Effects Of Bacterial-phosphorus Coupling On Spatial Distribution Of Phosphorus And Photosynthetic Characteristics In Alfalfa

Objective:The inefficient use of phosphorus is a major problem in agricultural production,and the application of phosphorus solubilizing bacteria can improve plant phosphorus nutrition to a certain extent by alleviating the deficiency of available phosphorus content of soil,and at the same time,alleviate the depletion of phosphorus resources and phosphorus pollution caused by excessive application of phosphorus fertilizer.In order to explore the effects of different proportions of inoculation of Funneliformis mosseae and Bacillus megaterium on the spatial distribution and photosynthetic characteristics of phosphorus in alfalfa,and to clarify the relationship between phosphorus application and different inoculation ratios of each growth indicator,we aimed to screen the results.In order to identify the relationship between phosphorus application and the growth indicators under different inoculum ratios,the optimal inoculum ratio for alfalfa growth was selected,which provided a theoretical basis for improving the utilization efficiency of phosphorus fertilizer in soil and the development of compound biofertilizer.Methods:Two factor completely randomized block design was used in this study with alfalfa（Medicago sativa L.）.Inoculation and phosphorus application（the phosphorus fertilizer was monoammonium phosphate,containing 52%P₂O₅）were two factors,in which the inoculation was double inoculation,and5 levels were set,and the ratio of double inoculation were:3:7(the application 3 g·pot^-1of Funneliformis mosseae and 7 m L·pot^-1of Bacillus megaterium),4:6,5:5,6:4 and 7:3,respectively,which were abbreviated as J₁,J₂,J₃,J₄and J₅in turn.Two phosphorus levels were set for the application of phosphorus,namely P₂O₅:0（P₀）and 100 mg·kg^-1（P₁）.A total of 10 treatments were performed,and each treatment was repeated 10 times,with a total of 100 pots.（1）Plant height,stem thickness,stem to leaf ratio,growth rate,aboveground biomass,crude protein（CP）content,neutral detergent fiber（NDF）content and acid detergent fiber（ADF）content were measured to comprehensively evaluate the application effect of each bacterial phosphorus coupling treatment by the fuzzy similarity priority ratio method.（2）To determine the phosphorus content of the whole plant above ground,phosphorus content of different organs（stem,leaf,flower and root）,phosphorus content of different parts（upper 1/3phosphorus,middle 1/3 phosphorus and lower 1/3 phosphorus）,available phosphorus content of soil（rhizosphere and non rhizosphere soils）and phosphorus utilization efficiency,and to evaluate the optimal treatment by combining all indicators through the affiliation function analysis.（3）To determine the rate of mycorrhizal infestation,leaf anatomy（leaf thickness,vein thickness,palisade tissue thickness,spongy tissue thickness and palisade tissue thickness ratio）,net photosynthetic rate（P_n）,transpiration rate（T_r）,stomatal conductance（G_s）and intercellular CO₂concentration（C_i）and chlorophyll content,and to determine the optimal treatment for improving photosynthesis in alfalfa by principal component analysis.Results:（1）Under the same inoculation conditions,plant height,stem thickness,growth rate,total above-ground biomass and CP content of alfalfa were significantly greater（P<0.05）in P₁treatment than in P₀treatment,while stem to leaf ratio,NDF and ADF were significantly lower（P<0.05）in P₁treatment than in P₀treatment.Under the same phosphorus application conditions,plant height,stem thickness,stem to leaf ratio,growth rate,total above ground biomass,NDF and ADF of alfalfa were significantly greater in J₁,J₂,J₃and J₄treatments than in J₅treatment（P<0.05）,and CP content of alfalfa was significantly greater in J₄and J₅treatments than in J₁,J₂and J₃treatments（P<0.05）.Plant height,stem thickness,growth rate and above-ground biomass were highly significantly and positively correlated with each other in two of the four（P<0.01）,and NDF was highly significantly and positively correlated with ADF（P<0.01）.The top three treatments were J₁P₁,J₂P₁and J₄P₁,respectively,when the production performance indexes（agronomic traits,above-ground biomass and nutritional quality）of alfalfa were ranked by the fuzzy similarity priority ratio method.（2）Under the same inoculation conditions,plant phosphorus,stem phosphorus,leaf phosphorus,flower phosphorus,root phosphorus,upper phosphorus,middle phosphorus,lower phosphorus,available phosphorus content of rhizosphere and non rhizosphere soils and dry matter yield of alfalfa were significantly greater for P₁treatment than P₀treatment（P<0.05）.Under the same phosphorus application conditions,plant phosphorus,stem phosphorus,leaf phosphorus,flower phosphorus,root phosphorus,upper phosphorus,middle phosphorus and lower phosphorus of alfalfa showed significantly greater（P<0.05）for J₅treatment than J₁,J₂,J₃and J₄treatments,while dry matter yield showed significantly greater（P<0.05）for J₁,J₂,J₃and J₄treatments than J₅treatment.Among all bacterial phosphorus coupling treatments,total dry matter yield and phosphorus utilization efficiency of alfalfa reached the maximum in J₄P₁treatment with 49.31 g·pot^-1and 27.23%,respectively.The total dry matter yield of alfalfa was highly significantly and positively correlated with the available phosphorus content of rhizosphere soil and phosphorus utilization efficiency（P<0.01）.The phosphorus content of each organ of alfalfa showed as follows:flower>leaf>root>stem;the phosphorus content of each part of alfalfa showed as follows:upper part>middle part>lower part;the effective phosphorus content of soil showed as follows:rhizosphere soil>non-rhizosphere soil.（3）Mycorrhizal infestation rate,leaf thickness,leaf vein thickness,palisade tissue thickness,P_n,T_r,G_sand chlorophyll content of alfalfa were all greater in P₁treatment than in P₀treatment.Mycorrhizal infestation rate was greatest in J₂treatment;leaf thickness,leaf vein thickness,palisade tissue thickness and spongy tissue thickness were all greatest in J₅treatment;except for P_n,T_r,G_sand C_i,which were all greatest in J₂treatment;chlorophyll a,chlorophyll b and total chlorophyll content were all greatest in J₄treatment.In addition to P_n,T_r,G_sand C_iwere all maximum in the J₁treatment;chlorophyll a,chlorophyll b and total chlorophyll content were all maximum in the J₄treatment.Leaf thickness and palisade thickness were positively correlated with P_n,T_r,G_sand chlorophyll content.Principal component analysis yielded the top three treatments in order:J₁P₁,J₄P₁and J₂P₁.Conclusion:The application of Funneliformis mosseae and Bacillus megaterium in a 3:7 ratio at 100mg·kg^-1of phosphorus（P₂O₅）facilitated the colonization of mycorrhizal roots and promoted the uptake and utilization of soil phosphorus by plants to a greater extent,thus improving the anatomical structure of leaves and photosynthesis,and ultimately improving the production performance of alfalfa.