Impact de la densité des peuplements forestiers sur la biodiversité et le fonctionnement biologique de l’interface sol-végétation

De nouvelles contraintes exercées par le réchauffement climatique font actuellement pression sur les écosystèmes forestiers et vont imposer une adaptation de leur gestion sylvicole pour préserver leur santé. Ainsi, le plan forestier national incite les gestionnaires forestiers à appliquer un régime d’éclaircie plus intensif. Ceci devrait permettre d’atténuer le risque de stress hydrique accru par des sécheresses estivales qui s’annoncent plus fréquentes à l’avenir. Cependant, peu d’études ont encore évalué l’impact potentiel de cette dynamisation de la gestion forestière sur la biodiversité et le fonctionnement du sol : c’est l’objet de mon travail de thèse. Les premiers résultats montrent que la structure du réseau trophique du sol est impactée par l’intensification des éclaircies. En effet, l’abondance de plusieurs groupes d’organismes détritivores, à savoir les collemboles, les acariens et les diplopodes baisse lorsque la densité du peuplement diminue. La communauté des vers de terre est également affectée, l’abondance des endogés augmentant avec la réduction de la densité du peuplement alors que celle des anéciques diminue. Ceci pourrait avoir des conséquences importantes en termes de décomposition des litières et de recyclage des nutriments, fonctions essentielles pour la fertilité des sols forestiers

Rhizosphere control of soil nitrogen cycling: a key component of plant economic strategies

Understanding how plant species influence soil nutrient cycling is a major theme in terrestrial ecosystem ecology. However, the prevailing paradigm has mostly focused on litter decomposition, while rhizosphere effects on soil organic matter (SOM) decomposition have attracted little attention. Using a dual(13)C/N-15 labeling approach in a 'common garden' glasshouse experiment, we investigated how the economic strategies of 12 grassland plant species (graminoids, forbs and legumes) drive soil nitrogen (N) cycling via rhizosphere processes, and how this in turn affects plant N acquisition and growth. Acquisitive species with higher photosynthesis, carbon rhizodeposition and N uptake than conservative species induced a stronger acceleration of soil N cycling through rhizosphere priming of SOM decomposition. This allowed them to take up larger amounts of N and allocate it above ground to promote photosynthesis, thereby sustaining their faster growth. The N-2-fixation ability of legumes enhanced rhizosphere priming by promoting photosynthesis and rhizodeposition. Our study demonstrates that the economic strategies of plant species regulate a plant-soil carbon-nitrogen feedback operating through the rhizosphere. These findings provide novel mechanistic insights into how plant species with contrasting economic strategies sustain their nutrition and growth through regulating the cycling of nutrients by soil microbes in their rhizosphere

La gestion de la fertilité des sols forestiers est-elle à un tournant ? Une conclusion transitoire

La gestion de la fertilité des sols forestiers est-elle à un tournant ? Une conclusion transitoir

Is the management of forest soil fertility at a turning point? A brief conclusion

Is the management of forest soil fertility at a turning point? A brief conclusio

Étude de l’influence de la densité de peuplement sur le fonctionnement du sol dans des chênaies par la mesuredes rapports isotopiques du carbone (13C/12C) et de l’azote (15N/14N) en EA-IRMS

International audienc

Impacts of forest stand density on the biodiversity and functioning of the plant-soil interface

Global warming imposes new constraints on forest ecosystems and induces forest management adaptation. French forest policy currently advocates more intensive thinning so as to mitigate the increased risk of water stress from more frequent summer droughts. However, few studies have yet assessed the potential impact of these intensive forest management practices on soil biodiversity, which is the subject of my thesis. The first results show that the soil food web structure is affected by more intensive thinning. Indeed, the abundance of several soil detritivore groups, i.e. springtails, mites and millipedes is negatively impacted by stand density reduction. The earthworm community is also affected, as endogeic abundance increases with stand density reduction while anecic abundance decreases. This could have important consequences in terms of litter decomposition and nutrient cycling, which are essential functions for forest soil fertility

Plant economic strategies of grassland species control soil carbon dynamics through rhizodeposition

