Single Superphosphate-P Fractions Influence Bacterial Communities in Wheat Rhizosphere Soil Under Microcosm Conditions

The water-insoluble phases of phosphate fertilisers have traditionally been considered an agronomic concern because of their phosphorus bioavailability. However, this chemical behaviour with respect to its solubility in soil could be an advantage from the point of view of environmental sustainability, since these phosphates are more stable in soil and less susceptible to blockage by adsorption. Hence, plants could have a long-term bioavailable form of phosphate for their nutrition, which may increase the agronomic performance of the crop. However, these phosphate-based chemical molecules need to be solubilised either by the action of organic compounds exuded by plants or by microorganisms with the capacity to solubilise phosphates. Through the application of phosphorus fractions of different solubilities from single superphosphate (SSP) in soils planted with wheat plants, here we studied plant growth and nutritional content as agronomic response, as well as the influence on the bacterial communities present in the rhizosphere and their potential functionality with 16S rRNA metabarcoding and predictive functional metagenomics (PICRUSt), respectively. Results indicate a direct effect on rhizosphere bacterial communities depending on the nature and solubility of the phosphorus fractions applied. The increase in the amount of bioavailable phosphorus in soil (+1.57 to 2.21 mg·kg⁻¹), together with the increase in bacterial genes related to phosphate solubilisation after the application of slightly soluble or non-soluble forms of phosphate, suggests that the presence of these forms of phosphate may stimulate the activity of phosphate-solubilising microorganisms. However, by comparing rhizosphere soil samples with bulk soil, we observed that plant roots may have a higher impact on edaphic bacterial biodiversity and functionality than the application of different forms of phosphates. Therefore, future studies are needed to elucidate direct actions of phosphate forms on rhizosphere microorganisms, indirect effects caused by plant root exudates and the combined effect of both.