Mycorrhiza Inoculation and Soil-Available Phosphorus Modulate Phosphorus Acquisition Strategies of Alfalfa

The comprehensive understanding of how arbuscular mycorrhizal fungi (AMF) and phosphorus (P) application affect P uptake by alfalfa roots is poorly understood. We analyzed the effects of AMF inoculation and P application levels on the trade-offs and variations in P acquisition strategies of alfalfa by assessing plant growth, root morphology, organic acid contents, phosphatase activity, root transcriptome, and soil P forms. AMF inoculation increased five organic acid contents (glutaric acid, 3-hydroxymethylglutaric acid, gallic acid, 2-hydroxyphenylacetic acid, and fumaric acid) and phosphatase activity synthesized by alfalfa and secreted by the roots and hyphae, which activated soil Ca₂-P, Fe-P, and organic P, promoting root growth for P absorption and biomass. Without AMF inoculation, plant biomass, P accumulation, and root growth increased with increasing P application, and organic acid contents were higher at P20 and P40 (20 and 40 mg P kg^-1 dry soil). AMF colonization and phosphatase played a crucial role in P acquisition under P-limiting conditions.