Effect of various microorganisms on phosphorus uptake from insoluble Ca-phosphates by cucumber plants

Rhizospheric microorganisms can increase P availability to plants. The objective of this work was to elucidate the effects of two plant growth promoting rhizobacteria and biocontrol agents (Bacillus subtilis QST713 and B. Amyloliquefaciens FZB24), a biocontrol agent (the fungus Trichoderma asperellum T34), and Aspergillus niger CBS513.88 on P uptake from insoluble Ca-phosphates by plants. An experiment involving microbial cultivation in liquid media and three involving cultivation of cucumber plants in a siliceous growing medium fertilized with 40 mg P kg(-1) as phosphate rock (PR), a calcareous medium supplied with the same fertilizer, and one fertilized with KH2PO4 or PR at 200 mg P kg(-1) were conducted. In spite of the observed PR solubilization in liquid culture, not all the microorganisms improved P uptake by plants from this P source. The effect of each microorganism also differed depending on the plant-growing medium, revealing that its P-solubilizing activity was affected by pH and P concentration in the medium. Overall, best results were obtained with Bacillus subtilis QST713 which increased P uptake from the siliceous growing medium and the calcareous medium fertilized with 200 mg P kg(-1). Improved P nutrition of plants was the result not only of increased P solubilization, but also of enhanced ability of plants to absorb P. The other microorganisms studied provided less promising results despite the P mobilizing strategies they exhibited in the liquid culture (acidification and organic anion exudation). Therefore, these cannot be the only mechanisms contributing to P uptake by plants. Our results support the ability of B. subtilis QST713 to enhance the use of PR as a P source for calcareous soils or to improve uptake of residual P in the form of sparingly soluble Ca-phosphates.