Reforestation impact on soil bacterial biodiversity antagonists of fungal pathogens in Amazon biome

Abstract

Forest restoration is one of the key approaches employed to reverse environmental degradation, although there is no consensus regarding the best model to be applied for this purpose. It has been hypothesized that overall bacterial density and the diversity of bacterial antagonists in replanted areas are modulated by the reforestation model employed. To test this hypothesis, we have compared the bacterial density in soil samples from ten different replanted forest areas and from a native forest for two consecutive years, evaluated the antagonist activities of bacterial isolates from these areas against Fusarium oxysporum and Rhizoctonia solani, and identified the bacterial antagonists by 16S rRNA gene sequencing. All timber and non-timber species employed in the reforestation models were native to the Amazon biome, with the exception of eucalyptus, and additional cover plants were employed in three of the areas. No significant differences were found in bacterial densities between replanted forest areas in either of the years, but significant between-year differences were detected in five of the plots. Twenty of the 220 bacterial isolates tested showed antagonistic activity and of these three were classified within the phylum Proteobacteria and 17 were classified as Firmicutes and belonged to the genera Bacillus, Brevibacillus, Burkholderia, Paenibacillus and Variovorax. The largest microbial diversities were found in soil samples from plots that had undergone natural reforestation or had been reforested by broadcasting or mechanical sowing of a mixture of seeds from timber and non-timber species along with leguminous cover plants.