The Effects Of Different Eucalyptus Stands On Soil Fungal Community Structures And Functions

Large-area intensively planted fast-growing Eucalyptus plantation has become a strong guarantee for human social and economic development.However,some studies showed that Eucalyptus planting has caused certain adverse effects on the ecosystem,such as reduced biodiversity,decline in soil fertility,and serious soil erosion,which caused the deterioration of ecological environment and the reduction of biological productivity.These studies have questioned and argued about the promotion of Eucalyptus planting,and even against its planting.The soil fungal community structure,function and dynamics were closely related to the above-ground forest stand structure,the size of the forest nutrient pool,the availability of nutrients,biodiversity and even the health of the entire forest ecosystem.Therefore,the soil fungal community structure and functional diversity of Eucalyptus plantations is of important for revealing the interaction between eucalyptus and fungi and the regulation mechanism in nutrient cycling,as well as evaluating the productivity of Eucalyptus,improving the anti-risk ability of Eucalyptus plantation,especially for the sustainable management of Eucalyptus forests.In this study,two locations of typical southern subtropical Eucalyptus plantations were selected as the experimental sites.One was located in Eucalyptus Research Center,Chinese Forestry Academy,Zhanjiang,Guangdong.The other one was located at Tropical Foretry Experimental Center,Chinese Forestry Academy,Pingxiang,Guangxi.The study included Eucalyptus plantations with different species,plantation ages,rotation periods and managements(close to nature),e.g.mixed with nitrogen-fixing plants.The aims of the the thesis are to study 1)the effects of different Eucalyptus plantatons on soil fungal community and functional structures(FUNGuild functional groupings and enzyme activities mainly involved in carbon,nitrogen,phosphorus,sulfur and other cycles)by using high-throughput sequencing and fluorescence microplate technology,2)how the total amount of soil microorganisms(the number of fungi and bacteria)and the soil physical and chemical properties changes,3)the relationship between the stand structure and soil fungal community,the role of plants and soil fungi in regulating the nutrient cycling process of the ecosystem,4)the feedback of soil fungi on the forest and the mechanism of plant and soil fungi in regulating the ecological process.The main results are as follows:1)The analysis of soil physical,chemical and biological properties showed that with the growth of plantation age,the number and structure of soil microorganisms have been optimized.The soil fertility has been restored to a certain extent,and the mineralization rate of the soil organic matter has shown a downward trend.The effect of fertility level was not significant.Theβ-glucuronidase,β-glucosidase and N-acetyl-glucosamidase involved in carbon and nitrogen cycles were significantly different between the Eucalyptus species.The mixing of nitrogen-fixing tree species significantly improved the soil fertility level and physical structure.The organic matter mineralization rate involved in the carbon cycle showed a downward trend,while the acid phosphatase enzyme activity involved in the phosphorus cycle showed an upward trend.The fertility level and the organic matter mineralization rate after continuous planting had a certain downward trend,but the Eucalyptus planting period was extended.The bacterial ratio also dropped significantly,and the soil fertility gradually recovered.The interaction between the soil physical and chemical properties was through the change of microorganisms as an intermediate medium.2)Analysis of the response characteristics of soil fungal community diversity,composition and structure to stand structure changes showed that the forest age,mixing with nitrogen fixation tree specise and continuous planting generations were the important driving forces for soil fungal alpha diversity and community composition among forests.As the age of the Eucalyptus plantation increased,the richness of the fungal community increased while the diversity and uniformity declined.The relative abundance of Ascomycota,Pleosporales,Eurotiales and Chaetothyriales were decreased.The relative abundance of Basidiomycota,Agaricales,Sebacinales,Cantharellales,Russulales,Helotiales and Mortierella were increased in relative abundance.The mixing of nitrogen-fixing tree species significantly increased the richness and diversity of the fungal community.The restoration of soil fertility inhibited the growth of Basidiomycota,so that Ascomycota were dominant.The fungal species richness of the pure second-generation Eucalyptus grandis forest showed an upward trend compared with the pure first-generation forest.The abundance of Basidiomycota decreased,and the abundance of Ascomycota and Mortierella increased.At the same time,the extension of planting years could compensate the negative effect of soil fungal community degradation.Eucalyptus species had no significant effect on the fungal alpha diversity,but selectively enriched certain fungal groups,such as Agaricales,Coprinellus and Pisolithus with high host preference or specificity.3)The analysis of the composition and structure of the soil fungal community and the relationship with environmental factors showed that the changes of the forest stands mainly caused changes in the fungal community structure by driving the fungal groups involved in the carbon and nitrogen cycles.The soil organic carbon,as a key driving factor,mainly drove the community structure of soil fungi through changes in the quantity and quality of litter and root exudates.The total nitrogen was mainly related to the fungal groups with nitrification and denitrification ability and the production of saprophytic fungi as a nutrient source.4)The analysis of the response characteristics of the functional composition and structure of the soil fungal community to the change of the stand structure showed that compared with the mixed tree species and nitrogen-fixing tree species,the forest age and continuous planting generations were more important for the functional groups of soil fungi in Eucalyptus plantations.With the growth of the forest age and the stability and continuity of the understory habitat,the species richness and diversity of understory epiphytes,bryophytes,insects and epiphytic lichens were restored,which in turn led to bryophytes and insect parasitic flora and an increase in the abundance of animal pathogenic flora.At the same time,there were more allelochemicals and higher abundance of Mortierella and endophytic fungal can antagonize plant pathogen in Eucalyptus forests.The enrichment of E.urophylla × E.grandis on Agaricus,Coprinellus and undefined saprotroph with higher average host specificity index fully proves the high correlation between fungal community structure and functional community structure.With the extension of the planting age of mixed nitrogen-fixing tree species and continuous Eucalyptus planting,the diversity of understory plants and soil animals have been restored to a certain extent,which in turn led to the dung saprotroph,lichen parasite and algal parasite in the nitrogen-fixing mixed forest and the second enrichment in the next generation forest.At the same time,environmental factors had different driving options for different functional groups.The total nitrogen content of the soil affected the saprotroph(including undefined saprotroph,soil undefined saprotroph and litter undefined saprotroph)and symbiotroph(ectomycorrhizal and ericoid mycorrhizal)produced opposite trends.The increase in tree species after mixing of nitrogen-fixing tree species and the extension of eucalyptus forest age after continuous planting,the relative abundance of Clonostachys,Mortierella and endophytic fungi are relatively high,and they will have a certain ability to regulate plant pathogen.The main study results suggest that the effects of Eucalyptus forest age and continuous planting generations on the diversity,structure and function of fungal community were more significant than that of Eucalyptus species and mixed nitrogen-fixing species,respectively.At the same time,the environmental factors are also important driving factors for changes in the structure and function of the fungal community through C and N nutrient inputs.In addition,changes in C and N nutrient cycles caused by changes in soil fungal communities and functions will feed back and affect the growth status of above-ground Eucalyptus.The near-natural management of Eucalyptus plantations that extend the planting period and mix nitrogen-fixing tree species can increase soil fungal communities and stability of functional structure.