Selective enrichment of soil bacteria on the surface of an enhanced biochar

Biochar is being increasingly recognised as a supplement to agricultural soils as it improves plant productivity, and pollutant remediation as well as causing a decrease in green house gas emission. Several studies have shown that biochar also impacts onto soil microbiology in terms of biomass, activities and community structure, however the exact mechanisms underpinning these effects remain obscure. In order to better understand the role of microorganisms in biochar amended soil, we here investigate the direct micro-scale interactions of biochar particles and soil bacteria using electron microscopy and 16S rRNA gene-based analysis of bacterial diversity. We found that bacterial groups belonging to the taxa Bradyrhizobium, Streptosporangineae, Bacillales and Acidobacteriaceae have a greater relative abundance on biochar particles compared with the bulk soil phase that was either amended with biochar or not. Bacteria also form dense biofilms on the biochar surface and appear to interact in a way that could mediate direct electron transfer to the biochar structure. Based on the known functional properties of the surface-enriched bacteria and their interactions, we proposed certain biochemical reactions to occur on the biochar surface that would have a substantial impact on the nitrogen and carbon cycle of soil.