| Eucalyptus(Eucalyptus spp.)has been planted on a large scale in southern China over the last two decades because of its fast growth rate and good economic benefits.However,monoculture planting patterns combined with short rotation periods have resulted in a series of problems such as severe soil fertility loss,declining soil organic carbon and reduced biodiversity,which have reduced soil quality.It has been found that the effect of mixing eucalypts with nitrogen-fixing species is important for improving soil quality,but what effect does mixing eucalypts with broad-leaved species have on soil physicochemical properties,enzyme activity and soil microorganisms?How does the structure and diversity of soil bacterial and fungal communities develop after hybridisation?What are the changes in soil microbial stability in mixed forests?To address these questions,this study transformed Eucalyptus pure stands into mixed stands of Eucalyptus with Magnolia glauca,Michelia macclurei and Mytilaria laosensis in the main plantation area of Eucalyptus plantations in China,and used adjacent broadleaved evergreen forests and Eucalyptus pure stands as controls to The soil physical and chemical properties and enzymatic activities of the understorey were measured,the condition of the forest floor was investigated,and the structural and functional diversity of the soil microbial communities in the different forest floors were characterized using high-throughput(16SrRNA and ITS)sequencing molecular biology methods to reveal the changes in the soil physical and chemical properties of the different forest floors and their microbiological mechanisms.The main studies are as follows.(1)Mixing Eucalyptus with native species significantly improved soil acidification and increased soil sucrase and catalase activities,microbial biomass carbon,microbial biomass nitrogen and the ratio of microbial biomass carbon and nitrogen,but reduced soil urease and acid phosphatase activities.Principal coordinates analysis revealed that nitrate nitrogen,ammonium nitrogen,pH and soil organic carbon were key factors in the physicochemical properties of soils from different woodlands.Soil nitrate nitrogen,ammonium nitrogen and soil organic carbon were the key soil physicochemical factors that positively affected soil urease activity;soil pH and soil organic carbon were the key soil physicochemical factors that positively affected soil sucrase and peroxidase activities.(2)The dominant bacterial phyla among different stands were Proteobacteria,Actinobacteria,Acidobacteria and Chloroflexi.Soil sucrase activity,catalase activity,bacterial richness and diversity index were higher in Eucalyptus mixed forests with Magnolia glauca and Eucalyptus mixed forests with M.laosensis than in Eucalyptus pure forests,while urease and acid phosphatase activities were lower than in broadleaved evergreen forests.The functional predictions showed that the hybrid treatment significantly increased the relative abundance of soil bacterial metabolic functions compared to Eucalyptus pure stands.Line discrimination analysis showed that bacterial biomarkers with nitrogen fixation and phosphate solubilisation functions(Burkholderiales,Bradyrhizobium,Flavisolibacter)were enriched in the Eucalyptus and Ashwagandha mixed forests.Redundancy analysis showed that pH,soil fast-acting boron content,and soil water content were the main soil environmental factors driving changes in the structure of the soil bacterial community.(3)The main fungal communities among the different stands were Basidiomycota,Ascomycota,Rozellomycota and Mortierellomycota.The soil fungal community structure of the mixed forest system is clearly separated from that of the broadleaf evergreen forest and the Eucalyptus pure forest,with the mixed forest of Eucalyptus and M.laosensis located at the very edge.In FUNGuild,the number of Arbuscular mycorrhizal fungi was higher in mixed plantations and Ectomycorrhizal fungi was higher in mixed plantations than in Eucalyptus pure forests,and the number of Ectomycorrhizal fungi was higher in mixed plantations than in broadleaved evergreen forests.The soil fungal communities of Eucalyptus and M.glauca mixed forests were highly correlated with fast-acting phosphorus.The covariance network analysis revealed that the soil fungal community of Ectomycorrhizal fungi in Eucalyptus mixed forests had a less stable network structure compared with Eucalyptus pure forests,while Eucalyptus mixed forests with M.laosensis showed a more stable network structure.(4)Correlation heat map analysis showed a significant correlation between soil nutrients and major soil bacteria,with pH showing a highly significant positive correlation with the Mucorales phylum.Structural equation modelling showed that soil nutrients had a significant positive effect on soil enzyme activity and soil microbial biomass nitrogen,a highly significant negative effect on soil fungal community diversity and richness index.Studies have shown that soil nutrient factors have direct or indirect effects on the diversity of fungal communities.A significant negative effect of soil fast-acting phosphorus on soil acid phosphatase activity,a significant positive effect on the relative abundance of Arbuscular mycorrhizal fungi,a significant positive effect of soil acid phosphatase activity on the relative abundance of Arbuscular mycorrhizal fungi,a significant positive effect of Arbuscular mycorrhizal fungi on the relative abundance of Arbuscular mycorrhizal fungi,and a significant positive effect of Arbuscular mycorrhizal fungi on the relative abundance of Arbuscular mycorrhizal fungi.There was a significant positive effect of soil acid phosphatase activity on the relative abundance of mycorrhizae and a significant positive effect of relative abundance of mycorrhizae on soil microbial biomass N.The results of this study show that soil fast-acting phosphorus content regulates the relative abundance of mycorrhizal fungi and thus the amount of microbial biomass nitrogen in the soil. |
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