Rhizodeposition of organic carbon by plants with contrasting traits for resource acquisition: responses to different fertility regimes

Background and aims Rhizodeposition plays an important role in mediating soil nutrient availability in ecosystems. However, owing to methodological difficulties (i.e., narrow zone of soil around roots, rapid assimilation by soil microbes) fertility-induced changes in rhizodeposition remain mostly unknown. Methods We developed a novel long-term continuous 13C labelling method to address the effects of two levels of nitrogen (N) fertilization on rhizodeposited carbon (C) by species with different nutrient acquisition strategies. Results Fertility-induced changes in rhizodeposition were modulated by root responses to N availability rather than by changes in soil microbial biomass. Differences among species were mostly related to plant biomass: species with higher total leaf and root biomass also had higher total rhizodeposited C, whereas species with lower root biomass had higher specific rhizodeposited C (per gram root mass). Experimental controls demonstrated that most of the biases commonly associated with this type of experiment (i.e., long-term steady-state labelling) were avoided using our methodological approach. Conclusions These results suggest that the amount of rhizodeposited C from plants grown under different levels of N were driven mainly by plant biomass and root morphology rather than microbial biomass. They also underline the importance of plant characteristics (i.e., biomass allocation) as opposed to traits associated with plant resource acquisition strategies in predicting total C rhizodeposition

Evidence for alternative electron sinks to photosynthetic carbon assimilation in the high mountain plant species Ranunculus glacialis

International audienceThe high mountain plant species Ranunculus glacialis has a low antioxidative scavenging capacity and a low activity of thermal dissipation of excess light energy despite its growth under conditions of frequent light and cold stress. In order to examine whether this species is protected from over-reduction by matching photosystem II (PSII) electron transport (ETR) and carbon assimilation, both were analysed simultaneously at various temperatures and light intensities using infrared gas absorption coupled with chlorophyll fluorescence. ETR exceeded electron consumption by carbon assimilation at higher light intensities and at all temperatures tested, necessitating alternative electron sinks. As photorespiration might consume the majority of excess electrons, photorespiration was inhibited by either high internal leaf CO2 molar ratio (C-i), low oxygen partial pressure (0.5% oxygen), or both. At 0.5% oxygen ETR was significantly lower than at 21% oxygen. At 21% oxygen, however, ETR still exceeded carbon assimilation at high C-i, suggesting that excess electrons are transferred to another oxygen consuming reaction when photorespiration is blocked. Nevertheless, photorespiration does contribute to electron consumption. While the activity of the water -water cycle to electron consumption is not known in leaves of R. glacialis, indirect evidence such as the high sensitivity to oxidative stress and the low initial NADP-malate dehydrogenase (NADP-MDH) activity suggests only a minor contribution as an alternative electron sink. Alternatively, the plastid terminal oxidase (PTOX) may transfer excess electrons to oxygen. This enzyme is highly abundant in R. glacialis leaves and exceeds the PTOX content of every other plant species so far examined, including those of transgenic tomato leaves overexpressing the PTOX protein. Finally, PTOX contents strongly declined during deacclimation of R. glacialis plants, suggesting their important role in photoprotection. Ranunculus glacialis is the first reported plant species with such a high PTOX protein content

Contrasting Diversity Patterns of Crenarchaeal, Bacterial and Fungal Soil Communities in an Alpine Landscape

International audienceBackground: The advent of molecular techniques in microbial ecology has aroused interest in gaining an understanding about the spatial distribution of regional pools of soil microbes and the main drivers responsible of these spatial patterns. Here, we assessed the distribution of crenarcheal, bacterial and fungal communities in an alpine landscape displaying high turnover in plant species over short distances. Our aim is to determine the relative contribution of plant species composition, environmental conditions, and geographic isolation on microbial community distribution. Methodology/Principal Findings: Eleven types of habitats that best represent the landscape heterogeneity were investigated. Crenarchaeal, bacterial and fungal communities were described by means of Single Strand Conformation Polymorphism. Relationships between microbial beta diversity patterns were examined by using Bray-Curtis dissimilarities and Principal Coordinate Analyses. Distance-based redundancy analyses and variation partitioning were used to estimate the relative contributions of different drivers on microbial beta diversity. Microbial communities tended to be habitat- specific and did not display significant spatial autocorrelation. Microbial beta diversity correlated with soil pH. Fungal beta- diversity was mainly related to soil organic matter. Though the effect of plant species composition was significant for all microbial groups, it was much stronger for Fungi. In contrast, geographic distances did not have any effect on microbial beta diversity. Conclusions/Significance: Microbial communities exhibit non-random spatial patterns of diversity in alpine landscapes. Crenarcheal, bacterial and fungal community turnover is high and associated with plant species composition through different set of soil variables, but is not caused by geographical isolation

