The Effects Of Reforestation On Soil Microbial Diversity And Function In Yunnan

Facing the increasingly fragile ecological environment,reforestation is an effective measure to repair and maintain the function of soil ecosystem.Soil microbial community plays an important role in the“grain for green”project.Therefore,revealing the effects of reforestation on soil microbial diversity and function provides important information for the evaluation of the environmental effects of ecological projects such as“grain for green”,but relevant research is still scant.Yunnan is rich in ecosystems and climate types.The staggered distribution of agroforestry ecosystems is a significant feature of this region,which provides superior research conditions for systematically analyzing the effects of reforestation on soil microbial diversity and function.Based on the comprehensive consideration of ecosystem typicality and spatial uniformity,this thesis collected soil samples from 30 forests and their adjacent croplands in Yunnan.Amplicon and metagenomic sequencing were used to reveal the effects of reforestation on soil microbial diversity and function,soil microbial driven biogeochemical cycle related gene,heavy metal resistance gene,pathogens,and antibiotic resistance gene abundance.The main research results of this paper are as follows.（1）The microbial abundance of forest soils was significantly higher than that of croplands.The abundances of prokaryotes and fungi per gram of dry soil in forest soil were 2.19×10¹⁰ and 3.8×10⁸,respectively.In cropland soil,they were 1.27×10¹⁰ and9.06×10⁷,respectively.However,there was no significant difference in microbial richness between cropland and forest soils.Soil prokaryote communities in cropland and forest soils were mainly dominated by Acidobacteria,Proteus,and Bacteroides,which accounted for more than 70%of prokaryote communities.More than 95%of fungi were classified as Ascomycetes,unclassified fungi,and basidiomycetes.The community composition of prokaryotes and fungi showed significant differences between croplands and forests.The relative abundances of Archaea,Cyanobacteria,Armorobacteria,and Chytridiomycetes in cropland soils were higher than those in the forest,while the relative abundances of bacteria,Acidobacteria,and Basidiomycota were lower than those in the forest.（2）The copy number of most functional genes involved in carbon,nitrogen,phosphorus,and sulfur cycling in forest soils was significantly higher than that in croplands,and the total copy number was 75.69%,70.33%,82.83%and 105.13%higher than that in croplands,respectively.These functional genes mainly include:rbc L,kor A,amy A,abf A,pmo A,and mmox genes involved in the process of carbon cycle;nif H,nir K,nira,hao,and nir B genes involved in the nitrogen cycle;ppk,pho D,phn K,and pqq C genes involved in phosphorus cycle;cys J gene involved in the sulfur reduction and all functional genes involved in sulfur oxidation.In addition,the moisture,total nitrogen,total carbon,and ammonia nitrogen of cropland and forest soil samples showed a significant positive correlation with most functional genes in the process of carbon,nitrogen,phosphorus,and sulfur cycling.（3）There were abundant antibiotic resistance genes in cropland and forest soils,which mainly belonged to vancomycin resistant,multi drug resistant,and bacitracin resistant peptides.The total copy number of antibiotic resistance genes in forest soil was 62.39%higher than that in the croplands,but the richness was significantly lower than that in the cropland soil.The abundance of chloramphenicol and sulfonamide resistance genes in cropland soil was significantly higher than that in forests,while the abundance of most other antibiotic resistance genes was lower than that in the forests.There was no significant difference in copy number and richness of total heavy metal resistance genes between cropland and forest ecosystems.The copy numbers of integrons,insertion sequences,and mobile genetic elements in forest ecosystem were74.73%,205%,and 104.69%higher than those in cropland,respectively.There was no significant difference in the copy number of total pathogens between cropland and forest ecosystems,but the copy number of 11 pathogens detected in forest ecosystem was significantly higher than that in cropland.Exophiala dermatidis and venturia inaequalis were unique pathogens in forest ecosystem.（4）The copy numbers of multidrug,macrolide,bacitracin,tetracycline,and unclassified resistance genes in the forest soils showed a significant positive correlation with the content of soil total carbon and total nitrogen,while the copy numbers of antibiotic resistance genes in cropland did not show a significant correlation with environmental factors.The correlation between soil microbial community and antibiotic resistance genes in the forest was stronger than that in cropland ecosystem,and the correlation between antibiotic resistance gene and bacterial community was stronger than that in fungal community.The co-occurrence of antibiotic resistance genes,heavy metal resistance genes,and pathogenic microorganisms in cropland soils were significantly stronger than that in forest soils.Therefore,heavy metal pollution in cropland soils is more likely to lead to the enrichment of antibiotic resistance genes,and antibiotic resistance genes in cropland soils are more likely to lead to the emergence of drug-resistant pathogens.In addition,compared with croplands,the correlation between forest soil antibiotic resistance genes and mobile genetic elements was stronger,which indicates that reforestation significantly enhanced the transmission ability of soil antibiotic resistance genes.The findings in this thesis not only provide a theoretical basis for the formulation of cropland and forest ecosystem management strategies,but also provide data support for the evaluation of the environmental effects of major projects such as“grain for green”.