Mechanisms of increasing alfalfa growth and phosphorus uptake by inoculation with arbuscular mycorrhizal fungal under low phosphorus application level

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.