Transport Pathways of Biochar Enhanced Phosphorus Uptake to Root Surface: A Review

Phosphorus (P) deficiency is a major constraint to crop productivity, especially in tropical and subtropical regions where strong fixation by Fe and Al oxides limits its availability. Biochar has gained attention as a soil amendment capable of modifying soil properties to enhance P solubility, mobility, and uptake by plants. This review synthesizes understanding of the transport pathways through which biochar enhances P movement from soil to the root surface, focusing on diffusion, mass flow, desorption-resorption dynamics, and microbial mediation. The objectives were to 1) analyze how biochar influences soil pH, cation exchange capacity, and surface chemistry governing P availability; 2) evaluate its role in altering P transport mechanisms toward roots; and 3) assess interactions among biochar, soil microorganisms, and plant roots that support improved P acquisition. A systematic examination of peer-reviewed studies published over the past two decades was conducted. Our review revealed that biochar acts through multiple pathways, chemical (reducing fixation, contributing P), physical (enhancing diffusion and mass flow), and biological (stimulating microbes). In addition, transport processes are central, and that biochar’s role is not just for soil amendment but a facilitator of ion flux toward roots. Therefore, integrating the effect of soil chemistry, water dynamics, and microbial mediation increases the flux of ${\text{H}}_{\text{2}}{\text{PO}}_4^{？}$ and ${\text{HPO}}_4^{2？}$ . ions to the rhizosphere and improves root uptake efficiency, which is greater than any single mechanism studied in isolation. In conclusion, the review show that biochar’s impact is context-dependent, varying with soil type, biochar feedstock, and microbial community composition among others.