| At present,atmospheric nitrogen deposition has become an increasingly serious global problem,and the resulting ecological and environmental problems have become increasingly prominent.So far,studies on the effects of nitrogen addition on various components of grassland ecosystem have been widely carried out,but studies on the coupling relationship and synergistic evolution of plant-soil-microorganism under the background of nitrogen addition are still rare.Therefore,this study,based on the ecological stoichiometry theory,selected four typical herbaceous plants in the Bothriochloa ischaemum(B.ischaemum)community on the Loess Plateau as the research objects,including B.ischaemum,Stipa bungeana(S.bungeana),Artemisia gmelinii(A.gmelinii),and Cleistogenes squarrosa(C.squarrosa).A gradient nitrogen addition experiment with four levels of 0(N0),3(N3),6(N6)and 9 g N m-2 y-1(N9)was set up using a completely randomized design to analyze the response characteristics of plant,soil and microbial ecological stoichiometry of grassland communities and the coupling relationship between the three.In order to provide a scientific basis for the restoration and management of grassland ecosystem on the Loess Plateau under the background of nitrogen deposition.The main findings are as follows.(1)Nitrogen addition significantly changed the rhizosphere soil nutrient stoichiometry of different species,especially the available nutrient elements.The rhizosphere soil dissolved organic carbon(DOC)content of different species tended to increase and then decrease with increasing nitrogen addition level,and reached the highest value at the N3 level(B.ischaemum,123.30±27.30 mg/kg;S.bungeana,146.61±30.84 mg/kg;A.gmelinii,138.77±23.81 mg/kg;C.squarrosa,159.20±5.77 mg/kg).The rhizosphere soil available nitrogen(AN)content of different species increased linearly with the nitrogen addition gradient,while the rhizosphere soil available phosphorus(AP)content did not change consistently among different species.The rhizosphere soil total phosphorus(TP)content of different species tended to decrease after nitrogen addition,while the rhizosphere soil organic carbon(SOC)and total nitrogen(TN)content of each species did not change significantly.Compared with the rooted soil total nutrient stoichiometry ratio,the response of rhizosphere soil available nutrient stoichiometry ratio to nitrogen addition was more obvious for each species.(2)With the increase of nitrogen addition level,the rhizosphere soil microbial biomass carbon(MBC)and microbial biomass nitrogen(MBN)of different species generally showed a"single-peak"pattern of variation and reached the highest value at the N6 level.However,the rhizosphere soil microbial biomass phosphorus(MBP)of each species showed different degrees of decrease after nitrogen addition.The rhizosphere soil MBC:MBP and MBN:MBP of different species showed a trend of increasing and then decreasing with the gradient of nitrogen addition,while the variation of rhizosphere soil MBC:MBN was different among species.Correlation and redundancy analyses showed that there was a significant positive correlation between rhizosphere microbial stoichiometry and soil stoichiometry,with soil carbon and nitrogen being the main driving factors.Stoichiometric homeostasis analysis showed that there was a homeostatic relationship between rhizosphere soil microbes and soil resources.(3)Low N levels(N3)increased rhizosphere soil C-acquisition enzyme activities of different species,while high N levels(N9)exerted some inhibitory effects.Compared with the N0 level,the rhizosphere soil N-acquisition enzyme activity was significantly increased in all species at the N9 level(B.ischaemum,34.45%;S.bungeana,27.45%;A.gmelinii,39.15%;C.squarrosa,62.00%),forming a new short-term resource utilization strategy.The effect of fertilization factors on rhizosphere soil P-acquisition enzyme activity was not significant.The rhizosphere microbial communities of different species were always strong phosphorus limitation during nitrogen addition,while carbon limitation of rhizosphere soil microbes was significantly alleviated at all N9 levels,and there was a highly significant negative correlation between carbon limitation and phosphorus limitation(vector angle>45°).Correlation and redundancy analyses indicated that rhizosphere soil nitrogen and phosphorus elements played a dominant role in the changes of soil enzyme stoichiometry.(4)The biomass and cover of the Gramineae increased significantly after nitrogen addition,with B.ischaemum showing a"single-peak"change overall and S.bungeana and C.squarrosa showing a"linear"change;while the biomass and cover of the Compositae(A.gmelinii)decreased significantly.Correlation and redundancy analyses showed that rhizosphere soil nutrient content was an important driver of plant biomass change.Nitrogen addition significantly increased nitrogen content in plant tissues,while the effects on carbon and phosphorus content varied by species.Both shoot and root N:P increased significantly with increasing levels of nitrogen addition for each plant species,indicating a more phosphorus-limited plant growth condition(N:P>16).Plant nutrient stoichiometry at each N level also differed significantly among species due to differences in resource allocation and ecological adaptation strategies.During nitrogen addition,the stoichiometric homeostasis indices of most plant were strictly homeostatic,while the root N of B.ischaemum and A.gmelinii showed plastic homeostasis(1/H>0.75).(5)The response of the relative importance value(RIV)of plants to nitrogen addition varies with species,but the RIV of B.ischaemum is significantly higher than that of other species at all nitrogen levels.The correlation analysis showed that the plant-soil-microbe stoichiometry of the dominant species(B.ischaemum)showed a closer relationship,while the ecological stoichiometry of the companion species(S.bungeana,C.squarrosa,and A.gmelinii)showed some decoupling.The results of the partial least squares pathway model(PLS-PM)showed that N addition had a positive effect on the RIV of Gramineae(B.ischaemum,0.33;S.bungeana,0.79;and C.squarrosa,0.65)and a negative effect on the RIV of Compositae(A.gmelinii,-0.75),and that the direct or indirect pathways that produced the above total effects differed somewhat by species. |
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