The plant economics spectrum is increasingly recognized as a major determinant of plant species effects on terrestrial ecosystem functioning related to carbon cycling. However, the role of plant economic strategies in the effects of living root activity on soil organic carbon (SOC) dynamics through rhizodeposition remains unexplored, despite SOC being the largest terrestrial carbon pool. Using a continuous C-13-labelling method allowing partitioning of plant and soil sources to carbon fluxes and pools, we studied here the linkages between plant economic strategies and SOC cycling processes in a 'common garden' greenhouse experiment. It includes a panel of 12 grassland species selected along a gradient of economic traits and belonging to three functionnal groups (C3 grasses, forbs and legumes). All species induced an acceleration of native SOC mineralization but this rhizosphere priming effect (RPE) substantially differed across species and varied eleven-fold by the end of the experiment (from +26% to +295% relative to unplanted soil). Interspecific variation in RPE was primarily linked to plant photosynthetic activity associated to species economic strategies of light and CO2 resource acquisition and processing. Fast-growing acquisitive species, such as legumes, featured large RPE, in relation with their high canopy photosynthesis coupled to high leaf photosynthetic capacity and large net primary productivity allocated above-ground. This large RPE was further associated with high root metabolic activity, rhizodeposition and soil microbial activity. In contrast, fine-root growth and economic traits related to soil resource foraging ability were poor predictors of RPE. The formation of new root-derived SOC varied nine-fold across species and was similarly positively related to the net primary productivity allocated above-ground. Fast-growing acquisitive species with a high photosynthetic activity induced a disproportionately large RPE relative to SOC formation. Synthesis. Overall, our study demonstrates that rhizodeposition is a major mechanism through which plant economic strategies of grassland species control soil carbon dynamics. Acquisitive versus conservative species were associated with high versus low rates of photosynthesis and rhizodeposition, in turn leading to fast versus slow SOC turnover. This emphasizes the importance of considering rhizosphere processes for understanding plant species effects on soil biogeochemistry

Habitat diversity associated with island size and environmental filtering control the species richness of rock-savanna plants in neotropical inselbergs

International audienceDisentangling the multiple factors controlling species diversity is a major challenge in ecology. Island biogeography and environmental filtering are two influential theories emphasizing respectively island size and isolation, and the abiotic environment, as key drivers of species richness. However, few attempts have been made to quantify their relative importance and investigate their mechanistic basis. Here, we applied structural equation modelling, a powerful method allowing test of complex hypotheses involving multiple and indirect effects, on an island-like system of 22 French Guianan neotropical inselbergs covered with rock-savanna. We separated the effects of size (rock-savanna area), isolation (density of surrounding inselbergs), environmental filtering (rainfall, altitude) and dispersal filtering (forest-matrix openness) on the species richness of all plants and of various ecological groups (terrestrial versus epiphytic, small-scale versus large-scale dispersal species). We showed that the species richness of all plants and terrestrial species was mainly explained by the size of rock-savanna vegetation patches, with increasing richness associated with higher rock-savanna area, while inselberg isolation and forest-matrix openness had no measurable effect. This size effect was mediated by an increase in terrestrial-habitat diversity, even after accounting for increased sampling effort. The richness of epiphytic species was mainly explained by environmental filtering, with a positive effect of rainfall and altitude, but also by a positive size effect mediated by enhanced woody-plant species richness. Inselberg size and environmental filtering both explained the richness of small-scale and large-scale dispersal species, but these ecological groups responded in opposite directions to altitude and rainfall, that is positively for large-scale and negatively for small-scale dispersal species. Our study revealed both habitat diversity associated with island size and environmental filtering as major drivers of neotropical inselberg plant diversity and showed the importance of plant species growth form and dispersal ability to explain the relative importance of each driver

Plant N economics and the extended phenotype: Integrating the functional traits of plants and associated soil biota into plant‐plant interactions

International audienceThe nitrogen (N) economics of plants are generally described in terms of functional traits and how these affect N availability in a given environment. However, recent studies have shown that plant symbionts play a crucial role in plant N economics. A plant together with its symbiont can be considered as a meta-organism, the holobiont. Plant-associated symbionts are shaped by the plant, thereby extending the plant's phenotype. Decomposers also play an important role in plant N economics yet are usually not included in the plant holobiont. In this review, we show the important roles that both symbionts and decomposers play in plant N economics. We focus on how plants respond to fluctuating N availability in a complex interaction network, which includes the plant's strategies and its interactions and feedback loops with the soil biota and with neighbouring plants, through competition for N by exploitation and interference. Synthesis: Plant N economics and the outcome of plant-plant interactions in a community cannot be fully described solely through the functional traits of plant individuals. Properties emerging from the interaction network bring new insights into plant N economics. Further research is now needed to gain a deeper understanding of plant N economics and resource economics in plant communities by integrating a broader extended plant phenotype

Impact des pratiques culturales de l’agriculture conventionnelle, l’agriculture biologique et l’agriculture de conservation sur les microorganismes et les nématodes du sol

Impact des pratiques culturales de l’agriculture conventionnelle, l’agriculture biologique et l’agriculture de conservation sur les microorganismes et les nématodes du sol. 11. Journées d'Etude des Sols (JES