Effects Of Thinning On Community Characteristics And Assembly Mechanisms Of Soil Microbial In A Larix Olgensis Plantation

Larch（Larix spp.）is a major afforestation specie in northeast China and plays an important role in ecological construction and forestry development.However,unreasonable management will lead to low yields and land degradation in larch plantations.Soil microorganisms play an extremely important role in regulating nutrient cycling,participating in substance turnover and energy flow,decomposing plant residues,and maintaining land productivity.Thinning as the major management measure of the plantation,can promote the growth of stand and improve the environment in the forest by reducing the nutrient competition of the remaining trees.Previous studies have shown that different treatments of thinning residues（residues retention and residues removal）and different thinning intensities lead to changes in the understory microenvironment,soil properties,and vegetation community distribution.However,the response of soil microbial communities to such changes is still unclear,and in particular,the impact on the functional microbial communities involved in the nitrogen cycle is poorly understood.Therefore,it is particularly important to reveal the diversity,composition,and community assembly mechanism of soil microorganisms.To this end,Mengjiagang National Forest Farm,Jiamusi City,Heilongjiang Province was used as the research area,soil of Korean larch（Larix olgensis）plantation under different management of thinning residues（residues retention RT,residues removal RM）and different thinning intensities（CK,control;LT,quartic thinning with low-intensity;MT,triple thinning with medium-intensity;HT,twice thinning with high-intensity）as the research objects,soil properties of Larix olgensis plantation were determined in the 0-10cm,10-20 cm soil layer,relevant marker genes for soil microbes（bacterial 16S r RNA,nitrogen-fixing microbes nif H gene,denitrifying microbes nir S and nos Z genes）in the 0-10 cm soil layer were amplified.The community composition,diversity,and community structure characteristics of these soil microbial were thoroughly analyzed by high-throughput sequencing and bioinformatics techniques.Combined with soil properties,phylogenetic characteristics,molecular ecological network,and community construction process,the analysis of stability and response mechanism of soil microbial community structure under different management of thinning residues and different thinning intensities.The main results are as follows:（1）The soil property of the Larix olgensis plantation changed significantly under different treatments of thinning residues.The soil property（except C:N）in the 0-10cm soil layer were higher in the residue retention than in the residue removal treatment.The soil TC,TN,TP,AP and NO₃^-in the 10-20cm soil layer were all higher in the residue retention treatment than in the residue removal treatment.Residue retention treatments reduced soil bacterial community alpha diversity.LEf Se analysis showed that different treatments of thinning residues significantly affected the bacterial taxa involved in carbon and nitrogen metabolism,the Bacteroidetes as copiotrophic bacteria were significantly enriched in residue retention treatment,and the Rhizobiales with nitrogen fixation were significantly enriched in residue removal treatment.Adonis test and redundancy analysis showed that there were some differences in soil bacterial community structure among different treatments of thinning residues,and soil TC and TN were the main driving factors of bacterial community structure changes.（2）Different treatments of thinning residues significantly affected soil nif H,nir S and nos Z gene abundances,community composition and diversity in Larix olgensis plantation.The abundances of nif H and nir S genes were significantly reduced in the residue retention treatment soils,while the abundance of nos Z genes did not change significantly.Theα-diversity analysis showed that the nif H gene community richness and diversity were increased,while the nir S and nos Z gene community richness and diversity were decreased in thinning residue retention treatment.The soil nir S and nos Z gene sequences were mainly from Proteobacteria.Different treatments of thinning residues had a greater impact on the composition of the nir S gene community,while the nos Z gene community composition had a lesser effect.Proteobacteria and Actinobacteria were the dominant nitrogen-fixing taxa in the nif H gene community at the phylum level,and the residual treatment significantly decreased the relative abundance of non-symbiotic nitrogen-fixing bacteria such as Azoarcus,Herbaspirllum and Dechloromonas.Mantel correlation analysis showed that soil TC,TN and MBC were the main environmental factors that significantly affected the community structure of nif H and nir S genes in different treatments of thinning residues,while NH₄⁺and NO₃^-were the main environmental factors affecting the nos Z gene community.（3）The interrelationship between nodes in soil bacterial and functional microorganisms network of Larix olgensis plantations under different thinning residues treatments is mainly"synergistic".Proteobacteria were the main keystone taxa in the bacterial network,and they were mainly enriched in the residue retention treatment.Nitrogen cycle function microbial molecular ecological network analysis showed that compared with nos Z genes,the nif H and nir S gene community networks were more complex and vulnerable to external interference.Network of nos Z gene community were more stable and less susceptible to environmental perturbations.Key species in the nif H gene network were enriched in soils of thinning residues removed,resulting in close interactions between species designed to resist nitrogen-deficient soil environments.Rare species play an important role in maintaining the stability of the nos Z gene community.The phylogenetic null model analysis showed that the soil bacterial community construction process under different treatments of thinning residues was mainly driven by deterministic processes.Thinning residue retention increased the proportion of homogenizing dispersal in the stochastic process of soil nif H gene community assembly and the proportion of heterogeneous selection in the deterministic process of nir S gene community assembly,but had no effect on the nos Z gene community assembly process.