Effect And Mechanism Of Phosphate-Solubilizing Bacteria And Their Co-Activation With Chinese Milk Vetch On Insoluble Phosphorus

Astragalus sinicus（Chinese milk vetch,CMV）is the most important leguminous green manure crop in paddy fields in southern China with the ability to activate and absorb insoluble phosphorus by roots.Phosphate-solubilizing bacteria（PSB）can activate insoluble phosphorus in soil into available bioavailable phosphorus which can be absorbed and utilized by plants.It is of great significance to improve the phosphorus resources utilization efficiency by inoculating phosphorus-solubilizing bacteria into soil during CMV growing season and exerting the biological potential of co-activation of insoluble phosphorus by CMV and PSB.In this study,two strains of PSB were used to carry out simulation and pot experiments in the southern red soil paddy soil commonly used in phosphate rock powder.PSB,phosphate rock powder particle size and PSB,CMV varieties were set up respectively.The composition of soil alkaline phosphatase functional gene（phoD）bacterial community under the combined action of the two was analyzed by molecular biology methods,and the effect and mechanism of PSB and their co-activation with CMV on insoluble phosphorus were studied.The main results were as follows:（1）In the incubation experiment,Acinetobacter calcoaceticus（P1）and Acinetobacter pittii（P2）could activate insoluble phosphorus in soil and phosphate powder,with no significant activation effect.During the whole culture period,the average available phosphorus of inoculated PSB treatment under 0.18 mm,0.10 mm and 0.05 mm particle sizes was 13.4～14.7 mg·kg^-1,which was higher than that in the non-inoculated（P0）treatment and increased by 31.1%～53.1%（p<0.05）.The active phosphorus of inoculated PSB treatment increased by 15.0%～46.7%,and stable phosphorus decreased by 1.2%～12.2%.Compared with the P0 treatment,inoculation of PSB significantly reduced pH value by 0.18～0.35 units（p<0.05）and increased acetic acid and propionic acid contents by 5.2%～13.7%and 45.9%～127.5%.Alkaline phosphatase content and phoD abundance in P1 treatment increased by 6.5%～13.4%and 24.0%～98.6%（p<0.05）,respectively,and acid phosphatase activity in P2 treatment increased by 12.8%～17.2%（p<0.05）which indicating that P1 mainly secreted alkaline phosphatase and P2 mainly secreted acid phosphatase.The structural equation model showed that inoculation of PSB and particle size of phosphate rock powder could directly increase the content of available phosphorus,and PSB had a greater impact on available phosphorus.（2）In a pot experiment,the cooperation of A.calcoaceticus（P1）with different varieties of CMV could activate insoluble phosphorus,increased the soil active phosphorus pool,and improved the phosphorus absorption of CMV.The phosphorus uptake in P1 treatment was 16.3～26.6 mg·pot^-1,which was 48.5%～125.8%higher than that under P0 treatment（p<0.05）.The soil available phosphorus in P1 treatment was 7.1～8.4 mg·kg^-1,which was higher than that in P0 treatment,and increased by10.5%～17.4%（p<0.05）.Active phosphorus increased by 2.3%～12.1%,while moderate active phosphorus and stable phosphorus decreased by 3.5%～12.6%and 0.9%～9.4%,respectively.P1 treatment also increased the contents of organic acids such as oxalic acid and phosphatases activities.Among all the varieties,the oxalic acid content of the Yujiang Daye increased by 21.2%（p<0.05）,and the activities of acid phosphatase and alkaline phosphatase were increased by 2.5%and 1.8%（p<0.05）,respectively.The results of structural equation model showed that both CMV and PSB could directly promote the phosphorus absorption of CMV,and PSB had a greater effect on the phosphorus absorption of CMV.（3）In a pot experiment,inoculation of A.calcoaceticus（P1）increased the abundance of the inoculated A.calcoaceticus and phoD functional gene in soil of different varieties of CMV,and changed the microbial community structure of phoD thus promoting the activation of insoluble phosphorus in soil.Compared with P0 treatment,the abundance of A.calcoaceticus and phoD functional gene in P1 treatment increased by 56.3%～260.5%and 56.4%～107.4%,respectively,among which,the abundance of A.calcoaceticus and phoD functional genes in Yujiang Daye variety increased by 220.8%and 107.4%,respectively（p<0.05）.The chao 1 index,observed species and shannon index of phoD microbial community in Yujiang Daye variety increased 19.4%,12.3%and 23.7%,respectively（p<0.05）.Under the concomitance effect of A.calcoaceticus and varieties of CMV,the main phoD microorganisms in soil were Proteobacteria and Actinobacteria,and the family levels were Xanthomonadaceae,Streptomycetaceae and Rubrobacteraceae.Co-occurrence network analysis showed that the species significantly related to phosphorus availability in the core module belonged to Streptomycetaceae,Xanthomonadaceae,Rhodobacteraceae and Acetobacteraceae,respectively.Inoculation of A.calcoaceticus could increase phosphorus availability in soil by changing the abundance of phoD-containing Streptomycetaceae and Xanthomonadaceae phosphate-solubilizing microorganisms in soil.