Effects And Mechanism Of Calamagrostis Angustifolia Expansion On Litter Decomposition And Soil Nutrients In The Alpine Tundra Of Changbai Mountains

Global warming has resulted in the expansion of lower elevation plants to higher elevation ecosystems successfully,which has a significant impact on litter decomposition.The slight change of litter decomposition will have a significant impact on the greenhouse gases such as CO₂in the atmosphere and soil C reserves,and may lead to further changes in the global climate.The alpine tundra ecosystem is fragile and sensitive to climate change.There are large soil carbon reserves in tundra,and tundra plays an important role in regulating global carbon balance.In recent 50 years,the alpine tundra of the Changbai Mountains has been warming at an average rate of 0.28℃/10a,and herbaceous plants such as Calamagrostis angustifolia（C.angustifolia）have expanded to native shrubs-tundra.The alpine tundra of the Changbai Mountains has become an ideal research place to study the carbon cycle caused by climate change and vegetation change.Therefore,we set up five sample plots of C.angustifolia with different expansion degrees in tundra,and investigated the influencing factors and mechanisms of vegetation change on litter decomposition by comparing litter input,litter layer composition and quantity,litter decomposition environment and litter decomposers.In addition,we compared the corresponding changes in soil nutrients and soil C emissions.This study can provide a scientific basis for elucidating the mechanism of vegetation change in response to climate change on litter decomposition and C and N cycling,and lay the foundation for predicting further changes in the alpine tundra after the expansion of low elevation plants and predicting future climate change.The main conclusions are as follows:（1）The expansion of C.angustifolia significantly increased the annual input biomass of litter layer,and simultaneously changed the shrubs litter layer into the mixed shrub and herb litter layer,which increased the initial litter N content,decreased the initial litter C/N and lignin/N ratio,thus the tundra litter was easier to be decomposed.（2）The expansion of C.angustifolia significantly reduced the existing litter amount in litter layer.Mixed shrub and herb litter have non-additive effects on litter layer decomposition and nutrient release.Moreover,the greater the expansion degree of C.angustifolia,the stronger the non-additive effects,and the easier it is to produce synergistic effect.The expansion of C.angustifolia promoted litter decomposition mainly in the growing season.Although the expansion of C.angustifolia had no significant effect on the nutrient release pattern of litter,it would accelerate the nutrient release rate of litter,and affect the soil and vegetation.（3）The expansion of C.angustifolia significantly increased soil moisture and p H,which resulted in significant increases in microbial PLFAs,enzyme activity and the abundance of soil mesofauna.In addition,the expansion of C.angustifolia changed the structure of litter decomposer communities and enlarged the gap of litter decomposition caused by litter chemical properties.The decomposition rate of C.angustifolia litter was 1.99 times higher than that of Rhododendron aureum（R.aureum）when only litter inputs changed.The change of environmental factors and litter decomposers caused by the expansion of C.angustifolia increased the decomposition differences of C.angustifolia and R.aureum,and the decomposition rate of C.angustifolia litter was 2.49 times higher than that of R.aureum litter.（4）Environmental differences（atmospheric temperature,soil temperature and humidity）are the main reasons affecting the difference in litter decomposition between the growing and non-growing seasons decomposition stages.In the field in-situ simulation experiment,low atmospheric temperature,soil temperature and humidity inhibited the litter decomposition during the non-growing season.The laboratory simulation experiment had no significant effect on litter decomposition in growing season,however,it increased litter decomposition rate in non-growing season.（5）Litter decomposers and litter chemical properties were the main reasons that affect the decomposition differences among different expansion degrees of C.angustifolia in the same decomposition stage.However,litter decomposers and litter chemical properties changed significantly in the process of litter decomposition,and their relative importance to litter decomposition changed with time.The expansion of C.angustifolia increased the litter decomposition rate at the early stage（0-240 d）by promoting litter N,P and soluble sugar contents.The expansion of C.angustifolia increased the litter decomposition rate at the middle stage（240-360 d）by increasing fungi,cellulase enzyme activity and collembola abundance to jointly promote the cellulose release.And,the expansion of C.angustifolia increased the litter decomposition rate at the late stage（360-720 d）by decreasing the lignin content and the ratio of lignin/N,increasing bacterial communities and polyphenol oxidase enzyme activity to promote the lignin release.（6）When the expansion of C.angustifolia only changes the chemical properties of litter layer,the loss of litter N and P increases,the N and P cycle of ecosystem is promoted,and the turnover of soil nutrients increases,which provides nutrients for C.angustifolia growth and promotes the further expansion of C.angustifolia.However,the change of chemical properties of litter layer only temporarily improves soil organic carbon content,and has no effect on the reserves of soil organic carbon,indicating that chemical properties of litter layer are not the main influencing factors to control soil organic carbon and CO₂emissions.The expansion of C.angustifolia.changes the chemical properties of the litter layer while changing the litter decomposers and decomposition process of litter,which not only increases the turnover of soil nutrients,but also reduces the soil organic carbon.The increase of microbial PLFAs,βG enzyme activity and collembola abundance play a major role in this process.In addition,it also increases the emission of CO₂from soil,and microbial PLFAs,fungal PLFAs and the abundance of soil mesofauna play a major role in this process.In conclusion,although the expansion of C.angustifolia increased the carbon input of litter layer,it accelerated the decomposition of litter by changing the litter chemical properties,decomposition environment and litter decomposers,increasing the effective soil nutrient and soil C emission,and decreasing the soil organic carbon storage.The increase of soil available nutrients is beneficial to the growth of C.angustifolia and promotes the continuous expansion of C.angustifolia.It can be inferred that climate warming,plant invasion and increased C emissions constitute a positive feedback,which continuously strengthens this change process and further aggravates climate warming.