Phosphorus (P) is an essential nutrient and limits plant growth in many soils. Since global commercially available P stocks are finite and fertilizer production costs are likely to increase, novel management strategies to secure plant P nutrition are needed. A sustainable approach is to use P-rich biochar derived from the pyrolysis of organic waste such as sewage sludge. However plant P availability in thermally treated waste is usually reduced.
Plant P availability of biochar-P can be potentially increased by means of phosphate-solubilizing microorganisms (PSM). PSM-based biofertilizers have been marketed but the mechanisms are poorly understood. The solubilization profiles of phosphate-solubilizing Penicillium strains for biochar-P were determined in vitro in order to select strains that can efficiently solubilize biochar-P and investigate the underlying mechanisms. Based on this screening, P. aculeatum (Pa) was selected and tested in two pot experiments. Wheat inoculation and subsequent persistence of Pa in biochar-amended soil resulted in an improved growth and P uptake by wheat, thus increasing the fertilizer value of biochar.
As a major part of P solubilized outside the rhizosphere is adsorbed to charged surfaces, the presence of an active mycelium of arbuscular mycorrhizal (AM) fungi was anticipated to increase plant uptake of PSM-released P outside the rhizosphere. Wheat response to co-inoculation of Pa and the AM fungus Rhizophagus irregularis was studied in a pot experiment at two levels of biochar fertilization. The inclusion of radioactive P in a root-free compartment showed that, despite the overall AM-induced growth depressions, AM transferred considerable amounts of P to the plant. Pa actively colonized the rhizosphere and bulk soil, both in the presence and absence of AM and enhanced root colonization by AM-colonization. On the other hand, AM hyphal length density was not enhanced but also not inhibited by the presence of Pa. This suggests that AM and Pa can be combined without showing antagonistic interactions. The application of biochar at a low rate also increased AM-colonized root lengths. However, these effects were not translated to increased P uptake by wheat. Wheat negative responsiveness to AM may have masked any additive effects.
Another aspect that must be taken into consideration before the application of a biofertilizer is the potential effects on the indigenous soil microbial communities, which play a key role in plant nutrient acquisition. Biochar-mediated changes of the physical and chemical properties of soil can also impact the soil microbiome. Therefore a controlled pot experiment in compartmented system was set up and 16S rRNA amplicon sequencing was performed to investigate whether biochar and/or Pa affect the bacteria community structure, and if these changes are transient and if they differ in the rhizosphere and bulk soil. Biochar added in the bulk soil-compartment stimulated specific groups of taxa both at the early and late growth stage and induced a shift in the community even in the rhizosphere soil situated in a distant compartment, whereas Pa affected the communities at a much lesser extent and only temporarily.
Taken together, these results open up for new approaches using P-solubilizing Penicillium fungi to increase the fertilizer value of P-rich biochar, but also call for future studies to investigate variations upon different experimental conditions (use of different waste materials, soils, plant species or even microbial strains). Future work on understanding the mechanisms behind the AM-Pa-biochar interactions will promote the potential of combining these in commercial biofertilizers.