Synergistic Response Mechanism Of Plant-soil-microbial Carbon And Nitrogen Of Stipa Bungeana Trin.to N And P Addition

The rapid increase of nitrogen(N)deposition has a great impact on human society,economy and ecological environment.The grassland ecosystem,which can protect biodiversity and ensure the safety of animal husbandry,is also threatened by N deposition.However,evaluating the effects and mitigating the negative ones caused by N deposition on system is just about to start by taking the ecosystem components,plant,soil,and microbes as a system.In this study,Stipa bungeana Trin.,a typical pasture in Loess Hilly Region,was selected as the research object,with NH₄NO₃ and KH₂PO₄applied artificially.From analyzing plant and soil properties in the system,exploring stoichiometric homeostasis of organisms,then defining the microbial community which could regulate functional metabolism,and illustrating the dynamic characteristics of N distribution between plant and soil,and the dynamic changes of limiting elements in soil microbial community,the effect of N and P addition on plant-soil-microbe system of Stipa bungeana Trin.were comprehensively studied.Therefore,these help us to illustrate the response of each component in plant-soil-microbe system,and the elemental cycle among them under the background of N deposition,as well as evaluate the coupling effects of phosphorus(P)addition,which provides a theoretical basis for maintaining quality of pasture ecosystem.The main results are as follows:(1)The addition of N and P affected plant mineral elements,photosynthetic products and soil chemical properties of S.bungeana Trin.The single N addition had little effects on the contents of plant mineral elements,only having significant effects on contents of total N of aboveground plant,and contents of total N and total phosphors of belowground plant of S.bungeana Trin.However,the higher N and P addition(N2P2 treatment)significantly increased the mineral element content of S.bungeana Trin.,especially in the belowground part,which was increased by 10.26%-53.49%.Single low N addition(N1,2.5 g m^-2 a-¹)reduced the contents of photosynthetic products(soluble sugar,starch and nonstructural carbon)by 52.02%-56.36%.However,different N and P addition resulted in significant difference in the content of nonstructural carbohydrates of aboveground and the underground part of S.bungeana Trin.Single N addition only had significant effect on soil nitrate nitrogen.However,N and P addition had significant effects on soil chemical properties.However,the change trend of each index was different.In addition,N addition had no significant effect on the total glomalin-related soil protein(GRSP),but increased the content of easily extractable GRSP,and affected the organic carbon components(Cf2,Cf3 and Cf4)and the stability of soil organic carbon.The results showed appropriate N and P rate(N1P1treatment)could increas total and easily extractable GRSP by 55.17%and 81.30%,respectively;however,high N and P addition(N2P2 treatment)reduced the content of easily extractable GRSP by 11.67%and decreased the stability index of soil organic carbon by 27.89%.(2)The addition of N and P significantly affected the stoichiometry of the system,but did not change the homeostasis of consumers.Single N addition did not significantly change soil C:N ratio,N:P ratio and soil microbial stoichiometries,and led significant differences in stoichiometries of the aboveground and underground parts of plants.Due to different amount of applied N and P,there were significant differences in stoichiometries of soil,plants and microbial biomass.Due to the unequal trends of carbon,nitrogen and phosphorus content and stoichiometry among plants,soil and microorganisms,the transformation of microbial limiting elements was caused.N limitation for soil microbial community of S.bungeana Trin.could be alleviated by single N addition,and changed into P limitation in vigorous growth stage.On N1 level,the elemental limitation(N or P)for microbial community depended on amount of P added,while,on N2 level,P limitation could not be alleviated by P addition.Overall,soil microbes and plants could maintain stoichiometric homeostasis under N and P addition.(3)Soil bacterial and fungal communities can regulate microbial metabolism under N and P addition.The single N addition changed soil bacterial community structure,and affected fungal community structure to a certain extent(no significant difference between N0P0and N2P0 treatment).The addition of N and P did not only significantly affect the richness indices,but also significantly affected the relative abundance and composition of bacterial and fungal communities.Soil bacterial community was affected by soil nutrients,plant mineral elements and their stoichiometries,but the degree of being affected was stronger than soil fungal community.In addition,the regulation of soil enzymatic properties by bacterial community was also stronger than that of soil fungal community.Bacterial communities,Bacteroidetes,Nitrospirae and Tectomicrobia,and fungal communities,Pyrenochaetopsis and unclassified?c?Leotiomycetes were affected by soil nutrient properties,and had regulation effects on soil enzymatic properties,thus driving the element cycle in the system.(4)The amount of N distributed to components of plant-soil system was affected by N addition level and the components of the system.Single N addition could increase the biomasses of S.bungeana Trin.However,with the growth of S.bungeana Trin.,the promotion effect of N addition on aboveground and belowground biomass decreased gradually,from 53.06%and 76.36%to 16.61%and 58.45%.Similarly,the promotion effect of N addition alone was weaker than that of N and P mixed addition(N2P2 treatment),and the aboveground and underground biomass were increased by 135.79%and 113.51%in the vigorous period.The addition of N and P had stronger promotion effect on the mineral content of S.bungeana Trin.than that of single N addition.In addition,the plant mineral elements contents of were changed by the growth stages of S.bungeana Trin.Along the growth of S.bungeana Trin.,the contents of organic carbon and total nitrogen in aboveground parts of plants decreased.Overall,the distribution of added ¹⁵N determined by components of the system,with the amount of N in aboveground parts higher than that in the underground parts and soil.N addition level basically determined the allocation amount to each component.However,the mixed addition of N and P had a weak effect on the distribution amount of N.(5)N and P addition and growth stages of S.bungeana Trin.affected elemental limitation for soil microbial community.The effect of N and P addition on soil microbial community limitation elements was less than that of S.bungeana Trin growth stage.Along the growth of S.bungeana Trin,C and N limitation for soil microbial community was gradually strengthened,and the N limitation was the strongest on 30th days.From 65th days on,the limitation of C and N was obviously alleviated.The changes of elemental limitation for soil microbial community were related to changes of soil microbial metabolic substrate(soil available nutrients).Soil microbial community could alleviate the limitation of N or P elements on soil microbial community by changing soil enzymatic properties,with the growth of S.bungeana Trin.,showing a trend from strong to weak and then strong.The above results indicated that the soil in the Loess Plateau is poor,the response of plant-soil-microbe system to single N addition was weaker than the mixture of N and P.N and P addition promoted the biomass and increased mineral element content of S.bungeana Trin.,providing material basis for animal husbandry and ensuring forage quality.In addition,the responses of various properties of organisms(plants and microbes)to N and P addition were not consistent,driving the elemental cycle in the system,but did not change stoichiometric homeostasis of organisms at present.Therefore,under the background of global N deposition,appropriate P addition can be used to make coupling effects on components in the system,which is conducive to grassland safety and sustainable development of S.bungeana Trin.in the Loess Plateau.