Phosphorus Addition Alters Rhizosphere Phosphorus Mobilization Pathways in Arbuscular Mycorrhizal and Ectomycorrhizal Tree Species

Abstract

Plants associated with arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi exhibit distinct strategies for acquiring phosphorus (P), yet how these strategies respond to increased soil P availability remains poorly understood. We investigated how P addition altered rhizosphere P fractions, root-derived organic acids, and microbial PLFA groups in representative AM and ECM tree species in a subtropical region. Under control conditions, rhizosphere soils planted with AM tree species contained greater bacterial and actinomycete PLFA abundances, whereas those of ECM tree species had higher organic acid concentrations and fungal-to-bacterial PLFA ratios. These patterns indicated greater reliance on bacteria-associated processes in AM species and on fungal activity and root-derived organic acids in ECM species for rhizosphere P mobilization. The P addition increased most rhizosphere soil P fractions but markedly decreased most organic acids in both mycorrhizal types. In contrast, bacterial abundances showed no significant response to P addition. These shifts indicated that increased soil P availability reduced the relative reliance on root-derived P mobilization and increased the potential reliance on readily available soil P. Nevertheless, AM tree species remained more strongly associated with inorganic P pools, whereas ECM tree species retained greater potential for organic P mobilization. Our results highlight contrasting rhizosphere P mobilization strategies between AM and ECM tree species and demonstrate that these strategies shift with soil P availability, providing new insights into mycorrhizal regulation of rhizosphere P cycling.