Comparative effect of ion uptake and root growth patterns of a forage legume and grass on rhizosphere acidification and solubilization of phosphate rocks

Dissolution of phosphate rocks (PR) in soils requires an adequate supply of acid (H+) and the removal of the dissolved products (Ca 2+ and H2PO4-). Plant roots may excrete H+ or OH- in quantities that are stoichiometrically equal to excess cation uptake in order to maintain internal electroneutrality. Extrusion of H+ or OH- may affect rhizosphere pH and PR dissolution. Differences in rhizosphere acidity and solubilization of three PRs were compared with triple superphosphate between a grass (Brachiaria decumbens) and legume ( Stylosanthes guianensis) forage species at two pH levels (4.9 and 5.8) in a P-a Ca-deficient Ultisol. The experiment was performed in a growth chamber with pots designed to isolate rhizosphere and non-rhizosphere soil. Assessment of P solubility wit chemical extractants led to ranking the PRs investigated as either low (Monte Fresco) or high solubility (Riecito and North Carolina). Solubilization of the PRs was influenced by both forage species and mineral composition of the PR. The low solubility PR had less, apatite and a higher content of calcite than the high solubility PRs, which led to increased soil pH values (>7.0) and exchangeable Ca and reduced relative agronomic effective (RAE) of the PR. Rhizosphere soil pH decreased under Stylosanthes but increased under Brachiaria. The greater ability of Stylosanthes to acidify rhizosphere soil and solubilize PR relative to Brachiaria is attributed to differences between species in net ion uptake and root surface area. Stylosanthes had a larger root surface area than Brachiaria, and an excess cation uptake, defined by a large Ca uptake and its dependence on N2 fixation, which induced a significant H+ extrusion from roots to maintain cell electroneutrality. Brachiaria had an excess of anion uptake, with NO 3- comprising 92% of total anion uptake. Nitrate and SO42- reduction in Brachiaria root cells may have generated a significant amount of cytoplasmic OH-, which could have increased cytoplasmic PH and induced synthesis of organic acids and OH- extrusion from roots. Nodulation and total N content in Stylosanthes increased as solubility of PR was enhanced by rhizosphere acidification. Soil acidification under Stylosanthes had a negative affect on total soil bacterial growth (CFU), but promoted P-solubilizing bacteria (PSB). Total bacterial growth was favored by high values of soil pH (pH > 7) and exchangeable Ca. Increased total soil microbial populations did not lead to corresponding increases in soil acidification or PR solubilization.