Phosphorus solubilization in the rhizosphere in two saprolites with contrasting phosphorus fractions

Little is known about the contribution of plants and microorganisms to the release of phosphorus (P) from minerals, despite the importance of P solubilization for plant nutrition and soil formation. Therefore, the aim of this study was to assess how plants affect P solubilization and the abundance of phosphorus-solubilizing bacteria (PSB) in the rhizosphere. We conducted an experiment in which we determined the release of P, Si, Fe, and Al from two contrasting saprolites (weathered rocks) that were kept in rhizoboxes with and without the grass Nassella trichotoma. In addition, we determined the pH and the concentration of organic acids in the solution over six weeks as well as the abundance and phylogeny of cultured PSB. In the strongly-weathered (SW) saprolite, a larger proportion of P was present in the NaOH-extractable P fraction, and thus was likely associated with Al and Fe oxides and hydroxides, than in the only moderately-weathered (MW) saprolite. The plants released more P, Si, and Fe from the MW saprolite than from the SW saprolite. In contrast, more Al was released from the SW saprolite than from the MW saprolite. The total P release was increased due to plants by a factor of 1591 in the MW saprolite and by a factor of 711 in the SW saprolite. P was preferentially released from the saprolites compared to Si. The release of elements went along with high consumption of protons, particularly in the MW saprolite, indicating high rates of mineral weathering. Citric acid, which can contribute to P solubilization, was present in the MW saprolite but not in the SW saprolite. The plants maintained a high abundance of PSB in the rhizoplane (surface of the roots) in the SW saprolite, but not in the MW saprolite, where concentrations of dissolved P were higher. In conclusion, the study shows that N. trichotoma strongly increased the release of P from saprolite and enriched PSB in the rhizosphere when growing in saprolite with low P availability. Taken together, our results suggest that plants can increase P solubilization by a factor of > 1000 by preferentially releasing P from saprolite and preventing its (re-)adsorption and (re-)precipitation.