Impact de la durée d'enneigement sur les cycles biogéochimiques dans les écosystèmes alpins

Alpine tundra store large carbon stocks in their soils. In these ecosystems, the local mesotopography determines snow cover distribution, a key variable, which affect the edapho-climatic conditions on the short term (direct effects) and, in the longer-term, select for contrasting plant and microbial communities at both ends of the topographical gradient (indirect effects). In the context of global change, where large changes in snow precipitations are projected, this study explores the controls exerted by snow cover on carbon fixation and carbon mineralization in alpine tundra. Edapho-climatic variables (water and temperature) were measured during several years and we used vegetation functional characteristics (using plant functional traits) to quantify the indirect effects of snow cover on biogeochemical cycles. Various approaches (in situ measurements, experimental manipulations and modeling) were used. This study demonstrates that carbon fixation along mesotopographical gradients is determined by plant functional traits, canopy properties and growing season length. A longer growing season may lead to a marked increase in primary production, if freezing events at snowmelt remain infrequent. In contrast, carbon mineralization is mainly dependant over soil organic matter quality. Shifts in plant functional traits, in particular those related to litter lignin content, will strongly impact the degradation process. Finally, the quantification of carbon and nitrogen fluxes in plants and at the plant-soil interface reveals a tight spatial and temporal coupling which is essential for species whose growth is limited by growing vegetation length. This coupling is reduced in plant communities which benefit from a longer growing season. The evolution of carbon fluxes and stocks in alpine ecosystems is discussed in the context of climatic changes.Les écosystèmes alpins, au même titre que les écosystèmes arctiques, séquestrent des quantités importantes de carbone dans leurs sols. Dans ces écosystèmes, la topographie locale détermine la répartition de la neige; un facteur qui, sur le court terme, affecte les paramètres physiques de l'environnement (effets directs) et qui, sur le long terme, a sélectionné des communautés végétales et microbiennes très différentes aux deux extrêmes du gradient de mésotopographie (effets indirects). Au regard des modifications futures des régimes d'enneigement prédits par les différents modèles climatiques, cette étude vise à explorer les contrôles directs et indirects exercés par l'enneigement sur la fixation du CO2 et la minéralisation du carbone organique dans les écosystèmes alpins. Les paramètres physiques des sols (eau et température) ont été mesurés pendant plusieurs années révélant les effets directs. Afin de quantifier les effets indirects de l'enneigement sur les flux biogéochimiques, nous avons utilisé les caractéristiques fonctionnelles des végétaux (leurs traits). Différentes approches (mesures in situ, manipulations expérimentales et modélisation) ont été employées. Cette étude démontre que la fixation du carbone le long des gradients de mésotopographie est à la fois déterminée par les traits fonctionnels végétaux, les propriétés des canopées et la longueur de la saison de végétation. Un allongement de la saison de végétation devrait entraîner une augmentation marquée de la production primaire si les événements de gel en début de saison de végétation demeurent limités. La minéralisation du carbone est au contraire largement dépendante de la qualité de la matière organique contenue dans les sols. Des changements de composition en traits fonctionnels de la végétation, notamment ceux affectant les concentrations en lignine des litières, devraient avoir un impact déterminant sur les vitesses de minéralisation de la matière organique. Enfin, l'étude des flux de carbone et d'azote dans les plantes dominantes et à l'interface plante – sol révèle un couplage temporel et spatial essentiel chez les espèces dont la croissance est limitée par la longueur de la saison de végétation. Ce couplage apparaît plus limité dans les communautés végétales bénéficiant d'une plus longue saison de végétation. L'évolution des flux et stocks de carbone au sein des écosystèmes alpins dans un contexte de changement climatique est discutée

Guide de détermination des habitats terrestres et marins de la typologie EUNIS - version 1.0

