Abstract:

This study explored the effects of inoculation with arbuscular mycorrhizal fungi （AMF） and phosphorus-solubilizing bacteria （PSB） on the photosynthetic characteristics and biomass of alfalfa under different phosphorous application levels. The ultimate aim of this research was to provide a theoretical basis for improving the efficiency of phosphorus fertilizer utilization for the production of high-quality， high-yielding alfalfa by formulating a scientifically based and appropriate fertilization system. A pot experiment was conducted with a randomized block design. There were 16 treatments in total， consisting of four inoculation treatments ［uninoculated control （CK， J₀）， single inoculation with Bacillus megaterium （Bm， J₁）， single inoculation with Funneliformis mosseae （Fm， J₂）， and double inoculation （Bm×Fm， J₃）］ and four phosphorus （P₂O₅） application levels ［0 （P₀）， 50 （P₁）， 100 （P₂） and 150 mg·kg^-1 （P₃）］. The net photosynthetic rate （P_n）， transpiration rate （T_r）， stomatal conductance （G_s）， intercellular CO₂ concentration （C_i）， light energy use efficiency （LUE）， water use efficiency （WUE）， chlorophyll ［Chl （a+b）］ content， and biomass of alfalfa leaves were measured. Correlation analyses were conducted to clarify the relationship between each photosynthetic index and the biomass of alfalfa. The best combination of bacteria and phosphorus conditions in terms of biomass accumulation and photosynthetic characteristics of alfalfa was identified using a membership function analysis. It was found that， under the same inoculation treatment， the P_n， T_r， G_s， LUE， WUE， Chl （a+b）， and biomass all initially increased and then decreased with increasing phosphorus levels， with the maximum values in the P₂ treatment. The values of these parameters were significantly higher in the phosphorus application treatments than in the no-phosphorus treatments （P<0.05）. The value of C_i gradually decreased with increasing phosphorus levels， and its value was significantly lower in the phosphorus application treatments than in the no-phosphorus treatments （P<0.05）. Under the same phosphorus application level， the P_n， T_r， G_s， LUE， WUE， Chl （a+b）， and biomass of leaves were significantly higher in alfalfa plants inoculated with AMF and PSB than in uninoculated plants （P<0.05）. The parameters P_n， T_r， G_s， LUE， WUE， and Chl （a+b） showed maximum values in the J₃ treatment， while C_i showed the minimum value in J₃ and was significantly lower in inoculated plants than in uninoculated ones （P<0.05）. Correlation analyses showed that C_i was significantly negatively correlated with P_n， T_r， G_s， LUE， WUE， Chl （a+b） and biomass （P<0.05）， and the other parameters were significantly and positively correlated with each other （P<0.05）. A multivariate evaluation ranked the treatments on the basis of the comprehensive membership function values， and the top three were J₃P₂>J₂P₂>J₃P₁. Therefore， a phosphorus application rate of 100 mg·kg^-1 and double inoculation with AMF and PSB can significantly increase photosynthesis and the chlorophyll content of alfalfa leaves， thereby increasing the biomass of alfalfa.

Key words: alfalfa, AMF, phosphorus-solubilizing bacteria, phosphorus fertilizer application, photosynthetic characteristics, biomass