Evaluation of Bacillus seed coatings on soybean phosphorus uptake in an oxisol fertilized with 32 P-labeled hydroxyapatite

Phosphorus (P)-solubilizing microorganisms (e.g. Bacillus species) paired with modern seed coating techniques are promising biological tools to promote crop growth via introduction of beneficial microorganisms that effectively colonize the rhizosphere. This study evaluated P uptake of four soybean seed treatments in an oxisol fertilized with isotopically (32P) labeled hydroxyapatite. Seeds included: no coating (NC), fungicidal coating (FC), Bacillus coating (BC), and fungicidal + Bacillus coating (FBC). Bacterial strain B. velezensis EX180863 was used in the study. Treatments also included soil with and without lime amendment (as Ca(OH)2), pH ~ 6 and ~ 4.8, respectively. Soybeans were grown in a growth chamber. A non-destructive method was used to measure 32P uptake from labeled fertilizer during the soybean vegetative stages by autoradiography. Plants were harvested before entering reproductive stages and plant performance, together with various post-harvest soil properties, were evaluated at harvest. Total P and radioisotope (32P) in plant tissues were quantified using different techniques to distinguish soil- and fertilizer-derived P. Without lime, BC and FBC treatment assimilated more 32P than NC, starting from late vegetative stage (V4). Assuming the ratio of 32P/31P was constant when P moved from the labeled hydroxyapatite to plant tissues, BC and FBC contained 50–60 % more fertilizer derived P than NC at harvest. Post-harvest soil pH decreased by ~ 0.2 units from BC and FBC compared to NC. In contrast, when limed, soybean plants from BC seeds did not take up more P, but the bacteria enhanced root biomass. Effect of B. velezensis (EX180863) coatings on soybean growth were pH dependent with FBC providing additional benefits compared to BC alone: when unlimed, P uptake was maximized under FBC while FBC had the greatest root biomass when liming the soil.