Metagenomic assesment of the potential microbial nitrogen pathways in the rhizosphere of a Mediterranean forest after wildfire

Wildfires are frecuent in the forest of the Mediterranean Basin and have greatly influenced this ecosystem. Changes to the physical and chemical properties of the soil, due to fire and post-fire conditions result in alterations of both the bacterial communities and the nitrgen cycle,. We explored the effetcs of a holm-oak forest wildfire on the rhizospheric bacterial communities involved in the nitrogen cycle. Metagenomic data of the genes involved in the nitrogen cycle showed that both the undistrubed and burned rhizospheres had a conservative nitrogen cycle with a larger number of sequences related to the nitrogen incorpotation pathways and lower number for nitrogen output. However, the burned rhizosphere showed a statistically significant increase in the number of sequences for nitrogen incorporation (allantoin utilization and nitrogen fixation) and a significantly lower number of sequences for denitrification and dissimilatory nitrite reductase subsystems, possibly in order to compensate for nitrogen loss from the soil after burning. The genetic potential for nitrogen incorporation into the ecosystems was assessed through the diversity of the nitrogenase reductase enzyme, wich is encoded by the nifH gene. We found that nifH gene diversity and richness were lower in burned than in undisturbed rhizospheric soils. The structure of the bacterial communities involved in the nitrogen cycle showed a statistically significant increase of Actinobacteria and Firmicutes phyla after the wildfire. Both approaches showed the important role of Gram-positive bacteria in the ecosystem after a wilfire.