Augmentation of the phosphorus fertilizer value of biochar by inoculation of wheat with selected Penicillium strains

Due to uncertainty about the accessibility, amount and quality of the remaining phosphate rock stocks, a replacement of mineral phosphorus (P) fertilizers by sustainable alternatives is desirable. One possibility is to use P rich biochar derived from pyrolysis of organic wastes such as sewage sludge; however, plant P availability is usually reduced in thermally treated products. This study aimed to increase the plant availability of biochar-P by means of phosphate-solubilizing microorganisms (PSM). The solubilization profiles of four Penicillium strains for biochar-P and for calcium, iron and aluminum phosphate were determined in liquid cultures containing each respective P source. The pH, soluble P and organic anion (OA) production were measured in the culture filtrate at the end of the incubation. Subsequently, two efficient Penicillium strains were selected to investigate if the observed in vitro P solubilization could benefit wheat growth in pot experiments. The Penicillium strains differed in their ability to solubilize the four P sources and in their OA production pattern. Addition of individual major OAs to Penicillium-free liquid suspensions showed that especially citrate was closely associated with solubilization of biochar-P. Under semi-sterile soil conditions, the inoculated P. aculeatum established well and this fungus significantly increased both wheat shoot biomass and P content in the biochar-amended treatments. These results open up for new approaches using P-solubilizing Penicillium fungi to increase the fertilizer value of P-rich biochar.