Changes In Microbial Community Structure And Function During Root Decomposition Of Grassland Plants

As the main decomposer of grassland roots,microorganisms participate in the decomposition and transformation of soil organic matter and nutrient cycling,and are the main drivers of carbon and nitrogen cycling.Due to the different carbon and nitrogen content of the root system itself,the dominant decomposer groups and functions are also different.Therefore,to clarify the effects of root decomposition on soil microorganisms in grassland with different carbon and nitrogen contents,an indoor incubation experiment was conducted with the roots of gramineous forage（Bothriochloa ischaemum）and leguminous forage（Lespedeza davurica）over three decomposition times（90,270 and 450 days）.By studying the nutrient changes of roots and soil in the process of root decomposition of gramineous and leguminous forages,and clarifying the nutrient release process of root decomposition of the two species,we can establish the correlation between the nutrient changes of root decomposition of the two species and soil nutrients.By measuring the dynamic changes of rhizosphere microbial community structure and functional characteristics,we can explore the changes of soil microbial community structure and function during the root decomposition of the two species.This provides a scientific basis for the restoration and reconstruction of grassland vegetation on the Loess Plateau.The main results are as follows:（1）From 90 to 450 days,the TOC and TN in two species decreased continuously,showing a release pattern.From 90 to 450 days,TOC in B.ischaemum released 13%,TN content released 40%.TOC in L.davurica released 26%and TN released 27%.Soil organic carbon（SOC）and total nitrogen（STN）increased rapidly in the early stage of decomposition（90-270 days）,but did not change in the later stage（270-450 days）.The SOC content increased by 12%during the whole decomposition process.The SOC in L.davurica increased slowly in the early stage and increased significantly in the later stage.The SOC content increased by 13%during the whole decomposition process.The change trend of STN content in two species was similar.During the whole decomposition process,the STN of B.ischaemum increased by 83%,and that of L.davurica increased by 160%.（2）For bacteria,the richness index（Chao1 and ACE index）increased with the decomposition of root residues from 90 to 450 days,and the growth of L.davurica was more.There was no significant change in Shannon and Simpson indexes in B.ischaemum.The Shannon index L.davurica increased significantly from 270 to 450 days,and the Simpson index did not change significantly.Except for the 270^th day,there was no significant difference in the richness and evenness index in two species at the same time.For fungi,the four indexes（Chao1,ACE,Shannon,Simpson）of the two species decreased rapidly in 90-270 days,and then increased slightly in 270-450 days,among which the four indexes in L.davurica decreased more in 90-270 days.The results suggested that the dominant soil bacteria were Proteobacteria,Chloroflexi,and Acidobacteria,and the dominant soil fungi were Ascomycota and Basidiomycota.On the 90^th day,the relative abundance of Chloroflexi in B.ischaemum was dramatically higher than that in L.davurica,while differences in the relative abundance of Proteobacteria and Acidobacteria were not observed.The relative abundance of Proteobacteria in the B.ischaemum was markedly lower than that in the L.davurica,while the reverse occurred for Chloroflexi and Acidobacteria on the 270^th day.On the 450^th day,the relative abundance of Chloroflexi in B.ischaemum was markedly lower than that in the L.davurica soil（P<0.05）.The main bacterial indicator species in B.ischaemum were Acidobacteria,Chloroflexi,Proteobacteria,Firmicutes,and the main fungal indicator species in B.ischaemum were Ascomycota,Glomeromycota,Basidiomycota,Mortierellomycota.The main bacterial indicator species in L.davurica are Actinobacteria,Cyanobacteria,Firmicutes,Proteobacteria,Chloroflexi,etc.The main fungal indicator species in L.davurica are Ascomycota,Basidiomycota,and Glomeromycota.STN was the most important factor affecting soil microbial community structure.（3）In bacteria,the abundance of carbohydrate and amino acid metabolism genes in L.davurica was higher in the early stage of decomposition(on the 90^th day),while the reverse was true at 270^th days.The trophic modes of the fungi in the two species were dominated by saprotrophs and pathotroph-saprotroph-symbiotrophs,which accounted for approximately 50%or more of the total amount.In two species,the abundance of saprotrophs increased rapidly from 90 to 270 days.The abundance of pathotroph-saprotroph-symbiotroph in B.ischaemum decreased significantly from 90-270 days,and the abundance of pathotroph-saprotroph-symbiotroph in L.davurica decreased significantly from 90-270 days,and then increased significantly.