic position in the microbial food web, probably resulting from their different feeding strategies. [Copyright &y& Elsevier]

Microbial communities living in Sphagnum are known to constitute early indicators of ecosystem disturbances, but little is known about their response (including their trophic relationships) to climate change. A microcosm experiment was designed to test the effects of a temperature gradient (15, 20, and 25 °C) on microbial communities including different trophic groups (primary producers, decomposers, and unicellular predators) in Sphagnum segments (0-3 cm and 3-6 cm of the capitulum). Relationships between microbial communities and abiotic factors (pH, conductivity, temperature, and polyphenols) were also studied. The density and the biomass of testate amoebae in Sphagnum upper segments increased and their community structure changed in heated treatments. The biomass of testate amoebae was linked to the biomass of bacteria and to the total biomass of other groups added and, thus, suggests that indirect effects on the food web structure occurred. Redundancy analysis revealed that microbial assemblages differed strongly in Sphagnum upper segments along a temperature gradient in relation to abiotic factors. The sensitivity of these assemblages made them interesting indicators of climate change. Phenolic compounds represented an important explicative factor in microbial assemblages and outlined the potential direct and (or) indirect effects of phenolics on microbial communities. Les microorganismes des sphaignes sont connus comme indicateurs précoces des perturbations environnementales. Or, peu d'études portent sur l'influence de la température sur ces communautés. Dans cette étude, l'effet d'un gradient de températures (15, 20 et 25 °C) sur les microorganismes des sphaignes, incluant différent groupes trophiques (producteurs primaires, décomposeurs, micro-prédateurs), a été testé dans les différentes parties des sphaignes (0 à 3 cm et 3 à 6 cm du capitulum). Les relations entre les microorganismes et les facteurs abiotiques (pH, conductivité, température, polyphénols) ont également été analysées. La densité et la biomasse des amibes à thèque ont augmenté dans les parties supérieures des sphaignes. Une modification de la structure de leur communauté a aussi été mise en évidence. La biomasse des amibes à thèque est apparue liée à celle des bactéries et à la biomasse additionnée des autres groupes, suggérant des effets indirects de la température sur les relations trophiques. Les analyses par redondance ont révélé que la structure des communautés microbiennes variait entre les différents traitements dans les parties supérieures des sphaignes. Ainsi, les communautés microbiennes des sphaignes apparaissent comme un outil intéressant quant au suivi in situ des changements climatiques. Les polyphénols ont également été identifiés comme un facteur explicatif important de la structure des communautés microbiennes, montrant ainsi leurs effets potentiels sur les microorganismes des sphaignes. [ABSTRACT FROM AUTHOR]