Objectives Phosphorus (P) is easily adsorbed and fixed by soil, resulting in low P availability in soil. This research investigated the interactive effects of AMF and low levels of P fertilizer on alfalfa growth and P-uptake to identify optimal methods for increasing P efficiency in alkaline soil.

Methods A pot experiment was conducted using loessial soil and alfalfa (Medicago sativa) as test materials. The soil was treated with different P levels (0, 5 and 20 mg/kg, code as P0, P5, P20), and with AMF inoculation (20 g/L Glomus mosseae BGC YN02; +AMF) or without (–AMF). After 120 days of plant growth, the growth indices of alfalfa, P concentrations in alfalfa tissues, pH values, alkaline phosphatase activities, available P contents, and microbial biomass P in the rhizosphere and non-rhizosphere soil were analyzed. The composition and concentration of rhizosphere organic acids were also measured.

Results In +AMF treatments, alfalfa roots were colonized by mycorrhiza, but P application rate had no significant effect on AMF root colonization. Both P application and AMF colonization significantly increased the dry mass and P concentrations of shoots and roots. The plant dry mass and P concentration in P20+AMF treatment were significantly higher than in the other treatments. The total amount of rhizosphere organic acids, especially citrate and acetate, in +AMF treatments decreased with increase in P application rate, but they were still significantly higher than those in −AMF treatments. Phosphorus and AMF application significantly decreased soil alkaline phosphatase activity, but increased soil available-P and microbial biomass P contents. Alkaline phosphatase activity and microbial biomass P in rhizosphere were significantly higher than in non-rhizosphere. P20 trentment significantly decreased P-uptake efficiency and P-utilization efficiency, and +AMF significantly increased P-uptake efficiency.

Conclusions AMF has a good symbiotic relationship with alfalfa roots in alkaline loessial soil. Low level of P application (≤ 20 mg/kg) has no significant effect on AMF root colonization. Both P application and AMF colonization could significantly stimulate the growth and P uptake of alfalfa. In low available P soil, inoculating AMF could broaden absorption area of root, increase alkaline phosphatase activity in rhizosphere and secretion of organic acids (especially citrate and acetate), and increase P-uptake efficiency.