Rhizospheric Biological Processes And Underlying Mechanisms Of Interspecific Facilitation In Maize/peanut Intercropping System

Intercropping practice plays important roles in maintaining biodiversity and stability,and promoting crop yield in agroecological system.It is very necessary to explore the underlying mechanisms of yield advantage under intercropping sysem,which provides the theoretical basis and practiccal guide for the development of modem agriculture.Maize/peanut intercropping system shows the significant yield advantage.However,there is little information about the mechanisms of yield advantage under maize/peanut intercropping.Therefore,in this study,five treatments were set,including maize monoculture(MM),peanut monoculture,maize(Zea mays L.)and peanut(Arachis hypogaea L.)intercropped without any separation(NS),by half separation(SS)using a nylon net(50 ?m)and complete separation(CS)by using a plastic sheet in the udergound parts,respectively.Firstly,the changes in yield,agronomic traits,photosynthetic physiological characteristics and soil physicochemical properties between different treatments were analyzed to find out the key factor contributing to yield advantage.Then,the shifts of soil microbial community structure and function diversity were analyzed by using BIOLOG?PLFA?qRT-PCR?T-RFLP and metabonomics technologies.The purpose of this study is to explore the relationship between microbial community and yield advantage in maize/peanut intercropping regime,and the effects of root exudates on the growth of key specific microbes.The main conclusions are listed as follows.(1)The results from field trial showed that the contents of chlorophyll content,photosynthetic efficiency of maizes were higher in NS and SS than in CS and MM at four different growth stages.There is no significant difference between CS and M.The contents of chlorophyll content of peanuts were higher in NS and SS than in CS and PM at four growth stages.At the later stage,the photosynthetic efficiency of peanuts was higher in NS and SS than in CS and PM and There is no significant difference between CS and M.Moreover,we found that the agronomic traits and yields of maizes were higher in NS and SS than in CS and PM,and the yields of peanuts were higher in NS and SS than in CS.The number of root nodules of peanuts was significantly higher in NS and SS than in CS and PM.The land equivalent ratios for NS and SS treatments were all higher than 1,showing an obvious interspecific facilitation.(2)The results from the detection of ecological factors in field trial showed that electrical conductivity in maize soils(112 mS·m-1?144 mS·m-1)were higher than that in peanut soils(57 mS·m-1?75 mS·m-1).The electrical conductivity in both maize and peanut soils declined at the later stage.Furthermore,the electrical conductivity in maize soils under CS treatment was slightly higher than that under NS and SS treatments.The electrical conductivity in peanut soils showed an opposite trend.We also found that soil available nutrients(available nitrogen(Avail N)and available phosphorus(Avail P))and enzymatic activities(soil urase and phosphomonoesterase)in both crops were improved in NS and SS treatments as compared to CS.These results mentioned above suggested that the composition of root exudates and microbial community in rhizosphere had changed under intercropping and then affect the soil enzymatic activities and available nutrients.The agronomic traits,photosynthetic physiological characteristics and soil physicochemical properties between different treatments were also detected in pot experiment and showed the consistent trend with the results from field trial.(3)The results from BIOLOG and PLFA showed that maize/peanut intercropping could significantly promote the amount of microbial biomass and function diversity in both crops.The amount of bacterial biomass was significantly higher in in NS and SS than in CS.The Gram-positive bacteria(G+)in maize soils were significant higher in NS and SS than CS,while the Gram-negative(G-)was significant higher in peanut soil.The ratio of normal saturated to monounsaturated PLFAs was significantly higher in rhizosphere of peanut under CS treatment than in any other treatments,which is an indicator of nutrient stress.Redundancy analysis and cluster analysis of PLFA showed rhizospheric microbial community in rhizospherix soils of both plants tended to be consistent in NS and SS treatments.The urase and Avail N were higher in the soils of both plants under NS and SS regimes,and positively correlated with bacteria,fungi(F)and total PLFAs,while negatively correlated with G+/G-and NS/MS.Furthermore,the qPCR quantification in situ showed that intercropping could promote the copies of nitrogen cycling-related genes(amoA(AOA)?amoA(AOB)?narG?nirK?nirS?nosZ?nifH)in rhizopspere soil of both crops plant.