| Alpine grasslands having been suffered from mutual influences of natural causes changes and human activities and thus kinds of degraded landscape came out on the Qinghai-Tibet Plateau.In addition,the soil nitrogen plays key roles in degraded grasslands restoration.Nitrogen addition is an effective method to explore the mechanism during the process of degraded grasslands structure and function restoration.Nitrogen addition not only changes the soil conditions but also influences the structure and function of plant community and soil microbial community.In the past few decades,there have been countless studies on the relationship between alpine degraded land and nitrogen.However,alpine degraded grassland has high vulnerability,high sensitivity,low stability,and harsh environmental conditions.The driving mechanism that land degradation and nitrogen addition playing on plants and microbial community was extremely complex.What changes woud happen to plant communities,microbial structure and functions on degraded grassland after nitrogen addition?Based on the field survey on the Qinghai-Tibet Plateau,we selected five different degraded grasslands(non-degraded-I,lightly-II,moderately-III,severely-IV,extremely-V degraded grassland)in the alpine region above4600m elevation according to vegetation coverage,soil bareness,and gravel ratio,etc..Plant community survey was carried out through 225 quadrats,and then implemented 5 nitrogen addition regimes(N0:0,N1:5,N2:10,N3:20,N4:30,g N·m-2·a-1).This study analyzed the plant community assembly characteristics and the relationship between plant community,soil microbial community structure changes and environmental factors of degraded grassland after two growth seasons with different levels of nitrogen addition.And thus we expected to provide theoretical basis for further understanding of the biological change process of alpine degraded grassland and its restoration and functional maintenance.The main results were showed as follows:(1)Alpine grassland degradation drove the replacement of plant species,and the structure of plant functional groups indicated grassland degradation.Grassland degradation increased the plant richness and gravel mass ratio,in which 23?36?34?31 and 32 species were recorded from I to V,and the gravel mass ratio increased from 0 to 34.59%.However,grassland degradation decreased community productivity(from 122.70 to 17.77 g·m-2)and community coverage(from 89.49%to 20.47%).The phylogenetic diversity index of I was significantly lower than that of rest degraded sgrasslands(P<0.05).Obvious plant replacement processes occurred among the communities with degradation intensifying.The internal structure of the sedge functional group had an obvious ecological process of replacing Kobresia with Carex becoming one of the dominant species in V.Meanwhile,the sedge group coverage and productivity declined whereas that of grasses,forbs and legumes showed an unimodal change trend.The dominance of shrubs and cushion plants in V had increased significantly,which also provided support for the maintenance of the community structure and productivity in V.Therefore,changes in the structure and composition of plant functional groups may indicate alpine grassland degradation.In addition,the net relatedness index(NRI)and nearest taxon index(NTI)of the community indicated a"clustering-random-overdispering-random-clustering"phylogenetic structure along the degradation.(2)Nitrogen addition drove unimodal changes in vegetation coverage of degraded grasslands,changed the niche characteristics of dominant species,and enhanced the impact of soil environmental factors on the structure of plant communities.The coverage increased firstly and then decreased with the increasing in nitrogen addition.N2 promoted the coverage of I and II,III,and IV to their peaks,increasing the coverage of the communities by 22.78%,33.47%,17.36%and 72.00%,respectively.And N1 promoted the V to reach the highest coverage,which increased by 24.24%.For the coverage of I,N4had not yet reached nitrogen load,but N4 inhibitted the coverage in degraded grassland.Nitrogen addition increased the niche overlap in I and II with an increase of 6.87%-26.25%,and reduced the niche overlap in V with a decrease of 5.92%-30.84%.Meanwhile the niche overlap in III and IV fluctuated with different nitrogen addition.After nitrogen addition,the Faith's PD index had a decreasing trend,the NRI index and NTI index changed weakly.The changes of environmental factors along the degradation had a higher impact on the plant communities than nitrogen addition,which mainly affected the coverage and biomass,but Faith's PD was less affected.