From Legacy Phosphorus to Resource via Immobilization and Activation for Enhanced Fertilizer Efficiency

Phosphorus (P) is an essential macronutrient central to nucleic acid synthesis, photosynthesis, and energy metabolism. Although agricultural soils contain substantial total P reserves, only a minute fraction is plant-available, leaving more than 40% of global croplands P-deficient. Fertilizer supplementation is therefore indispensable; however, most applied phosphate is rapidly immobilized through precipitation with Ca, Al, or Fe minerals or by strong sorption to soil matrices, resulting in crop recovery efficiencies typically below 25%. This inefficiency has generated extensive residual and legacy P pools that can buffer crop demand yet also contribute to eutrophication when mobilized into surface waters. This review explores the dynamics of these hidden soil P reserves and evaluates emerging strategies to transform them into renewable agronomic resources. Engineering approaches including biochar amendments and bioelectrochemical systems such as soil, sediment, and plant microbial fuel cells, can restrict dissolved P losses, induce redox-mediated immobilization, and support controlled P recycling. Complementary biological pathways mediated by phosphate-solubilizing microorganisms and rhizospheric enzyme systems reactivate immobilized or organic P, thereby improving plant nutrient acquisition and fertilizer-use efficiency. Framed within a phosphorus cascade perspective, this synthesis highlights how integrating soil chemistry, microbial ecology, and environmental engineering can enhance P utilization, sustain crop productivity, and mitigate nutrient driven water pollution.