Effects Of Nitrogen And Phosphorus Addition On Stoichiometry Of Leymus Chinensis-soil-rhizosphere Microbe

Ecological stoichiometry is a theory that studies the concentrations and ratios of key elements like carbon(C),nitrogen(N),and phosphorus(P)in different biotic and abiotic components of ecosystems.Grassland ecosystems are important components of terrestrial ecosystems,and C:N:P ecological stoichiometry of different components is a common method to evaluate the quality and health of grassland ecosystems.The rhizosphere is an important interface for nutrient and energy exchange between above-ground and underground components of the ecosystems,and it is also a special ecosystem.The rhizosphere micro-ecosystem theory refers to the overall system of interactions among various factors in the soil micro-domain around the plant root system.The rhizosphere micro-ecosystem theory is to study the law of interactions among the micro-domains of the root-soil interface and plant-soil-microbe and its environmental factors,and to explore the material circulation,energy conversion and regulation processes in the micro-domains of the rhizosphere.N and P are key elements that can limit the plant production of grasslands,yet N deposition from atmosphere and fertilization of human can significantly alter the C?N and P cycling,with important consequences for C:N:P stoichiometry of different components of grasslands.However,we know little about the stoichiometric characteristics of multiple components in the grassland rhizosphere micro-ecosystem,such as plant-soil-rhizosphere microbe and their response laws and mechanisms under the input of exogenous nutrients(such as N and P addition).In this study,we performed a completely random pot control experiment with Leymus chinensis,a dominant species in Songnen grassland.There were four treatments: blank control(CK),N addition(N),P addition(P)and N+P addition(N+P).This experiment investigated the responses of key nutrient elements(C,N,P)in plants(L.chinensis),soil,rhizosphere microbe and enzymes in the rhizosphere micro-ecosystem to the treatment of N and P addition,and analyzed the mutual coupling relationships and change of the above components.The main research conclusions obtained are as followed:(1)The addition of N and P had significant effects on the physical and chemical properties of the soil.N addition significantly reduced the soil conductivity and water content,and significantly increased soil organic carbon(SOC)and N content.The additionof P and N+P significantly reduced soil pH and water content,and increased soil P content.The addition of N,P,and N+P significantly reduced soil N:P and C:P.(2)The responses of stoichiometric characteristics of rhizosphere soil microbe to the addition of N and P were different.The addition of N,P and N+P significantly reduced the microbial biomass C and N content.It showed that a large amount of N and P input in the short term resulted in insufficient C in the soil to maintain the rapid initial growth rate of microorganisms,making it limited by C rather than N and P,causing a large number of deaths of microbes in the rhizosphere.Thus,rhizosphere microbes were in a disadvantaged position in the nutrient competition with plants.(3)Rhizosphere soil enzyme activity only responded significantly to N addition,but not to P addition.With the addition of N,the soil enzyme activities associated with the C(?G,?G,?x,and CBH)and P(aP)cycles all showed an increasing trend,with ?G activity not changed significantly.According to the microbial resource allocation strategy,a large amount of N addition in a short period of time causeed the microbes to be limited by C and P,and stimulated the microbes to secrete C and P cycle related enzymes to meet their needs for C and P nutrients.At the same time,N addition significantly increased NAG activity,which was contrary to the resource allocation theory.In this study,the soil N content was poor.In the short term,a large amount of N input increases the soil N content,which increased the N allocated to synthetic NAG and produced a new N allocation strategy in the short term.(4)With the addition of N and P,the leaf,stem and root biomass,plant height and leaf photosynthetic rate of L.chinensis increased significantly,and the C,N and P contents and stoichiometric ratios of leaves,stems and roots also changed differently.It showed that the growth of L.chinensis was limited by N and P in the soil,and the effect of N limitation was stronger.Although N addition can alleviate the effects of N limitation,it will change the limitation on plant growth to P limited.P addition will aggravate the extent to which the grassland is limited by N,thereby destroying the balance of plant C:N:P stoichiometry.(5)Redundancy analysis(RDA)results showed that,with the addition of N and P,soil characteristics had the highest effect on rhizosphere soil enzyme activity,and rhizosphere microbial characteristics had the least effect.It showed that although the extracellular enzymes were secreted by plants and microbe,but in soil,their activity was more affected by abiotic factors(physical and chemical properties of soil).Soil N content,available nitrogen(AN)content and pH had the most significant effects on enzyme activity,indicating that soil N concentration was an important factor affecting soil hydrolase activity.The leaf,stem,root biomass,leaf P content,stem N:P and root N content of L.chinensishad significant effects on soil enzyme activity,indicating that plant biomass,N and P element concentrations were also the main influencing factors of soil enzyme synthesis and secretion.(6)With the addition of N and P,in general,in the rhizosphere micro-ecosystem,the relationship between stoichiometric characteristics of L.chinensis-soil was the strongest,the relationship between L.chinensis-rhizosphere microbe and soil-rhizosphere microbe was strong,and the relationship between enzyme stoichiometry with L.chinensis,soil and rhizosphere microbe was weak.It showed that the addition of nutrients disrupts the balance of C:N:P stoichiometry of microbe,and made the C:N:P stoichiometric relationships between rhizosphere microbe and L.chinensis,rhizosphere microbe and soil tend to decoupled.At the same time,the increase of N and P content in the soil with the addition of N and P will cause its dependence on the release of nutrients by enzyme hydrolysis to decrease.Compared with the below-ground part(root)of L.chinensis,the stoichiometry correlation between soil and rhizosphere microbe with the above-ground part(stem,leaf)of L.chinensis was stronger,indicating that the plant nutrition allocation strategy will affect the stoichiometric relationships among each organ,soil and rhizosphere microbe.In summary,N and P addition exerted significantly different impacts on C:N:P stoichiometric properties of plants,soil,and rhizosphere microbes.More importantly,N and P addition tended to decouple the inner C:N:P relationships among the three components,potentially reducing the community stability of rhizosphere microbes.Therefore,if fertilization is used to improve grassland,it is necessary to consider the element balance and nutrient relationships among the components in the rhizosphere micro-ecosystem to improve the efficiency of grassland improvement measures.At the same time,this study also enriched the rhizosphere micro-ecosystem theory and provided an experimental basis for the application of rhizosphere micro-ecosystem theory in stoichiometry.