PGPB Colonizing Three-Year Biochar-Amended Soil: Towards Biochar-Mediated Biofertilization

Biochar porous structure can be considered a shelter for soil beneficial microorganisms, as they can be protected against grazers or competitors. At this purpose, bacteria which colonize biochar-amended soil can be isolated, characterized, and tested in combination with chars deriving from different feedstocks, which can be used as inoculum carriers. In this work, cultivable bacterial strains were isolated from a three-year maize-biochar-amended soil. Morphologically different bacterial isolates were characterized by amplified ribosomal DNA restriction analysis (ARDRA) followed by 16S rDNA sequencing of those presenting different profiles. Twelve strains were characterized by multiple plant growth–promoting properties, as siderophores and acid-3-indolacetic production, and ACC deaminase activity. An environmental scanning electron microscope was utilized to visualize in vivo bacteria biofilm formation on plant roots and biochar derived from a different feedstock. Most of the identified bacterial strains were found to be homologous with plant growth–promoting bacteria, among those Arthrobacter, Pseudomonas, Microbacterium, Bosea, and Variovorax genera, capable to synthetize high levels of IAA, produce siderophores and ACC deaminase. In addition, Bioch1, Bioch2, Bioch4, and Bioch7 strains were shown to colonize biochar deriving from a poplar wood feedstock and to create biofilms on plant roots. The results of this study indicate that beneficial soil bacteria can colonize a long-term biochar-amended soil and open the interesting possibility for the selected bacteria to be utilized as biochar carrier–mediated biofertilizers in agricultural soils.