Exopolysaccharides from <i>Rhizobium tropici</i> Enhance the Solubility of Soil Legacy Phosphorus

Long-term application of phosphorus (P) fertilizers in the intensive croplands has led to the accumulation of residual phosphorus in soils, referred to as legacy phosphorus. Most reported observations on the role of extracellular polymeric substances in enhancing P availability have been based on microbially excreted mixtures, rather than on relatively pure exopolysaccharides, as used in the present study. The objective of this work was to investigate the effects of purified exopolysaccharides (EPS) produced from Rhizobium tropici on the solubility of soil legacy phosphorus and on their interactions with soil, clay minerals, and Fe/Al oxides. A Mississippi River Delta agricultural soil, two clay minerals (montmorillonite and kaolinite), and Fe/Al oxides were spiked with phosphate and treated with the purified EPS and then incubated under laboratory conditions at a field capacity moisture. Results show that EPS strongly interacted with phosphate, enhancing its complexation and promoting the solubilization of legacy soil phosphorus. EPS facilitated P transformation from the soil residual phosphorus fraction and HCl-extractable P (Ca–P) fractions into the water-soluble form, thereby increasing phosphorus bioavailability. Therefore, the current study demonstrates the beneficial role of EPS from Rhizobium tropici in improving the solubility and bioavailability of soil legacy phosphorus in agricultural soils where substantial phosphorus has accumulated, particularly in intensive cultivated crop fields. Application of EPS may therefore increase P fertilizer efficiency, reduce phosphorus losses through runoff, and mitigate water body eutrophication.