Effects Of Mixed Nitrogen Fixation Strain Inoculation On Sugarcane Growth And Soil Microbial Community

Sugarcane is the most importamt sugar crop in China and the most important cash crop in Guangxi.For the past few years,with the increasing overuse of chemical fertilizer,a lot of problems have been brought to the farmland soil.Soil microorganisms can improve the soil ecosystem and promote the circulation and transformation of soil nutrients.A large number of studies have proved that plant growth promoting rhizobacteria(PGPR)can improve the soil rhizosphere microbial community,colonize the plant,and promote plant growth and development.Therefore,it is of great significance to study the effects of PGPR on sugarcane growth and rhizosphere soil microenvironment to enhance soil fertility effectively and the environment,and regulate soilecosystem.In this paper,4 strains of PGPR CA1(Bacillus mycoides),CN11(P.entomophila),DX120E(klebsiella sp.)and WZS021(Streptomyces chartreusi)were selected for single and combination test.By using the biological characteristics of PGPR strains,the optimal combination was obtained through factor analysis,and then pot planting test was conducted.Three optimal strain combinations(CA1+CN11,CN11+WZS021,WZS021)and PAL5(Gluconacetobacter diazotrophicus)were selected to apply in two sugarcane varieties(B8,ROC22).The changes of plant height,chlorophyll content,enzyme activity in sugarcane and rhizosphere soil microbial community were measured at different time after inoculation.The effects of different PGPR inoculation treatments on the growth of sugarcane at different stages and the activity of rhizosphere soil enzymes,the soil nutrients and the structure and function of soil bacterial community were discussed.The main results were as follows.1.The chlorophyll content in both sugarcane varieties increased after PGPR treatment.The chlorophyll content in the variety ROC22 changed in a unimodal curve which showed increasing firstly and then decreasing within whole growth duration.The chlorophyll content in the variety B8 kept rising.After the inoculation treatment for 120 d,the plant height in the two varieties was significantly higher than that in the control,but the difference between the PGPR treatments was not significant.Thedry weight of the above and underground parts of sugarcane increased,and the dry weight of CA1+CN11 and PAL5 increased most significantly.2.The activities of nitrate reductase(NR),glutamine synthetase(GS),glutamic pyruvate transaminase(GPT)and glutamic oxaloacetate transaminase(GOT)in leaves of the two sugarcane varieties were higher than those in the control.The highest nitrate reductase activity in B8 was found in CN11+WZS021 treatment,which was 10.68% higher than that in control.For ROC22,the nitrate reductase activity in CA1+CN11treatment was significantly higher than that in control by 17.77%.The activity of glutamine synthase in the two sugarcane varieties increased with time and then decreased,and reached the maximum at 90 d.The enzyme activity of GOT and GPT in the two sugarcane varieties showed a single peak change,and that of GOT reached the maximum at 90 d while that of GPT reached the maximum at 60 d.3.The contents of available potassium,hydrolyzed nitrogen and available phosphorus were significantly increased in the PGPR treatments as compared with the control.The highest content of available nutrients in soil was found in PAL5 treatment,followed by CA1+CN11 treatment.The activities of catalase,alkaline phosphatase and urease in soil were significantly increased.The highest soil catalase activity for B8 variety was found in WZS021 treatment,which was 1.07 times higher than that in control.The highest enzyme activity for ROC22 variety was observedin CN11+WZS021 treatment,which was 62.61% higher than that in control.The highest activity of alkaline phosphatase in the soil for B8 variety was found in WZS021 treatment,which was 2.45 times higher than that in control.The highest activity of alkaline phosphatase in the soil for B8 variety was found in WZS021 treatment,which was 2.45 times higher than that in control.The highest activity of alkaline phosphatase in the soil for ROC22 was observed in PAL5 treatment.The activity of urease in the soil for PGPR treatment was higher than that in control.The highest activity of urease was found in PAL5 treatment,which was 88.59% higher than that in control,and 39.10%-78.14%increases were observed in the other PGPR treatments.The highest activity of urease for ROC22 was found in CA1+CN11 treatment,which was 64.77% higher than that in control.4.The full-length 16 S r RNA sequence of sugarcane rhizosphere bacteria was determined by Illumina Mi Seq high-throughput sequencing technology,so as to analyze the diversity and structure of sugarcane rhizosphere soil bacterial community in different PGPR treatments.based on the quality data of sequencing,a total of 294,239 valid sequences and9,803 OTU numbers were obtained from the sequencing at the early stage(seedling stage)after PGPR treatment.At the later stage(elongation stage),424373 effective sequences and 12215 OTU numbers were obtained from the soil.Alpha diversity analysis show that the diversityindex for WZS021 treatment was the highest in ROC22 at the early stage PAL5 treatment was the highest in ROC22 at the later stage,and the diversity index of soil for B8 was the highest in CA1+CN11 treatment.The composition of bacterial community in rhizosphere soil was analyzed,and it was found that the dominant phyla in the soil samples of the two stages are Actinobacteria,Chlorofexi,Proteobacteriaes and Firmicutes,and the dominant phyla all contain the genera of Microvirga and Bacillus.In summary,PGPR can improve the rhizosphere soil environment,increase soil nutrient content,and increase soil enzyme activity and soil organic matter.At the same time,the application of PGPR can affect the structure and diversity of soil bacterial community,promote the colonization of dominant rhizosphere bacteria,and thus affect the growth and development of plants.Therefore,the results of this study provide a theoretical basis for the growth promotion by applying different PGPR strains to increase and stabilize sugarcane yield and improve the efficiency of fertilizer while reducing the application rate in sugarcane production.