Ce guide est un outil d’accompagnement à l’identification des habitats avec la typologie EUNIS. Il permet de mieux appréhender cette typologie d’habitat et d’améliorer la rigueur et la reproductibilité des interprétations et identifications réalisées sur le terrain comme préalable aux inventaires, cartographies et suivis. À terme, cela permet d’entrevoir une bancarisation plus efficace des informations sur la distribution des habitats.Sont proposés :• une présentation de la typologie EUNIS (Partie A) ;• des clefs de détermination pour identifier les grands types d’habitats jusqu’au niveau 3 d’EUNIS ; ce qui est le plus souvent possible à toute période de l’année sans relevé floristique (Partie B) ;• des descriptions illustrées pour vérifier l’identification réalisée (Partie C) ;• en complément, les habitats qui peuvent représenter des objectifs particuliers de conservation sont indiqués (Annexe).Ce guide s’adresse au gestionnaire d’espaces naturels (terrestres et marins) pour évaluerles effets d’une action de restauration ou d’une pression anthropique sur les habitats d’un site, au chargé de mission qui identifie les enjeux sur un territoire avant d’y penser une stratégie de préservation de la biodiversité, à un service de l’État qui souhaite connaître si des objectifs particuliers de conservation existent vraisemblablement sur un habitat, à l’étudiant qui analyse les relations espèces/habitats... Ce guide est destiné à l’écologue et au naturaliste, sans connaissance approfondie en botanique ou en phytosociologie

Land use in subalpine grasslands affects nitrogen cycling via changes in plant community and soil microbial uptake dynamics

International audience1. Nitrogen (N) cycling is a key process determining ecosystem functioning in subalpine grasslands where traditional mowing and manuring are being abandoned. However, the roles of the plant and microbial communities in mediating changes in N availability are still poorly understood. 2. We inoculated 15 subalpine grassland fields with dual-labelled ammonium nitrate (15 NH 4 + , 15 NO 3)) during July 2005 and used pool dilutions over 1 month to calculate inorganic N fluxes into the microbial pool and uptake in plant communities by grasses, forbs and legumes. The effects of current land abandonment were assessed by comparing manured and mown terraces (ancient crop-lands) with other terraces where these practices have ceased, and mown versus unmown unterraced meadows. 3. Rapid cycling of inorganic N and high soil N availability in forb-dominated manured and mown terraces resulted from fast plant N uptake and low microbial C:N ratio. In grass-dominated unmown terraces, N cycling was slower and N retention was greater; microbial N uptake remained similar to that in the other terraces, although a higher C:N ratio suggested a shift towards fungal dominance. 4. In unterraced meadows, pH was low due to reduced mixing of soil with the underlying calcare-ous rock. Soil [NH 4 + ] was high and [NO 3) ] low, but current management had no effect on N pool size, although plant N uptake was greater in the mown than unmown fields. This may be partially explained by high N retention by dominant Festuca paniculata tussocks. The microbial N pool and N uptake were both low and the microbial C:N ratio was high, suggesting that fungi slowed N cycling and reduced the influence of mowing on N turnover. 5. Synthesis. In these marginal long-term grasslands, with low productivity and high biodiversity value, changes in ecosystem function associated with reduced management intensity were mediated through slower N cycling. This response was expressed as more gradual nutrient uptake but greater retention by unmown plant communities, slower microbial uptake and smaller soil N pools. In contrast to more productive ecosystems, such as northwestern European grasslands, reduced management is detrimental to both biodiversity and the maintenance of soil-related ecosystem services. These costs will need to be balanced against potential benefits, such as carbon storage

Guide de détermination des habitats terrestres et marins de la typologie EUNIS - version 1.0

Ce guide est un outil d’accompagnement à l’identification des habitats avec la typologie EUNIS. Il permet de mieux appréhender cette typologie d’habitat et d’améliorer la rigueur et la reproductibilité des interprétations et identifications réalisées sur le terrain comme préalable aux inventaires, cartographies et suivis. À terme, cela permet d’entrevoir une bancarisation plus efficace des informations sur la distribution des habitats.Sont proposés :• une présentation de la typologie EUNIS (Partie A) ;• des clefs de détermination pour identifier les grands types d’habitats jusqu’au niveau 3 d’EUNIS ; ce qui est le plus souvent possible à toute période de l’année sans relevé floristique (Partie B) ;• des descriptions illustrées pour vérifier l’identification réalisée (Partie C) ;• en complément, les habitats qui peuvent représenter des objectifs particuliers de conservation sont indiqués (Annexe).Ce guide s’adresse au gestionnaire d’espaces naturels (terrestres et marins) pour évaluerles effets d’une action de restauration ou d’une pression anthropique sur les habitats d’un site, au chargé de mission qui identifie les enjeux sur un territoire avant d’y penser une stratégie de préservation de la biodiversité, à un service de l’État qui souhaite connaître si des objectifs particuliers de conservation existent vraisemblablement sur un habitat, à l’étudiant qui analyse les relations espèces/habitats... Ce guide est destiné à l’écologue et au naturaliste, sans connaissance approfondie en botanique ou en phytosociologie