Background and aims: We sought to understand biochar's role in promoting plant phosphorus (P) access via arbuscular mycorrhizas (AM), focusing on whether P solubility and biochar-P proximity altered AM enhancement of P uptake in a mycorrhizal crop legume. Methods: A greenhouse study compared feedstock-derived P with 50 mg P pot -1 of sparingly soluble FePO 4 (Fe-P) or soluble NaH 2PO 4 (Na-P) at different proximities to biochar (co-pyrolyzed, mixed with biochar, mixed with soil) on Phaseolus vulgaris P uptake, specific root length (SRL), AM colonization, AM neutral lipids, and microbial biomass-P. Results: Biochar increased AM colonization by 6% ( p2*) with AM hyphae. Biochar-P proximity did not alter P uptake, but shifted uptake towards AM for Fe-P and roots for Na-P. Soluble P located on biochar increased total plant+microbial P ( p<0.05). Biochar reversed ( p<0.05) reductions in SRL induced by AM. Conclusions: Biochar enhanced AM's access to sparingly soluble P, and root/microbial access to soluble P. Biochar augments sparingly soluble P uptake at scales larger than biochar particles, perhaps by reducing P sorption or facilitating root/hyphal exploration.
