A functional approach of plant-microbe relationships in the context of nitrogen cycling. Case of mountain grasslands.
Plant communities are strongly linked with soil microbial communities through symbiotic or resource competitive interactions. Plant functional traits have been often used to study these relationships and have highlighted, for example, plant litter or root exudate effects on soil microbial communities. However, few studies have taken into account both leaf and root functional traits to elucidate plant-microbe relationships. My thesis work has focused on functional approach of aboveground and belowground plant traits and microbial functional parameters of nitrogen (N) cycle. My main objective was to understand subtle mechanisms implied in plant-microbes relationships and their impacts on ecosystem functioning. These researches have greatly benefited from experiments based on a gradient of nested scales ranging from individual to ecosystem, with an intermediary step using artificial plant communities in controlled conditions. Methodologies varied from isotope labeling of nitrogen fluxes to enzymatic activities and abundances of key microbial genes of N cycling. I have shown close relationships between plant functional traits and microbial functional parameters. I found that root functional traits were the plant functional traits to be principally implied in relationships with microbial communities related to N cycling, particularly those related to denitrifying activities. I have also shown that ecosystem functioning is the result of the influence of plant and microbial interactions, and that the influence of one or the other was modulated by soil nutrient availability. Finally, my results suggest that responses of plant species to nutrient availability, in turn influence microbial communities related to N cycling to favor either recycling or retention of nitrogen nutrients.