(3)The soil microbial community structure of different degraded alpine grasslands responded differently to nitrogen addition.Proteobacteria,Actinobacteria,Acidobacteria and Chloroflexi were the dominant bacteria in the study area.Nitrogen addition decreased Chao1 and richness indices of I,II,III,and IV.Whereas these indices in V increased firstly and then declined with nitrogen addition.N4 led to convergent changes in the bacterial Chao1 and richness indices of grasslands with different degradation degrees.The bacterial Chao1,richness,Shannon,and Simpson indices of I were less affected by nitrogen addition(P>0.05).N1 and N4 drove the dispersal of bacterial community structure in I and II.The bacterial community structures in IV and V treated by nitrogen additions differed greatly,but they were similar in III among nitrogen additions.The key environmental driving factors of bacterial communities under N0 and N4 were urease and ammonium nitrogen;plant community factors and soil microbial biomass were the key driving factors in N2;and soil moisture was the key factor under N3.Ascomycota and Basidiomycota were the dominant fungi in the study area.The soil fungal community structure was scattered among degraded grasslands.The abundance of Ascomycota responded differently to nitrogen addition increasing in different degraded grasslands,and Basidiomycota showed a reverse trend compared with Ascomycota.N2 promoted the diversity of soil fungi in all degraded grasslands.N4 decreased the fungi diversity in II,III and IV,but it can promote the fungi diversity in I and V.Nitrogen addition increased the similarity of fungi community structure and weakened the driving effect of environmental factors on the fungi relative abundance along the grassland degradation.However,the fungi community structure still showed obvious regional distribution regimes.(4)The main functional types of soil bacteria were Carbohydrate metabolism,Amino acid metabolism,and Membrane transport predicted by Tax4Fun2.Nitrogen addition increased the relative abundance of Metabolism and reducing the Environmental Information Processing,Cellular Processes and Human Diseases significantly.The functional structure of soil bacteria in III was dispersal among nitrogen additions.N3 or N4 caused the bacterial functional structure in IV and V to be separated from N0 and N1,and the functional structure of bacteria in I and II was relatively stable.The N1 and N2reduced environmental factors number that significantly affected the bacterial functional structure in the study area.FUNGuild function prediction showed that there were 7 fungal trophic modes in the study area,and 50 guild level functional groups were fully identified.Degradation or nitrogen addition promoted facultative trophic types whereas monotrophic trophic types were usually inhibited.N1 and N2 reduced the degree of fungal functional groups being regulated by the environment.(5)The soil bacteria and fungi molecular ecological network along the degradation was constructed based on random matrix theory(RMT).Soil bacteria and fungi were mainly positive correlations on different degraded grasslands.Degradation increased the proportion of competitive relationships among bacterial communities(from 19.21%to 23.71%),while reduced competition among fungal communities(from 32.47%t o 16.67%).The bacterial network structure of I and IV was more unstable,while the fungal network structure of III was more susceptible to environmental changes.Soil TP and C/N had a significant effect on the fungal network structure,and TP also effected the bacterial network structure of IV and V greatly.The influence of environmental factors on the network structure of soil fungi was stronger than that in bacteria.The significant effects of total soil nutrients,microbial biomass,enzyme activities and plant community factors on the soil fungal network structure mainly occurred in III,IV and V,while the significant effects of soil available nutrients mainly occurred in I.The study revealed the differences of plant communities structure along the grassland degradation on the Qinghai-Tibet Plateau,and proposed that plant functional groups could indicate alpine grasslands degradation.In addition,results revealed that there was a nitrogen load in the degraded alpine grasslands,5?10 g N·m-2·a-1 can effectively promote the grasslands vegetation coverage.And the results improved the understanding of the community structure changes of soil bacteria and fungi under different nitrogen addition in alpine degraded grasslands on the Qinghai-Tibet Plateau;and revealed the response mode of soil microbial functions in the alpine degraded grasslands after nitrogen addition. |
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