Phosphorus Cycling: Prospects of Using Rhizosphere Microorganisms for Improving Phosphorus Nutrition of Plants

Abstract

Phosphorus (P) is the second most essential nutrient after nitrogen for plant growth and development. The available form of P is generally low even in fertile soils throughout the world due to high reactivity of soluble P with calcium, iron, or aluminum. Thus, major portion of applied phosphatic fertilizers in the soil is fixed into insoluble unavailable forms and the availability of P in the rhizosphere is limited. Organic matter is another important reservoir of immobilized P that accounts for 20–80 % of soil P. Phosphate-solubilizing bacteria and fungi convert insoluble fixed phosphates (both organic and inorganic) into a plant utilizable HPO₄ ²⁻ and H₂PO₄ ⁻ form. The mechanisms of phosphorus solubilization are production of organic/inorganic acids and H⁺ excretion by microorganisms. For mineralization of organic compounds in the soil, nonspecific acid phosphatases, phytases, and C–P lyases enzymes are involved. Inoculation of these phosphate-solubilizing bacteria in soil has been found to increase uptake of inorganic phosphorus, plant growth, and grain yield of different crop plants. Some of the bacterial genes involved in phosphate solubilization and mineralization of organic P sources have been characterized. The genetic engineering of bacterial strains to enhance phosphate solubilizing capacity will help in further improving the efficacy of biofertilizer inoculants for increasing crop productivity in sustainable agriculture.