| The structure and function of forest microbial communities and the effect of plant-microorganism interaction with soil on carbon and nitrogen cycling in forest ecosystems are one of the most advanced scientific issues in ecology.As an important component of the ecosystems,mycorrhizal,rhizosphere and non-rhizosphere soil fungi are considered to play important roles in the biodiversity maintenance,succession,and functioning of forest ecosystems.Moreover,the structure and function of the microbial community also respond to environmental heterogeneity.The subalpine forest distributed in western Sichuan experienced extensive forest harvesting during the 1950s to 1980s.At present,they are in the stage of comprehensive restoration which forming forest communities with different succession stages.It provides an ideal platform for the development of plant population dynamics,community diversity,and the construction mechanism of underground mycorrhizal,rhizosphere and non-rhizosphere fungal communities,as well as the effects of plant-soil-fungal interaction on carbon and nitrogen cycle patterns during forest succession.Hence,this study was carried out in secondary forests and well-preserved primary old forests in the subalpine areas of western Sichuan based on the space-substitution-time method.Firstly,community ecology investigation was carried out by setting monitoring plots and the mycorrhizal and rhizosphere soil samples of dominant tree species as well as the bulk soil samples were collected.Soil physiochemical properties of rhizosphere and non-rhizosphere soil at different depths(0?10 cm and 10?30 cm),as well as microbial enzymes that significantly affect soil carbon and nitrogen cycle,were measured.Secondly,The biomass of soil bacteria,fungi,and arbuscular mycorrhizal(AM)fungi were determined based on phospholipid fatty acid method,and the fungal species composition of ectomycorrhiza(EM)and rhizosphere of symbiotic dominant trees,and community-level EM,AM and bulk soil was determined using Illumina high-throughput sequencing technique.Thirdly,several mathematical statistical methods were implemented to quantify species diversity and phylogenetic structure of trees,shrub and grass species in forest communities at different succession stages,and then making a comparison of key soil parameters related to carbon and nitrogen cycling modes during forest succession,and then analyzing the dependence of the composition of ectomycorrhizal and rhizosphere fungi communities on the host tree during secondary forest restoration,and ultimately partitioning the effects of plant diversity and environmental factors on fungal community composition of EM,AM and bulk soil fungi at the community level.Moreover,the microbial molecular ecology network was constructed to systematically reveal the driving ecological processes of coexistence patterns of fungal communities.Lastly,this work attempt to decouple the plant-soil-fungus interactions to reveal how the ectomycorrhizal tree species influence the soil carbon and nitrogen cycling modes through regulating the underground microorganisms and their enzyme activities in the process of succession under mycorrhizal perspective.The main conclusions are as follows:1.Species diversity of trees,shrubs and grasses is abundant(totally with 37 families,85genera and 112 species)in different succession stages of subalpine forests in western Sichuan.In the process of forest succession,the species composition of tree layer is characterized by phylogenetic overdispersion,suggesting biotic competition plays a dominant role in tree species composition.The shrub layer and herb layer are affected by community characteristics,topography and soil environmental factors.The species composition of the shrub layer shows significant phylogenetic clustering during succession,indicating that environmental filtering has an important force in community construction at this layer.There was no significant difference between the herbaceous layer and random effect(null model)in neither succession stages,which demonstrates that the neutral theory is more suitable for explaining the coexistence pattern of herbaceous species.2.The key parameters of soil carbon pool,nitrogen pool and nitrogen flux in the subalpine forest ecosystems of western Sichuan showed significant during succession,which including total soil carbon(F=6.054,P=0.015),total nitrogen(F=4.365,P=0.038),non-organic nitrogen(F=4.047,P=0.045),ammonium nitrogen(F=4.735,P=0.031)and ratio of non-organic to organic nitrogen(F=16.960,P<0.001).The key parameters of soil carbon and nitrogen cycling correlated closely.The increasing of soil carbon and nitrogen stocks resulted in the increasing of microbial biomass.Soil nonorganic nitrogen and its turnover feedbacked soil carbon and nitrogen stocks.Soil microbial biomass plays a positive role in the conversion of soil organic nitrogen into inorganic nitrogen.3.The dominant species composition and population dynamics of subalpine secondary forest communities in western Sichuan have different effects on the construction of symbiotic EM fungal communities.In forest communities with Abies faxoniana and Betula utilis as dominant species,the main influencing factors of EM fungal community composition are leaf productivity,root biomass,total biomass and soil nitrate nitrogen(41.2%variance explained by the biotic and abiotic factors).The influencing factors of the rhizosphere fungal community are like those in EM fungal community,and both biotic and abiotic factors explained 20.3%variation of the fungal community.However,in forest communities with A.faxoniana and Betula albosinensis as dominant species,EM fungal community composition was not significantly affected by host or soil environmental factors,while the rhizosphere fungal community composition was significantly affected by the relative density of dominant species,total biomass and dissolved organic carbon,ammonium nitrogen,nitrate nitrogen content and carbon-nitrogen ratio(totally explained 30.1%variation).Similar to the factors affecting the structure of EM fungi community,the biotic and abiotic environmental factors explain 20.3%of the variability of rhizosphere fungi community structure of dominant species in A.faxoniana and B.utilis forest communities,while the forest community with A.faxoniana and B.utilis as dominant species is different from the corresponding factors that affecting the structure of EM fungi community.The population's relative density,total biomass,soil dissolved organic carbon,ammonium nitrogen,nitrate nitrogen content and soil C:N ratio significantly shaped rhizosphere fungal community composition(interpretation rate was 30.1%).4.At the community level,the community composition of mycorrhizal fungi(EM vs.AM)showed different patterns during forest succession in the subalpine of western Sichuan.The species composition EM fungi community was not significantly different between successional forests,while AM fungal community changed significantly during forest succession.The two PCo A dimensional coordinate axes accumulatively explained 44.62%of the variability of AM fungi communities.Variance partitioning analysis(VPA)showed that plant community diversity and soil environmental factors were the main driving factors(30.79%)of the diversity of AM fungal community during forest succession.The explanatory ability for the structural variability of EM fungi community was relatively weak(11.76%)and the explanatory degree(5.7%)for the soil fungi community was almost negligible.Molecular ecological network analysis showed that a modularity displayed in the species composition of these fungal communities,and the positive and negative correlation connectivity and the number of links among OTUs changed significantly during forest succession,which providing a visual evidence for mutual benefit or competitive exclusion among fungal communities.5.The variation of the relative dominance of ectomycorrhizal tree species during forest succession is significantly related to the soil fungi-bacteria ratio(R~2=0.634,P<0.001),and also to the stoichiometric characteristic of key microbial enzymes and their stoichiometry(i.e,C:N?active,R~2=0.487,P=0.002).Variance partitioning analysis(PA)showed that Nonactive was closely related to different functional fungal groups(AM,EM,bulk soil fungi explained75%of total variation),suggesting that ectomycorrhizal trees most likely to altering rhizosphere and non-rhizosphere fungal community,and then influence soil microbial components and their biomass result in changing of enzyme activity and eventually influence soil carbon and nitrogen cycling. |
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