Spatiotemporal Variation And Interspecies Differences In Root Exudates Of Four Tree Species In Northeast China

Root exudates are important mediators of plant-soil-microbe interactions,material exchange and information transfer,are the internal driving factors that constitute the different micro-ecological characteristics of the rhizosphere,and are the main regulators of rhizosphere communication.However,due to the limitation of collection and research methods,the previous studies on root exudates are most focus on short-cycle plants such as crops,little study on forest mature tree species root exudates,especially the research on the spatial-temporal distribution pattern and interspecies differences of forest root exudate input,limited accurate understanding and assessment of the characteristics and laws of the substrate cycle and biogeochemical cycle of forest ecosystems.Hence,in this study,we conduct a field experiment on four tree species(Pinus koraiensis,Larix gmelini,Pinus sylvestris var.mongolica,and Betula platyphylla)at four latitudes(Caohekou,Lushuihe,Maoershan,and Liangshui)in Northeast China.We collected the root exudates using a non-soil system.We investigated the spatiotemporal variation and interspecies differences of root organic carbon exudation rates and components and the factors that influence root exudation rates.The results were as follow:(1)The different representation methods(specific root weight,length,and surface area)root organic carbon exudation rates were different among latitudes,species,and months.Weight-basis exudation rate varies significantly among different tree species and months(P<0.05),length-basis and surface area-basis exudation rate varies significantly among different latitudes and months(P<0.05),the changes of length-basis and surface are-basis exudation rate among latitudes and species were similar.Only the exudation rate of B.platyphylla changes between months,which is higher in May to October than in September to October(P<0.05).(2)We found four phenolic compounds from four tree species root exudates at four latitudes,included benzoic acid,catechol,caffeic acid,and catechin.Benzoic acid and catechol were widespread in exudations.The benzoic acid and catechol concentrations were significantly different among latitudes and tree species,and there were significant changes on benzoic acid and catechol concentrations between months:the benzoic acid concentrations in the root exudates of four tree species were lower in May to October than in September to October(P<0.05),the catechol concentration of four tree species in May to October was higher than in September to October(P<0.05).Caffeic acid is contained in root exudaties in May to October,but only in P.koraiens,L.gmelini,P.sylvestris var.mongolica in Liangshui site in September to October.Catechin were detected only in the root exudates of four tree species in September to October.(3)Among the fine root traits,specific root length was influenced by species(P<0.05),specific root diameter was influenced by month(P<0.05),specific root surface area and tips were influenced by latitude,species,and month(P<0.05).Among the soil characteristics,soil total organic carbon and pH were influenced by latitude and species(P<0.05),soil total nitrogen and total phosphorus contents were influenced by latitude,species,and month(P＜0.05).(4)Multiple linear regression analysis results showed that soil total nitrogen content,soil pH,and mean annual precipitation were negatively correlated with the root organic carbon exudation rate,and the fine root specific surface area and mean annual temperature were positively correlated with the root organic carbon exudation rate.The linear fitting results showed that the exudation rate was negatively correlated with soil organic carbon content(Adj.R2=0.090,P=0.046),indicating that the input of root exudates may did not increase soil organic carbon content.The root organic carbon exudation rate does not show regular changes between the spatial and temporal scales and tree species,but the chemical composition of root exudates has obvious spatial and temporal and interspecies specificity.Studying the dynamics of forest root exudation input and its regulating factors at the regional scale can deepen the understanding of biogeochemical cycles,rhizosphere biological activities,plant growth and development process,and plays an important role in the estimation of underground carbon flux in terrestrial ecosystems and the rhizosphere micro-ecosystem soil carbon-nutrient cycling model.