Mechanism Of Arbuscular Mycorrhizal Fungi In Improving Aluminum Tolerance Of Medicago Sativa

Alfalfa（Medicago sativa） is an excellent perennial legume which plays an important role in grass-crop rotation system, vegetation restoration and livestock farming. However, the widely distributed acidic soil limits the cultivation and promotion of alfalfa in the south of China. Therefore, expanding planting area by improving acid soil adaptability of alfalfa has great significance for the sustainable development of agriculture, animal husbandry and grass industry in the south of China. Lupulina（M. lupulina） alfalfa is annual or over-year living herb with wild and unique species adaptability. It is recognized that M. lupulina adapts well to the acidic soil than M. sativa. Besides plant resistance to acid soil, the symbiotic relationships with Rhizobium and arbuscular mycorrhizal fungi（AMF） might also benefit its wide distribution in acid soil. Therefore, in the current study, the arbuscular mycorrhizal fungus were separated from rhizosphere of M. lupulina that distributed in acid soils and were inoculated to M. sativa, aiming to estimate the efficiency of AMF inoculation in improving acid tolerance of M. sativa and provide theoretical basis for expanding planting area of M. sativa in acid soils. Soil active aluminum content ware adjusted to 3 levels(Al³⁺â… : 900mg/kg, Al³⁺â…¡:1000mg/kg, Al³⁺â…¢:1100mg/kg) by liming. The effects of the inoculation of AMF and Rhizobium on biomass, root morphology, nutrient absorption, rhizosphere soil nutrients and active aluminum content were analyzed under different soil Al³⁺ levels. The main results were as follows:AMF spores were isolated from rhizosphere soils of M. lupulina by wet sieve-sucrose centrifugation method. The appearance of the AMF spores were oval or round, with varying sizes from large and small and colors from light yellow to brown. Soil DNA was extracted and the gene fragment containing the AMF sequences were amplified with specific primers, cloned, sequenced and blasted from NCBI data bank. In total 5 AMF sequences were obtained, with Glomus the predominance genus.After lime application, soil active aluminum content decreased significantly; the plant height, branch number, aboveground biomass, underground biomass, root length, root surface area, and root volume of the two plant species were significantly improved. At the Al³⁺â… , although the soil Al³⁺ content was the lowest, the frequency of mycorrhiza（F%） of two plant species decreased when compared to Al³⁺â…¡ and Al³⁺â…¢. This suggested that added lime in acid soil could reduce the content of soil active aluminum, promote the growth and development of alfalfa, whereas excessive lime application would inhibit the efficiency of AMF infection.After inoculated with rhizobium alone, nodules could be observed only under Al³⁺â…¡ level under which higher above-ground biomass was also obtained. No nodulation was observed under the other two Al³⁺ conditions. This suggested that the efficient rhizobia inoculation could promote alfalfa growth and development in acid soil by improving the N fixation efficiency. However, the effectiveness of the nodulation was restricted by soil environments and low soil acidity or excessive lime application would limit nodulation.After inoculated with AMF alone, the aboveground biomass, underground biomass and plant phosphorus concentration in two plant species increased in most treatments. It suggested that AMF inoculation could effectively improve the absorption and utilization of nutrients by alfalfa from acid soils.After the mixed inoculation of rhizobium and AMF, the aboveground biomass, underground biomass, plant phosphorus content and nodule number of two plant species increased in most treatments. It suggested that the mixed inoculation of rhizobium and AMF could effectively improve the alfalfa aluminum resistance by improving the nodulation and the absorption and utilization of insoluble phosphate.Soil nutrient analysis indicated that the soils inoculated with AMF had higher contents of soil available phosphorus than those without AMF. It suggested that the AMF could activate the undissolved phosphorus and increase the contents of soil available P in acid soil environments. AMF inoculation reduced the soil active aluminum content only under Al³⁺â… level. This implied that the effects of AMF to the contents of active aluminum in acidic soil depended upon the soil environments.