Biochar induced soil microbial community change: Implications for biogeochemical cycling of carbon, nitrogen and phosphorus

It is generally accepted that biochar-C is largely unavailable to soil microbes but changes in soil physicochemical properties and the introduction of metabolically available labile-C compounds associated with the biochar may shift the soil microbial community structure. In the event that biochar becomes widely applied to agricultural soils as a soil conditioner, this research focuses on understanding how biochar application to soil influences bacterial community structure and biogeochemical function. A controlled pot experiment was designed to investigate temporal changes in soil microbial ecology and physicochemical parameters in the presence and absence of biochar. Bacterial communities were investigated in both the rhizosphere and bulk soil using TRFLP coupled with a 454 new generation sequencing database to make tentative taxonomic assignments to TRFLP peaks that showed statistically significant change over time. When comparing biochar amended soils with controls, temporal changes in bacterial family abundances that were > 5% included: Bradyrhizobiaceae (∼8%), Hyphomicrobiaceae (∼14%), Streptosporangineae (∼6%) and Thermomonosporaceae (∼8%), where the biochar had a positive influence – either promoting an increase in abundance or reducing the magnitude of loss, and; Streptomycetaceae (∼−11%) and Micromonosporaceae (∼−7%), where biochar was perceived to have a negative effect on bacterial family abundance. The Bradyrhizobiaceae and Hyphomicrobiaceae have significant involvement nitrogen cycling, with genera/species identified by 454 involved in nitrate (NO3−) denitrification through to N2. The data also suggests that organisms involved in nitrification of ammonium (NH4+) to nitrite (NO2−) are less abundant while mycobacterial nitrate reduction to NH4+ increases along with N2 fixation. Coupled to the observation that biochar can adsorb NH4+, these results provide an explanation for the reductions in N2O emissions observed from soils when biochar is added. Results from this study also indicate that biochar promotes phosphate solubilising bacteria, alters C-fluxes through increasing the abundance of bacterial families that can degrade more recalcitrant C compounds, and potentially decreases bacterial plant pathogens.