Study On Rhizosphere Nutrient Regulation Mechanism Of Dominant Plants In Degraded Alpine Meadow On The Eastern Edge Of Qilian Mountains

As one of the typical alpine ecosystems with the largest area and most widespread distribution,alpine meadows were of great significance in protecting the ecological environment.However,due to the unreasonable reclamation and utilization of human beings,its degradation was serious.The degradation of alpine meadows was directly related to the material circulation and energy flow in the grassland ecosystem.As a special kind of micro-ecosystem,the rhizosphere was not only the node of material and energy exchange between plants and the soil environment,but also active areas for the circulation of various nutrients and important sites for nutrient absorption and metabolism during plant growth and development.Therefore,the nutrient regulation and transformation mechanism in the grassland rhizosphere process was of great significance for maintaining grassland health and ecological restoration.However,there were relatively few studies on the nutrient interaction mechanism between plant-soil-nutrients in the rhizosphere process of degraded alpine meadow plants.In view of this,this study took four alpine meadows with different degradation degrees(Non-degraded,Lightly degraded,Moderately degraded,and Severely degraded)on the eastern edge of Qilian Mountains as the research object,and collected the dominant plant rhizosphere soil(RS<2 mm)and bulk soil(BS>2 mm)soil samples of each plot.Use LC-MS technology,soil enzyme microplate detection technology and High-Throughput Sequencing technology to conduct detailed research on plant-soil-microbial rhizosphere process,including soil nutrient status,microbial biomass,extracellular enzyme activity,root exudates and soil microbial community structure diversity and other aspects.At the same time,the mechanism of plant-soil-microbial nutrient regulation and transformation mediated by root exudates of dominant plants in degraded alpine meadow was revealed through the construction of molecular ecological network model.The main conclusions are as follows:(1)With the increase of grassland degradation,the vegetation height,coverage,and above-ground biomass in the four degraded alpine meadows showed a decreasing trend,showing ND>LD>MD>SD,and plant community diversity was the most abundant in severely degraded plots.The aggravation of grassland degradation led to the gradual decrease of nutrient content and microbial biomass carbon,nitrogen and phosphorus in rhizosphere and bulk soil.At the same time,the activities of ?-1,4-glucosidase and?-1,4-xylosidase involved in carbon cycle,?-1,4-N-acetylglucosaminidase involved in nitrogen cycle,Acid phosphatase involved in phosphorus cycle,and Dehydrogenase,Polyphenol oxidase and Catalase involved in nutrient activation in alpine meadow rhizosphere and bulk soil decreased with the aggravation of degradation degree,and the activities of Leucine aminopeptidase and Cellobiohydrolase involved in nitrogen and carbon cycles were improved.Meanwhile,all of these soil nutrient factors showed rhizosphere>bulk soil,and rhizosphere nutrient enrichment was occurred.(2)Along with the degradation degree,the C:N:P of soil was changed significantly,and resulted in a serious imbalance of C:N and severe N restriction.In the degraded alpine meadows,the ratio of log-transformed rhizosphere C-,N-and P-extracellular enzymes deviated from the 1:1:1 of global ecosystem,which indicated that nutrient supply was mainly restricted by N and followed by P.The contents of soil total nutrients in degraded alpine meadow was relatively high,but the contents of available nutrients in soil were low,which was the limiting factor hindering the growth of forage plants.(3)Grassland degradation leads to changes in rhizosphere and bulk soil microbial alpha diversity and community structure,which shows a decreasing trend with the aggravation of degradation.The dominant groups of rhizosphere and bulk soil soil fungi in the four degraded plots was Basidiomycota,Mortierellomycota and Ascomycota;the dominant bacterial groups are Proteobacteria,Actinobacteria,Verrucomicrobia and Acidobacteria.Using LEf Se to analyze different degraded alpine meadow plots,it was found that there were 40 different species of fungi and 33 different species of bacteria.Through the function prediction of the fungal community(FUNGuild)and the bacterial community(Tax4FUN),it was found that the fungal community of degraded alpine meadows were mainly ectomycorrhizal fungi,soil saprophytic fungi,and symbiotic mycorrhiza as the main response functional groups;The bacterial community was mainly characterized by amino acid metabolism and carbohydrate metabolism,and the functional characteristics of fungal and bacterial communities in rhizosphere and bulk soil were similar.The main reason for this result was directly related to the effective nutrients in the soil and the enzymes involved in the conversion of carbon,nitrogen and phosphorus nutrients.(4)The composition and quantity of metabolites secreted by the roots of dominant plants in the four alpine meadow plots with different degradation degrees were significantly different.The root secretions of Erucamide,Betaine and D-(+)-Maltose,Hexadecanedioic acid,and ?,?-Trehalose,4-Dodecylbenzenesulfonic acid,Azelaic acid and Dodecyl sulfate and other metabolites were the main chemical substances released by plants in order to adapt to their living environment.Using PCA and OPLS-DA to analyze the root exudates of plants in 4 degraded plots,it was found that the metabolites secreted by grassland plants were mainly phenolic compounds,organic acid compounds,carbohydrate compounds,amide compounds and carboxylic acid compounds.These metabolites can provide carbon source substrates for rhizosphere soil microorganisms,and participate in the regulation of soil nutrient carbon,nitrogen and phosphorus cycles through rhizosphere processes.(5)Based on the random matrix theory,a molecular ecological network model was established between soil microorganisms,soil nutrient functional groups of carbon,nitrogen and phosphorus and root exudates in different degraded alpine meadow soils.Through the network topology properties,both fungi and bacterial communities show the looseness and instability of the network structure,but the stability of bacterial communities was higher than that of fungi.At the same time,the substances such as Erucic acid,Deoxycorticosterone 21-glucoside,trans-3-Indoleacrylic acid,Cyclopentylacetic acid,L-Norleucine,Dodecyl sulfate and ?-Eleostearic acid secreted by the roots of dominant plants have higher connectivity with microbial communities such as Mortierella,Hyphomicrobium,Sebacinales and Phyllobacterium,as well as TN,TC,MBC,MBP,?-1,4-glucosidase,Acid phosphatase and ?-1,4-xylosidase.It shows that the metabolites secreted by plants affect the entire fungal and bacterial communities in the degradation process of alpine meadows,and the microbial communities involved in the C,N,and P nutrient cycles were regulated and assembled.