Effects Of Nitrogen Addition On Root Exudates And Rhizosphere Fungal Communities Of Four Forest Plants

Plant-soil-microbe interaction process mediated by root exudates is an important component of the biogeochemical cycle in forest ecosystems.Exploring the relationship between plants,soil and microbes can contribute to understanding effects of root exudates on plant and soil microbial communities and the underground processes,which is of significance for revealing the maintenance mechanisms of forest biodiversity.Although nitrogen deposition in China has stabilized gradually after a rapid increase,it is still at a high level at present.Moreover,plant root exudates and rhizosphere fungal communities have been increasingly explored,but it remains unclear how they respond to different nitrogen deposition levels,thereby changing plant-soil-microbial relationships.In order to explore underlying mechanisms of the plant-soil-microbial interaction mediated root exudates under the ongoing nitrogen deposition,four forest woody plants(Abies beshanzuensis,Abies ziyuanensis,Schima superba and Callicarpa membranacea)were selected.This study included collecting root exudates in situ and greenhouse potting experiments.High-throughput sequencing technologies combined with metabolomics analysis were used to quantify the composition of rhizosphere fungal communities and root exudates and the responses of plant-soil-microbial relationships to nitrogen addition.The aims of the study were:(1)to detect the relationships between plant root exudates and rhizosphere fungal functional groups and to estimate the roles of soil properties in this relationship;(2)to explore how nitrogen addition affected the composition of plant root exudates,rhizosphere fungal functional taxa and the plant-soil-microbe relationship.The main results were as follows:(1)Analyses of root exudates in situ showed that lipids and lipid-like molecules,organic nitrogen compounds and organoheterocyclic compounds were main components of root exudates among the four woody species,but their quantity differed significantly(P < 0.05);The most abundant taxa in the composition of the rhizosphere fungal community of A.beshanzuensis was the genus Sebacina,while the most abundant taxa in the rhizosphere fungal community composition of the other three species was the genus Russula;The ectomycorrhizal fungi in the rhizosphere fungal communities of the four species was primarily affected by soil physicochemical properties,while the arbuscular mycorrhizal fungi and plant pathogenic fungi were mainly affected by the unique root exudates of each tree species.(2)The results of the greenhouse potting experiment(to simulate nitrogen deposition)showed that the composition and the quantity of seedling root exudates in the four species responded differently to nitrogen addition(P < 0.05).Nitrogen addition significantly increased the expression of root exudates of A.beshanzuensis and A.ziyuanensis(by 83.5% and 66.9%,respectively),but significantly decreased the expression of root exudates of S.superba and C.membranacea(by 50.0% and 75.0%,respectively);Nitrogen addition significantly affected the rhizosphere fungal community composition of A.ziyuanensis,S.superba and C.membranacea,without significantly affecting that of A.beshanzuensis.Furthermore,rhizosphere ectomycorrhizal fungi of the four plant species were mainly affected by root exudates after nitrogen addition,while plant-pathogenic fungi were mainly affected by soil physicochemical properties except A.beshanzuensis.In summary,this study explored the interaction between root exudates and rhizosphere microorganisms through in situ sampling(including root exudates and rhizosphere soil)in a subtropical forest,and found that rhizosphere fungal community depended on the composition of root exudates and the soil physicochemical properties.Then,the greenhouse potting experiment showed that nitrogen addition significantly enriched the metabolic pathways of various amino acids and fatty acids,changed the composition and expression of root exudates.Moreover,nitrogen addition also changed the relationship among soil physicochemical properties,root exudates,and rhizosphere fungal community of different tree species.This study provides a scientific basis for revealing the potential ecological mechanisms of root exudates and rhizosphere microbial communities in forests under global nitrogen deposition.