Dissolution of phosphate rock in the rhizosphere of five plant species grown in an acid, P-fixing mineral substrate

The dissolution of phosphate rock (PR) was studied in the presence and absence of plants grown in an acid mineral substrate (alumina sand) that simulates an acidic, P-fixing soil. This substrate being devoid of P and Ca, PR was the only source of both P and Ca fbr the plants, In the absence of plants, the low pH of the substrate (pH about 4) resulted in dissolution of about 8 and 30% of the applied PR, at high and low levels of application (1.0 and 0.1 mg P as PR per g of substrate), respectively. Additional dissolution of PR occurred in the presence of plant roots as indicated by a significant decrease in total Ca content in the PR-amended substrate and a concomitant accumulation of dissolved P in the rhizosphere and plant material of the five species studied. The largest root-induced dissolution was achieved by ryegrass (Lolium rigidum) and rape (Brassica napus) and amounted to 19 to 32% of the PR present in the first two mm of the rhizosphere. The root-induced dissolution of PR in this zone was thus of the same order of magnitude as the substrate-mediated dissolution, and two to three times larger for the highest level of application. The depletion of dissolved Ca which occurred in the rhizosphere of all species and particularly rape suggests that Ca uptake may have promoted PR dissolution. Rhizosphere pH increased by 3 units for rape, while little or no change in pH occurred for the other species. Nevertheless, proton excretion was suspected to have occurred for all species but rape, with a subsequent neutralization of these protons due to PB dissolution. When supplied with PR in an acid, P-fixing mineral substrate, plant roots may thus mobilize some Pas a consequence of a combination of Ca uptake and proton excretion which significantly enhances PR dissolution.