Miscanthus biochar promotes growth of spring barley and shifts bacterial community structures including phosphorus and sulfur mobilizing bacteria

• Biochar soil amendment significantly promoted growth of spring barley.
• Biochar soil amendment significantly altered bacterial community structures.
• S and P mobilizing bacterial abundances significantly increased with biochar addition.
• Stepwise regression attributed 74% of plant growth to P and S mobilizing bacteria.
• Pyrolysis of Miscanthus grass produced biochar with sulfate ester as main S source.

Biochar has shown great potentials in plant growth promotion but its effect on soil bacteria that potentially support plant growth is less well understood. In this study, the effect of biochar soil amendment was investigated on the growth of spring barley, its rhizobacteria diversity and the abundance of sulfur (S) and phosphorus (P) mobilizing bacteria. Furthermore, the S oxidation status was determined in soil, feedstock and biochar. Soil amendment with biochar made from Miscanthus x giganteus resulted in significantly increased growth of barley and grain formation four to seven fold. Further significant increases were found for the soil pH and abundances of rhizosphere soil bacteria capable of growing with tri-calcium phosphate, phosphate-esters, phosphonates or aromatic sulfonates as sole source of S or P. A stepwise regression model attributed 74% of the variation in plant growth to the abundances of P and S mobilizing bacteria. 16S rRNA gene based fingerprint analysis revealed a significant shift in the bacterial community structure upon biochar amendment that correlated significantly with the above mentioned changes (pH, plant growth, bacterial abundances). Under biochar amendment, up to 100 times increases in genera Brevundimonas known for P cycling and Arthrobacter and Cupriavidus previously involved in sulfonate desulfurization were identified via quantitative PCR. S k-edge XANES confirmed that the key S source sulfonate was largely absent in the applied biochar, thus bacterial mobilization of sulfonate-S may have originated from the soil.