Preparation And Application Of Nitrogen Fixation And Compound Microbial Agents In Coastal Saline Alkali Land Of The Yellow River Delta

The Yellow River Delta region is a typical coastal saline-alkali land in China.It is considered to be one of the most effective ways to improve saline-alkali soil by using microbial agents.In this study,nitrogen-fixing bacteria（Azotobacter beijerinckii）were selected from the saline-alkali soil and applied with Chlorella pyrenoidosa.The effects of bacteria and microalgae on crop growth,soil quality and soil microbial community structure were investigated.The interaction effect of bacteria and microalgae in soil was analyzed.The compound microbial agents were prepared by combining nitrogen-fixing bacteria,phosphorus-solubilizing bacteria,potassium-solubilizing bacteria,and Chlorella pyrenoidosa,and the fermentation conditions were optimized in order to provide a new method for the improvement of saline-alkali soil.The main results are as follows:（1）Two strains of autotrophic nitrogen-fixing bacteria（GN3 and GN4）were screened from the soil of wheat rhizosphere in saline-alkali land.Bacteria GN3 has a strong nitrogen-fixation ability,and it also has the ability to dissolve phosphorus,produce bacterial protein,and produce growth hormone IAA.The GN3 were identified as Azotobacter beijerinkii.The results of co-culture of A.beijerinckii with C.pyrenoidosa and their microalgal substances showed that the live C.pyrenoidosa had promoting effect on the growth and nitrogen fixation of A.beijerinckii.（2）Pot experiment was conducted to explore the effects of combined application of bacteria and microalgae on wheat growth index,wheat stress tolerance,and soil quality in saline-alkali land.Among all treatments,the combined treatment group of A.beijerinckii and live C.pyrenoidosa（BA）showed the best effect in improving wheat plant biomass and enhancing salt tolerance.The dry weight of wheat plants in the BA group was increased by 66.7%compared with the control group（CK）at 90 days.The C.pyrenoidosa alone treatment group（A）was the best in promoting wheat root length,which increased by 58.5%compared with CK at 90 days.At 30 days,the BA group better promoted the nitrogen absorption of wheat seedlings,and the total nitrogen content was increased by 69.5%compared with CK group.The N/P and K⁺/Na⁺of wheat plant were increased by 157%and 12.9%,respectively,indicating that the combined treatment with bacteria and microalgae was beneficial to alleviate the nitrogen restriction and salt stress of wheat.At 30 days,the proline content of BA group was increased by 100%compared with CK group,which improved the stress resistance of wheat plants.Compared with bacteria and microalgae alone,BA treatment group reduced soil pH and increased soil available nitrogen content.The BA group was more beneficial for improving the soil available phosphorus content,which was 42.4%higher than CK at 30 days.（3）The effects of bacteria and microalgae on soil physical and chemical properties and soil community structure were investigated under the condition of no crop planting.All treatments significantly reduced soil electrical conductivity,and the combined A.beijerinckii and live C.pyrenoidosa treatment group（GN+A）was 72.8%lower than that of CK.The GN+A group was most beneficial for improving the soil available nitrogen content,which increased by 28.8%compared to CK.The soil available phosphorus increased by 15.3%,38.4%,and 77.6%,respectively,in the treatments of single A.beijerinckii（GN）,single C.pyrenoidosa（A）,and GN+A,indicating that the combined application of bacteria and microalgae was more beneficial to the improvement of soil nutrient content.The increase of soil organic carbon content in each treatment was not significant.The A group increased the activities of soil alkaline phosphatase and sucrase,and the A and GN+A groups increased soil urease activity.After 40 days of application of bacteria and microalgae,each treatment reduced soil microbial diversity,while the GN+A group increased soil microbial richness.The dominant phyla of bacteria in soil were Proteobacteria,unidentified＿Bacteria,Firmicutes,Actinobacteria,and Bacteroidota.The GN+A group increased the relative abundance ofγ-Proteobacteria compared with the treatment of A.beijerinckii alone.（4）The effects of different fermentation conditions on the semi-solid fermentation of compound bacteria and microalgae were investigated.The total bacterial colonies in the compound bacteria+C.pyrenoidosa fermentation group（FB+A）were increased by91.4%compared with that in the compound bacteria fermentation group（FB）,indicating that the mixed fermentation of bacteria and microalgae could improve the effective viable bacteria number.The optimal fermentation conditions for bacterial agents were as follows:water content 75%,pH 7,temperature 30℃,carrier ratio 2:1:1,loading capacity 80 g/250mL.The total number of colonies was 3.64×10⁸ cfu g^-1 after low temperature drying,which met the standards of agricultural microbial agents.In conclusion,this study showed that the combined application of bacteria and microalgae had good effects on wheat growth and soil quality improvement,and provided a new effective strategy for the preparation of compound microbial agents.