(4)Terminal Restriction Fragment Length Polymorphism(T-RFLP)technique was applied to evaluate the response of the soil bacterial communities to intercropping practice.The results displayed that the diversity and evenness indices of bacterial community in both crops were significantly improved in non-separated treatment(NS)and semi-separated treatment(SS)as compared to completely separated treatment(CS).Proteobacteria is dominant bacteria in CS maize soil.Proteobacteria gradually decreased in SS and NS maize soils while Firmicutes and Actinobacteria gradually increased in SS and NS maize soils.Firmicutes is dominant bacteria in CS peanut soil.Firmicutes gradually decreased while Proteobacteria gradually increased in SS and NS peanut soils.The results of cluster analysis and principal component analysis were consistent with PLFA,BIOLOG results.Further analysis found that under intercropping(SS and NS)beneficial bacteria(i.e.Bacillus,Burkholderia,Pseudomonas,et al.)were improved in maize and peanut soils.Among them,Bacillus,Brevibacillus and Paenibacillus mainly increased in maize rhizosphere.Burkholderia,Pseudomonas,Sphingomonas and Rhizobiun mainly increased in peanut rhizosphere.Meanwhile,real-time PCR assay confirmed the changes in the abundance of main bacterial phylum and beneficial bacteria(Bacillus,Burkholderia,Pseudomonas,Rhizobium).In conclusion,under maize and peanut intercropping,the improvement of bacterial community structure and diversity was conducive to soil microecosystem stability.Besides,the increase of beneficial bacteria can reduce soil-borne pathogens,improve the availability of soil nutrients and thus promote the growth of intercropping crops.(5)Root exudates extracted from monocultured maize(MM),monocultured peanuts(MP)and intercropped treatment(IT)under vermiculite cultivation were identified by using GC-MS.There are 114 compounds were detected.92,67 and 110 compounds were identified from MM,MP and IT,respectively.These compounds were classified as amino acids,phenolic acids,carboxylic acids,purine,diterpenes,amine,sugar,sugar acid,organic acid and other kinds of compounds.Further analysis found that the change tendency in diversity and evenness in different treatments was:IT>MM>MP.Principal component analysis and cluster analysis showed that the compositions of root exudates were significantly different among three treatments,especially between Mm and MP.Under intercropping,amino acids,phenolic acids,terpenoids,N-containing compounds and ratio of N-containing compounds to C-containing compounds(N/C)were significantly higher as compared to the monocultured.In particular,the N/C and amino acids in IT were 20.2 times and 29.6 times more than those in MP,and 2.5 times and 3.1 times more than those in MM.Further analysis was carried out to study the effects of root exudates on the growth of rhizosphere beneficial bacteria(Burkholderia cenocepacia H111,Rhizobium tropici strain RHM54,Rhizobium multihospitium strain CCBAU,pseudomonas aeruginosa IOMTU133 Pseudomonas sp.Pi 3-21,Sphingobium japonicum strain UT26S)isolated in this study.The results showed that the three kinds of root exudates could promote the growth of these beneficial bacteria to different degrees,showing the following order IT>MM>MP.This study suggested that the increase in the composition and diversity of root exudates under intercropping was closely related to the soil microbial community diversity and the increase in beneficial bacteria in soil.In conclusion,under maize/peanut intercropping,two crops can respond to the changes in environment through the schifts in the composition and quantity of root exudates.Meanwhile,the increase in the diversity of root exudates under intercropping can promote the microbial community diversity,which was conducive to the stability of agroecological system,the control of soil-borne disease,the improvement of soil fertility.Under intercropping,the contents of amino acids and phenolic acids and N/C ratio were relatively higher possibly due to the interactions of root exudates from maizes and peanuts,which in turn affected the rhizosphere microbial community structure,promoted the number of microorganisms and rhizosphere beneficial bacteria.All these factors promoted the crop growth and yield under intercropping.