Responses Of Soil Enzyme Activity To Vegetation Removal And Short-term Nitrogen Addition In The Changbai Mountain Tundra

The alpine tundra belt is distributed above forest boundaries,and is one of the most environmentally stressful areas in mountain ecosystems,which is very sensitive to climate change.During the past 20 years,the Rhododendron aureum-Georgi community,the typical shrub community,has been severely up-rooted by low elevation vegetation(Deyeuxia angustifolia),which are also affected by a combination of climate warming,atmospheric nitrogen deposition and vegetation interactions,in the Changbai Mountain tundra.Terrestrial vegetation studies cannot reveal which process contributes to the upwelling process of the Deyeuxia angustifolia,however,soil enzymes are direct participants in the biogeochemical cycling process of terrestrial ecosystems,responding rapidly to ecosystem dynamic changes and interactions.Most of the current research on soil enzymatic processes involved in the up-rooted of Deyeuxia angustifolia has been limited to existing field samples with different invasion levels,and lessly pays attention paid to whether the response mechanisms of soil enzymatic activity to change vegetation type and increase nitrogen deposition levels at the same mixed community are consistent,over the Changbai Mountain tundra.Most analyses are also limited to discussions of the physico-chemical properties of the soil for the influencing factors,and lacking consideration of the differences of vegetation themselves.In this study,we use the same mixed community,Rhododendron aureum-Georgi and Deyeuxia angustifolia mixed community,as the study subject,which is used for the changes of soil enzyme activity structure and function under different treatment conditions of the same vegetation community investigated by vegetation culling and nitrogen application treatment,in two elevation gradients(2050m and 2200m)of the Changbai Mountain tundra.The main conclusions are as follows:(1)Soil enzyme activity was higher at high altitudes than at low altitudes;nitrogen application promoted hydrolytic enzyme activity and inhibited oxidative enzyme activity,but this phenomenon was no significant difference with soil moisture limitation lower elevations.The decrease in soil enzyme activity was most pronounced in the Deyeuxia angustifolia communities after vegetation removal.(2)Soil enzyme activity was positively correlated with the total soil nitrogen,organic carbon,and soil moisture content,and this relationship was more pronounced in the Deyeuxia angustifolia communities,Rhododendron aureum Georgi and Deyeuxia angustifolia mixed communities,and at this elevation after nitrogen application.Changes in soil nitrate and ammonium nitrogen content and soil water content are important environmental factors regulating changes in soil enzyme activity.Differences in the relevance of available phosphorus and available potassium to different vegetation types are important environmental factors explaining differences in vegetation types.(3)Soil enzyme activity of different vegetation types shows that Deyeuxia angustifolia tend to have lower soil C,N and P limits and higher oxidase activity.Altered water content due to warming is an important environmental factor controlling the degree of upward invasion of Deyeuxia angustifolia and the competitiveness of Deyeuxia angustifolia and Rhododendron aureum-Georgi play a regulatory role.The rate of Deyeuxia angustifolia invasion in the low-elevation tundra will intensify with warmer temperatures and increased levels of nitrogen deposition,while the higher elevations will continue as the Rhododendron aureum-Georgi-Deyeuxia angustifolia mixed communities.(4)The chemometric ratio of enzyme activity,C: N: P,is approximately 1.33:1:1.43 and the ecosystem is clearly limited by phosphorus in Changbai Mountain.Soil ?G activity was significantly affected by soil C/P,N/P and NAG activity was significantly affected by soil C/N,C/P,N/P.(5)Vector results of soil enzyme activity showed that all vegetation types in the tundra belt remained maximally phosphorus restricted,and phosphorus limitation is further enhanced by enhanced nitrogen deposition.This study reveals the pattern of change in soil enzyme activity after vegetation removal and nitrogen addition in samples with the same degree ofDeyeuxia angustifolia encroachment,which has important theoretical implications for a better understanding of environmental changes(e.g.,increased temperature and nitrogen deposition levels)on soil biogeochemical processes,and also provides a scientific basis for ecological conservation in the Changbai Mountain National Nature Reserve..