Effect Of PGPR On The Rhizosphere Soil Microecology Of Alfalfa Under Saline-alkali Stress

Saline-alkali stress is a primary abiotic factor adversely impacting plant growth and development.This stress type can diminish plant growth and yields,and inhibit photosynthetic processes,ultimately leading to decreased production or even plant death.Plant growth-promoting rhizobacteria(PGPR)have been demonstrated to enhance plant growth under such abiotic stresses,either directly or indirectly.Yet,there is a research gap concerning the effects of PGPR inoculation on soil microecology in the rhizosphere under saline-alkali stress.This study inculated alfalfa in the saline-alkali field with four PGPR strains,L11(Serratia marcescens),C7(Alcaligenes faecalis),M11(Enterobacter ludwigii),and L12(Acinetobacter beijerinckii).The Biolog-Ecoplate method was used to study the impact of PGPR on the carbon source metabolic activity of the soil microbial community in the alfalfa rhizosphere.High-throughput sequencing of 16 S r RNA/ITS genes was employed to uncover changes in the diversity,composition,and functional potential of the soil microbial community in the rhizosphere after PGPR inoulcation.The impact of PGPR on the physicochemical properties and enzyme activities of the rhizosphere was also analyzed.This research illuminates how PGPR inoculation under saline-alkali stress can promote plant growth from a rhizosphere microecological perspective,providing a theoretical basis for the manipulation of rhizosphere microenvironments and the application of beneficial microbial fertilizers in saline-alkali conditions.The main research results are as follows:1.Impact of PGPR on the carbon source metabolism activity of soil microorganisms in the rhizosphere of alfalfa under saline-alkali stressPGPR inoculation significantly improved the metabolic activity of carbon sources within the microbial community of the alfalfa rhizosphere under saline-alkali stress.Among different inoculation treatments,the changes in the average well color development(AWCD)rates were highest in L11 + C7,followed by L11 + M11,L11 +L12,M11 + C7,L11,C7,M11,and L12.The functional diversity of the soil microbial community in the rhizosphere under saline-alkali stress markedly increased after PGPR inoculation,as reflected by trends in the Shannon and Simpson indices consistent with AWCD values.Furthermore,PGPR inoculation significantly altered the utilization patterns of different carbon sources by the rhizosphere microbes.Additionally,the inoculation enhanced the ability of the rhizosphere microbes to utilize carbon sources such as amino acids,amines,and polymers.Among these results,PGPR co-inoculation outperformed single inoculation treatment.2.Impact of PGPR on the bacterial community in the alfalfa rhizosphere under saline-alkali stressHigh-throughput sequencing of the 16 S r RNA gene indicated that PGPR inoculation amplified the diversity and abundance of the bacterial community in the alfalfa rhizosphere under saline-alkali stress,with a more pronounced effect observed in the co-inoculation compared to the single inoculation.The inoculation altered the composition of the rhizospheric bacterial community,with distinct differences noted between single and co-inoculation treatments.Following PGPR inoculation,the relative abundance of certain bacterial taxa known to facilitate plant growth,such as Flavobacterium,Mesorhizobium,and Enterobacter,increased in the rhizosphere,with a greater abundance observed in the co-inoculation treatment.PGPR inoculation also led to an increase in the abundance of certain functional genes or pathways that contribute to maintaining normal microbial functionality,promoting plant resilience against abiotic stress.These include replication,recombination,and repair,which fall under the category of information storage and processing;signal transduction mechanisms from the cellular processes and signaling category;coenzyme transport and metabolism from the metabolism category;glycine,serine,and threonine metabolism(ko00260)from the amino acid metabolism category;and ABC transporters(ko02010)from the membrane transport category.3.Impact of PGPR on the fungal community in the alfalfa rhizosphere under saline-alkali stressHigh-throughput sequencing of the ITS gene suggested that PGPR inoculation augmented the diversity and abundance of the fungal community in the alfalfa rhizosphere under saline-alkali stress,with a greater effect seen in the co-inoculation compared to the single inoculation.The inoculation reshaped the composition of the fungal community in the rhizosphere and increased the relative abundance of beneficial fungal genera such as Trichoderma,Chaetomium,Acremonium,and Ceratobasidium,which are known to bolster plant growth.Additionally,the inoculation decreased the relative abundance of harmful fungal genera with pathogenic potential,including Rhizoctonia,Colletotrichum,Gibberella,Sarocladium,Fusarium,and Alternaria.Concurrently,the inoculation increased the abundance of nutritionally functional fungi beneficial to plant growth and quality in the rhizosphere while reducing the abundance of pathogenic fungi capable of causing diseases in plants.4.Impact of PGPR on the physicochemical properties in the alfalfa rhizosphere under saline-alkali stressPGPR inoculation significantly increased the soil moisture content and decreased the bulk density of the soil in the alfalfa rhizosphere under saline-alkali stress,altering the soil physical characteristics.This inoculation notably reduced the rhizospheric soil p H and electrical conductivity while markedly enhancing its organic matter content,and available nitrogen,potassium,and phosphorus levels.Certain physicochemical characteristics in the rhizosphere post-inoculation showed a significant correlation with the diversity and abundance of the soil microbial community.Among the results,co-inoculation with PGPR proved to be more beneficial than single inoculation.5.Impact of PGPR on the enzyme activity in the alfalfa rhizosphere under saline-alkali stressPGPR inoculation drastically increased the enzyme activities in the alfalfa rhizosphere under saline-alkali stress.The alterations in the activities of sucrase,dehydrogenase,catalase,and alkaline phosphatase followed the sequence: L11 + C7 > L11 + M11 > L11+ L12 > M11 + C7 > L11 > C7 > M11 > L12 > Control.Likewise,the modifications in urease and protease activities mirrored this sequence: L11 + C7 > L11 + M11 > M11 +C7 > L11 + L12 > L11 > M11 > C7 > L12 > Control.Certain enzyme activities in the rhizosphere displayed a significant correlation with the diversity and abundance of the soil microbial community post-inoculation.According to these findings,co-inoculation with PGPR appeared to be more advantageous than single inoculation.