| The Changbai Mountain tundra is the most typical alpine tundra in East Asia.Under the background of current climate warming,the herbaceous plants,which were previously distributed at lower altitudes below the timberline,have encroachment into the alpine tundra where they are distributed in a pattern of patches.The upward migration of herbaceous species has been proved as a major threat to the survival of native tundra plants and has changed the distribution and community structure of native plants and soil microbes,regulated the soil physical and chemical properties,and affected the corresponding carbon and nitrogen cycle process.Recently,the global atmospheric nitrogen deposition due to human activities has increased rapidly,and has impacted the functional traits and biodiversity of both migrated and native plants in the tundra zone,leading to changes in soil nutrients and microorganisms.However,what is the physiological and ecological mechanism of Calamagrostis angustifolia encroached into the tundra and what are the effects on soil nutrient cycling are still unclear.Here,we focused on the migrated plant,Calamagrostis angustifolia,which has upward migrated into the tundra,and the native plant of tundra,Rhododendron chrysanthum,which has been threatened by Calamagrostis angustifolia.We have conducted a two-year field control experiment with the treatments of N addition and removing specific plant groups at two altitudes(2050 m and 2200 m a.s.l.)in the alpine tundra of Changbai Mountain.In order to assess the response of physiological and ecological traits of Calamagrostis angustifolia to environmental change,we determine the physiological and ecological properties of plants,such as leaf shape,plant height,stem diameter,leaf and stem nutrient content.In addition,soil physical and chemical properties,soil microbial biomass,soil microbial community structure and enzyme activity were also measured to explore the effects of nitrogen deposition and vegetation migration on soil properties and soil microorganisms.The main results are as follows:(1)The leaf morphology and growth of Calamagrostis angustifolia and Rhododendron chrysanthemum were influenced by nitrogen deposition.We found that the nitrogen application restrained the leaf length and promoted the growth of stem diameter and plant height of Calamagrostis angustifolia at low altitude,but no obvious changes were found at high altitude.The leaf length,leaf width and annual growth of Rhododendron chrysanthum decreased under nitrogen application,and the leaf area also significantly decreased at low altitude.The leaf length,leaf width and leaf area of Calamagrostis angustifolia at low altitude were significantly higher than those at high altitude.On the other hand,the leaf length,leaf width,leaf area,stem diameter,annual stem growth(plant height)and leaf dry matter content at low altitude tended to increase,while the specific leaf area tended to decrease after eliminating the Rhododendron chrysanthum.The treatment of eliminating the Rhododendron chrysanthum reduced the leaf length of Calamagrostis angustifolia at low altitude,but increased the leaf length of Calamagrostis angustifolia at high altitude,and had no influence on other morphological characteristics.Additionally,we found that the significant differences between altitudes were mainly presented in the leaf length,leaf width,leaf thickness,leaf area,stem diameter and plant height of Calamagrostis angustifolia,and the leaf width,leaf area,specific leaf area,stem diameter and annual growth of Rhododendron chrysanthum.All these traits were significantly higher at low altitude than at high altitude.(2)We didn't find the significant effects of nitrogen application on soluble sugar,starch and NSC in leaves and stems of Calamagrostis angustifolia.The content of NSC in leaves of Rhododendron chrysanthum slightly decreased with nitrogen application.The starch content in leaves and soluble sugar and starch content in branches of Rhododendron chrysanthum at low altitude significantly decreased with eliminating Calamagrostis angustifolia in 2018,while had no obvious changes in2019.Additionally,the treatment of eliminating Calamagrostis angustifolia,and the combination treatment of eliminating Calamagrostis angustifolia and nitrogen application significantly reduced the contents of soluble sugar,starch and NSC in the branches of Rhododendron chrysanthum at low altitude in 2018.Nitrogen application significantly increased the nitrogen content in leaves of Calamagrostis angustifolia and Rhododendron chrysanthum,but the N,P and K contents in leaves and branches of Rhododendron chrysanthum were not affected by the treatment of eliminating Calamagrostis angustifolia.(3)Compared to the control samples,the contents of soil ammonium nitrogen and nitrate nitrogen increased after nitrogen fertilization,especially at high altitude in2019.Further,eliminating different vegetation had discrepancies in soil ammonium nitrogen and nitrate nitrogen.Also,we found that nitrogen fertilization reduced soil available P content,while did not influence the soil C/N ratio,C/P ratio and N/P ratio.The elimination of Rhododendron chrysanthum reduced the ratio of soil C to N,C to P and N to P.Moreover,the soil properties showed higher values at high altitude than low altitude with the exception of soil p H and total potassium,which showed higher values at low altitudes.(4)Finally,we found that the total phospholipid fatty acids(PLFAs),fungi PLFAs,bacteria PLFAs,gram-positive bacteria PLFAs,gram-negative bacteria PLFAs and the ratio of fungi to bacteria decreased with nitrogen application.The nitrogen application promoted the activities of oxidase and hydrolase at high altitude,while inhibited the activity of hydrolase at low altitude.The total PLFAs content of the soil was enhanced by eliminating the Rhododendron chrysanthum and Calamagrostis angustifolia.Soil PLFAs were mainly affected by soil available P and soil C/N,but the soil enzyme activities were mainly affected by soil total nitrogen,organic carbon,soil water content and C/N.In summary,the treatment of nitrogen deposition and vegetation removal had important effects on the growth morphology and nutrient content of Calamagrostis angustifolia and Rhododendron chrysanthum,soil physical and chemical properties,soil microbes and soil enzyme activities.The effects of nitrogen addition and vegetation removal on the eco-physiological traits of Calamagrostis angustifolia-Rhododendron chrysanthum community relied on the variation of hydrothermal condition determined by the discrepancy of years and altitudes.Nitrogen deposition promoted the growth of Calamagrostis angustifolia,and the herb invasion plus nitrogen deposition made the inhibition of Calamagrostis angustifolia more obvious,providing a favorable condition for the expansion of Calamagrostis angustifolia.Nitrogen addition and vegetation removal changes soil microbes by changing available phosphorus and C/N,and soil enzyme activities are changed by soil total nitrogen,soil water content and C/N effects.Therefore,this study could provide an insight into the mechanism of herbaceous plant expansion and its impact on the ecosystem of the tundra in Changbai Mountain,and provide a theoretical basis for the study of vegetation succession and ecosystem change in alpine and subalpine regions. |
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