Community structures of N2 -fixing bacteria associated with the phyllosphere of a Holm oak forest and their response to drought.

Biological nitrogen (N) fixation is a key pathway in terrestrial ecosystems and is therefore critical for understanding the responses of ecosystems to global environmental changes. The free-living diazotrophic community is distributed along the canopy-to-soil profile, but the ecological significance of epiphyllic N2 fixers, despite their functional relevance, on plant foliar surfaces remains very poorly understood compared with the N2 -fixing community in forest litter and soils. We assessed the community structure of N2 fixers and overall bacteria by genetic fingerprinting (t-RFLP) to explore the seasonal successional patterns of the microbial community in the natural phyllosphere of a Holm oak (Quercus ilex) forest submitted to 12-year field experiment of rain exclusion mimicking the conditions of drought projected for the coming decades. Leaves of Holm oak were analysed in different seasons over a period of 1.5 years. The bacterial community of the phyllosphere did not correspond to the surrounding soil biome in the same area. These analyses provided field evidence for the presence of free-living diazotrophs associated with the tissues of leaves of Holm oak, the dominant tree species of many Mediterranean forests. The results also revealed that the community composition is affected seasonally and inter-annually by the environment, and that the composition shifts in response to climate change. Drought treatment increased the richness of the epiphyllic microbial community, especially during the summer. These changes were associated with higher C:N ratios of leaves observed in response to drought in semiarid areas. This epiphyllic microbiota that can potentially fix N2 extends the capacity of plants to adapt to the environment.