Responses Of Soil Properties And Common Species Leaf Functional Traits To Nitrogen Addition In Meadow Steppe Of Inner Mongolia

Since the industrial revolution,the dramatic increase in atmospheric nitrogen deposition has been the focus of global climate change research,and the types of nitrogen compounds in nitrogen deposition can vary widely depending on the relative contribution of emission sources.The amount,components,and duration of nitrogen deposition had important implications for the structure and function of grassland ecosystems.China has become the third largest nitrogen deposition area after Europe and North America,among which the temperate meadow steppe ecosystem in Inner Mongolia accounts for a large area of grassland in China.Studying the changes of soil physicochemical properties and leaf functional traits under nitrogen deposition is important to reveal the initial response of grassland ecosystems to nitrogen deposition.This thesis is based on the Erguna Forest-Steppe Transition Zone Ecosystem Research Station,Shenyang Institute of Applied Ecology,Chinese Academy of Sciences.The common herbaceous plants of Inner Mongolia temperate meadow steppe,Leymus chinensis,Thermopsis lanceolata,Potentilla bifurca and Cymbaria daurica were used for the study.Three nitrogen compounds types were selected from the platform,and each nitrogen compound type corresponded to six nitrogen addition rates.The study focused on soil physicochemical properties and plant leaf functional traits,firstly,the response of soil physicochemical properties and plant leaf functional traits to different nitrogen additions and nitrogen compounds types was investigated;then,the synergistic and trade-off relationships between plant leaf functional traits and soil physicochemical properties under N additions were analyzed;finally,the responses of leaf functional traits of two representative plants,Leymus chinensis and Thermopsis lanceolata,were compared under long-term(7 years)and short-term(2 years)nitrogen deposition simulations.The following main results were derived from this thesis.(1)Nitrogen addition rates and nitrogen compound types significantly affected soil physicochemical properties,and plant leaf functional traits were only affected by nitrogen addition rates.With the increase of nitrogen addition,soil ammonium nitrogen,nitrate nitrogen and total inorganic nitrogen content increased significantly,and soil acidification increased,with the most serious acidification phenomenon when ammonium sulfate was applied.Leaf functional traits of the four plants responded to nitrogen addition basically in the same way.With the increase of N addition level,leaf nitrogen content,nitrogen-phosphorus ratio and specific leaf area increased significantly,while leaf dry matter content decreased.The plant leaves changed their resource allocation to adapt to the increase of nitrogen addition,so as to obtain resources quickly.(2)There were significant synergistic and trade-off relationships between the functional traits of plant leaves and their physicochemical properties of the soil under nitrogen addition.Soil ammonium nitrogen content,nitrate nitrogen content,total inorganic nitrogen content and soil pH were closely related to plant leaf functional traits,while soil water content was not significantly related to them.The nitrogen content and nitrogen-phosphorus ratio of the four plants were positively correlated with soil ammonium nitrogen content,nitrate nitrogen content and total inorganic nitrogen content,but negatively correlated with soil pH.The relationships among plant leaf functional traits differed slightly by species,but most of the relationships among traits were conserved.For example,leaf nitrogen content was positively correlated with specific leaf area and negatively correlated with leaf dry matter content,and these relationships were not species-specific.(3)There were significant differences in the effects of long-term versus short-term nitrogen addition on plant leaf functional traits.Compared with short-term nitrogen addition,long-term nitrogen addition increased plant height,specific leaf area,leaf nitrogen content and leaf phosphorus content,and decreased leaf dry matter content,suggesting that plants adopt a " quick investment-return" strategy under long-term N deposition.In addition,it was found that traits were more closely related under long-term N addition,and both Leymus chinensis and Thermopsis lanceolata showed more synergies and trade-offs correlations in leaf functional traits under long-term nitrogen addition.In this thesis,the response of soil physicochemical properties and plant leaf functional traits to nitrogen deposition in temperate meadow grasslands of Inner Mongolia was investigated by simulating nitrogen deposition from two perspectives:nitrogen compound type and nitrogen additive quantity,with soil physicochemical properties and leaf functional traits as the focus of the study.The results of this thesis enrich the study of soil and plant responses to nitrogen deposition in temperate meadow grasslands,helps us to understand more precisely the adaptive strategies of temperate meadow grasslands in the context of nitrogen deposition,and provides a theoretical basis for management of grasslands.