Response Of Rhizospheric Microbiology To Plant Competition And Moisture Stress

Plant competition affects plant growth,morphology and survival,and promote plant community succession,water stress intensifies the competition effects between plants,and rhizosphere is a special ecosystem associated with plant,soil and microbial interaction.Currently,combined action of plant competition and moisture stress on the system of plant-rhizosphere-microbe has been rarely studied.Given this,we selected typical dominant grass(Bothriochloa ischaemum)and companion species(Setaria viridis and Stipa bungeana)as research object,which appeared at the middle and later succession stage in the loess hilly-gully region,and constructed potted plant competition system with different moisture gradient.The plant-rhizosphere-microorganism system was comprehensively and meticulously studied,including plant mineral elements,rhizospheric nutrient status,stoichiometric characteristics,extracellular enzyme activity,microbial community structure and diversity,and so on.The present study aimed to provide theory basis for deeply understanding vegetation restoration and succession,and provide practice guidance for ecological construction in the Loess Plateau of China.The main results were as follows:(1)Plant competition and moisture stress affect plant biomass and mineral elements and rhizospheric nutrient contents.Plant competition affected plant biomass and mineral content,mineral contents of Stipa bungeana were highest,especially plant nitrogen and phosphorus contents,but effect of moisture stress on mineral content had occasionality.Overall,mixture treatments had higher rhizospheric organic carbon,total nitrogen,water soluble ammonium nitrogen,organic carbon and phosphorus contents,and lower available phosphorus,nitrate nitrogen and water soluble nitrate nitrogen contents compared with the control treatment.Moisture stress significantly influenced rhizospheric available phosphorus,water soluble total nitrogen and organic carbon content,as well as pH,and rarely affected other nutrient indices.(2)Plant competition and moisture stress influence rhizospheric extracellular enzyme activity.Both plant competition and moisture stress significantly affected the activity of hydrolase in the rhizosphere,showing that appear of grass significantly increased the activity of carbon and nitrogen related hydrolase,but almost have no increase effect on phosphorus related hydrolase.Furthermore,activities of ?-1,4-glucosidase,?-1,4-xylosidase,?-1,4-N-acetylglucosaminidase in mixture treatments wete generally higher than monocultured treatments,carbon and nitrogen related hydrolase,phenol oxidase and FDA hydrolase activities under middle drought stress condition were higher compared to sufficient water condition,but phosphorus related hydrolase was the opposite.Rhizospheric enzyme activity of different treatments could be classified into three groups,that is,the monocultured treatments,the mixed treatments and CK,enzyme activity in monocultured and mixture treatments have similarity of their own,but different from each other.Moreover,Enzyme activity was tightly associated with chemical properties,and organic carbon,available phosphorus,nitrate nitrogen and water soluble materials have significant effect on enzyme activity.(3)Plant competition and moisture stress affect stoichiometric characteristic in plant-rhizosphere-microorganism system.Plant competition significantly affected plant,rhizospheric and enzyme activity stoichiometric characteristics,and the influence on plant and enzyme activity was more obvious than on rhizospheric stoichiometry.The emergence of Bothriochloa ischaemum is beneficial to increase plant and enzyme activity C:N and C:P,and the emergence of Stipa bungeana is good for the increase of plant,rhizospheric and enzyme activity N:P.Moisture stress condition occasionally affected plant and rhizospheric stoichiometry,but greatly influenced enzyme activity stoichiometry,showing higher enzyme activity stoichiometry in mild drought stress condition than in sufficient moisture condition.Moreover,linear positive correlation was found between enzyme activitiy C:P and plant C:P,enzyme activity N:P and plant C:P,and linear negative correlation was found between enzyme activity C:N and plant N:P,enzyme activity C:P and N:P.Enzyme activity N:P was negative and positive correlated with rhizospheric N:P and total phosphorus content.(4)Plant competition and moisture stress change rhizospheric microbial biomass and community structure.In the current competition year,mixture treatments,especially treatments of CB,GB and GCB,had significant higher total PLFA,bacterial PLFA and gram positive bacterial PLFA content than control treatment,as well as than monoculture treatments under mild drought stress condition.Whereas,plant competition increased microbial biomass compared with control treatment,but the significant increase have randomness and there was no obvious regularity.In the next competition year,plant competition have increase effect on the PLFA content,furthermore,both soil moisture and competition time changed microbial biomass and community structure,and soil moisture have greater influence on microbial community than the current year.Correlation analysis indicated that SOC,TN,NO3--N,NH4+-N,WNO3--N,AP and WSOC were intimately related with microbial community,which were important factors affecting microbial community,and also suggesting that development of microbial community was conductive to the improment of these nutrients.(5)Plant competition affects rhizospheric bacterial and fungal community structure or diversity.Plant competition influenced rhizospheric bacterial community structure but did not influence its diversity,and Proteobacteria and Bacteroidetes in mixture treatments had higher relative abundance than in monoculture and control treatments,and relative abundance of Actinobacteria and Acidobacteria was the opposite.Plant competition showed influence on fungal community structure,composition and diversity.The appearance of Bothriochloa ischaemum decreased the relative abundance of Ascomycota but increased the relative abundance of Basidiomycota.Bacterial and fungal community in monoculture,mixture and control treatments have similarity of their own,but different from each other.Rhizospheric organic carbon,water soluble organic carbon,nitrate nitrogen and available phosphorus content were the major factors resulting in the changes of bacterial community,and organic carbon,nitrate nitrogen and available phosphorus content were the important factors leading to the changes of fungal community,rather than the differences of grass species themselves.Results mentioned above indicated that plant competition and moisture stress have an influence on the whole system of plant-rhizosphere-microorganism,and plant mixture benefits the improvement of soil condition,enhancement of enzyme activity,and increase of microbial biomass.Rhizospheric extracellular enzymes,bacterial and fungal community for plans in different succession stages were similar,indicating that it is not synchronized for the changes of vegetation community and soil microbial community,which may be due to the similar ecological niche occupied by three grass in this study,and soil changed relatively slowly when affected by historical background.Therefore,plant diversity should be increased and avoid to plant the same and similar species during artificial vegetation restoration in the Loess Plateau,which will be in favor of the sustainable development of both ground and underground ecosystem.