Soil Microbial Community Structure And Enzyme Activities Of Subalpine-alpine Zone Along Altitudes And Their Response To Nitrogen Deposition On The West Slope Of Changbai Mountain

The Changbai Mountain is a mountainous ecosystem strongly affected by volcanic eruptions and has distinct vegetation zonations along altitude.It offers a natural laboratory for studying post-volcano ecosystem reconstruction and forest succession.In the past twenty years,herbaceous species such as Deyeuxia angustifolia(Kom.)Y.L.Chang growing in the subalpine forest vertical zone has encroached alpine tundra regions of the western slope of the Changbai Mountains and has formed the alpine meadow vegetation communities with initial scale and certain continuity.Previous studies have found that atmospheric nitrogen deposition may promote the invasion,however,research on this species is limited to aboveground studies evaluating changes of vegetation cover and plant community diversity.Studies evaluating changes in soil biodiversity and below-ground community structures are scarce.Soil microbial community composition and enzyme activities were determined by phospholipid fatty acid method(PLFA)and microplate method,respectively.We sampled soils in coniferous forest zone(CF),ermans birch forest zone(EB),alpine meadow zone(AM)and alpine tundra zone(AT)along the altitudinal gradient.The aim of our study is to examine the distribution of microbial phospholipid fatty acids(PLFA)and enzyme activities of soils along altitudes of Changbai Mountain and investigate factors that affect the PLFA and enzyme.And we also set up N addition plots in alpine tundra to explore the response of soil microorganism and enzyme activities to the nitrogen deposition in Rhododendron chrysanthum Pall communities and different encroaching levels communities of D.angustifolia.Main conclusions are:(1)The results showed that both the total microbial PLFAs and various types of microbes PLFAs initially increased and then decreased as the elevation increased.The rank of the PLFAs amount was EB > CF > AM > AT.The rank of the contribution rate of total microbial PLFAs was bacteria > fungi > actinomycetes,G-> G+.The total nitrogen(TN)showed a significant positive correlation with various types of microbies and was the main factor differentiating soil microbial community structure along the altitudinal gradient.(2)Both soil ?-glucosidase(?G)and acid phosphatase(AP)activities increased with elevation increasing.The AP activity was high in AT ecosystem.Soil water content,soil organic carbon,and C/N ratio were the main factors affecting the vertical distribution of soil enzyme activities.(3)The meadow processes of alpine tundra strongly affected the soil water content,soil total phosphorus and AP activity,while had little effect on soil microbial PLFAs and other enzymes activities.Soil microbial community structure and enzyme activity in Changbai Mountain vertical zone are not only affected by soil environmental factors and hydrothermal conditions,but also closely related to vegetation community composition and litter properties(4)Three treatments with different amounts of nitrogen addition inhibited the soil microbial activitives of R.chrysanthum,the dominant vegetation,and the inhibition effect was more obvious with the increase of nitrogen application rate.Low nitrogen input promoted the activity of soil phosphatase,which significantly increased the content of soil available phosphorus under N1 treatment.In addition,the activity of hydrolase and oxidase were inhibited with the increase of nitrogen addition.The depress of Soil p H,the accumulation of Al3+,NH4+,NO3-and the limitations of Ca2+ and Mg2+ after nitrogen addition would have adverse effects on microorganisms and enzymes such as reducing microbial biomass,enzyme activities and the content of soil organic carbon,total nitrogen,total phosphorus and available phosphorus.It could be concluded that long-term nitrogen deposition would reduce soil nutrient content under native shrub R.chrysanthum community.Soil depletion would go against the growth R.chrysanthum.(5)Three treatments with different types of nitrogen addition inhibited the biomass of soil microorganisms,and the inhibition degree was different.The inhibitory effect of NO3--N was stronger than NH4+-N,and three kinds of hydrolase enzymes and two kinds of oxidase enzymes were inhibited by separately applying ammonium nitrogen and nitrate nitrogen.The above results would due to the depress of Soil p H which was given rise to the release of H+ in the process of NH4+ transforming into NO3-.(6)The invasion of D.angustifolia could increase soil microbial biomass and activities of three hydrolytic enzymes related to C,N and P cycling.The more D.angustifolia invaded,the more significantly enzyme activity increased.The invasion of D.angustifolia had created a favorable environment for the growth of herbaceous plants by significantly promoting the decomposition of soil organic matter and the increase of available nutrients,which would further aggravate its own invasion.The soil organic carbon content gradually decreased with the increase of invasion degree,and the herbaceous invasion accelerated the decomposition rate of organic matter while reduced the carbon storage of tundra soil.(7)Nitrogen application significantly changed the microbial distribution pattern of the invasion plots.In addition to being absorbed by plants and promoting plant growth,the addition of NH4+and NO3-could also inhibit the activities of microorganisms and reduce their biomass in high invasive level plots.Nitrogen application promoted the activities of hydrolase and oxidase.But in in high invasive level plots,the growth rate of oxidase which related to the synthesis of organic matter and humusorganic matter was higher than that of hydrolase,and the organic carbon content reached highest in these samples.Nitrogen application increased the content of organic carbon,total nitrogen and total phosphorus in medium and high invasive plots.Nitrogen deposition,on the one hand,accelerated the storage and transformation of soil nutrients by promoting the growth of the D.angustifolia and the amount of Litter return.On the other hand,nitrogen deposition also could promote the accumulation of soil carbon and nitrogen nutrients and alleviat the carbon release caused by the invasion of herbs in tundra by inhibiting microbial activity and significantly promoting organic matter synthetase.This may enhance our understanding of the spatial distribution pattern and formation mechanisms of soil microorganisms,and can also provide theoretical basis for predicting the impact of atmospheric nitrogen deposition on nutrient cycling in mountain ecosystem.Our study can also analysis the encroaching mechanism of D.angustifolia from the angle of soil microorganism to fill the blank area of the study.