（4）The soil properties,bacterial community diversity,composition and structure in Larix olgensis plantations under different thinning intensities changed significantly.The nutrient content of 0-10cm surface soil increased significantly in the thinning treatment,and peaked mainly in the medium thinning treatment.In the 10-20cm soil layer,all indexes were the highest in the low intensity thinning treatment.At the phylum level,soil bacteria were dominated by Actinobacteria,Proteobacteria and Acidobacteria.The relative abundance of Actinobacteria was significantly increased in low,medium and high intensity thinning treatments.LEf Se analysis showed that the relative abundances of bacterial taxa involved in carbon metabolism（such as Bacteroidetes,Firmicutes and Planctomycetes）were significantly changed in soils with different thinning intensity treatments.Oligotrophic bacteria and disease suppressive bacteria were significantly enriched in the control soil.Theα-diversity of soil bacterial community increased with increasing thinning intensity,in which Simpson and Chao1 indices were significantly positively correlated with TC and TN.The Adonis test showed that the soil bacterial community structure was significantly different under different thinning intensity treatments.The results of redundancy analysis and random forest prediction model both indicated that TC was the most important environmental factor driving the change of bacterial community structure.（5）The abundance and community structure of soil nif H,nir S and nos Z genes in Larix olgensis plantation changed significantly under different thinning intensity treatments.The abundance of nif H and nir S genes in the thinning treatment soils was significantly decreased,while the abundance of nos Z gene was significantly increased.The abundance of nif H gene was the lowest in the medium thinning treatment;the nir S gene abundance was the lowest in the low thinning treatment;the nos Z gene abundance was the highest in the medium thinning treatment.The Shannon and Simpson indices of the nif H gene community were increased in the thinning treatment soils,while the Shannon and Simpson indices of the nir S gene community were decreased,while the diversity of the nos Z gene community had no significant change.At the phylum,class and genus levels,the nif H gene community was dominated by Proteobacteria,δ-Proteobacteria and Geobacter,respectively.Both the nir S and nos Z gene communities were dominated by Proteobacteria.The soil nif H and nir S gene community composition changed significantly under different thinning intensities,while the soil nos Z denitrifying microbial community composition changed little.There were significant differences in soil nif H,nir S and nos Z gene community structure among different thinning intensity treatments.TC,TN and AP are the main environmental factors affecting the community structure of nif H gene;TC,NO₃^-and AP are the main environmental factors driving the community structure variation of nir S gene;TN and AP are the main driving factors causing the community structure variation of nos Z gene.（6）The soil bacteria network nodes are mainly"synergistic"under different thinning intensities.Key species in the bacterial network mainly belong to Proteobacteria,followed by Firmicutes.The number of key species in the bacterial network was less in the thinning treatment,which reduced the tightness of connections between nodes in the bacterial network.Phylogenetic null modeling indicated that thinning treatment increased the proportion of homogenizing dispersal in the stochastic process of soil bacterial community assembly.Network parameters of nitrogen cycle functional microorganisms showed that the molecular ecological networks of the nif H and nir S gene communities were more susceptible to external disturbances,while the nos Z gene community were more stable.Number of negatively correlated links in the nif H gene community network was more than the number of positively correlated links,indicated strong competition between nodes.The key species in the nif H gene network were enriched in the control treatment,the number of key species in the nir S gene network in the thinning treatment was overall higher than that in the control treatment,and the distribution of key species in the nos Z gene network had no obvious regularity.Phylogenetic null model analysis showed that thinning treatment increased the proportion of undominated processes in the stochastic process of nif H nitrogen fixing microbial community assembly and increased the proportion of heterogeneous selection in the deterministic process of nir S and nos Z denitrifying microbial community assembly compared to the control.In summary,this study used molecular biological techniques to analyze soil properties,community characteristics and construction mechanisms of soil microorganisms（bacteria,nitrogen-fixing microorganisms and denitrifying microorganisms）in Larix olgensis plantation under different treatments of thinning residues and different thinning intensities.In terms of soil nutrient content,soil microbial community composition and nitrogen function gene abundance,the thinning residue retention treatment was superior to the residue removal treatment.Similarly,thinning intensity management measures with moderate intensity thinning is the best.With the increase of soil nutrient content,the proportion of random process in the assembly of nitrogen-fixing microbial community increased.On the contrary,the community assembly of denitrifying microorganisms（nir S）mainly showed that the proportion of heterogeneous selection process increased.These results lay a scientific foundation for further research on nutrient cycling in larch plantation soils,especially the processes and laws related to nitrogen cycling,and to provide a theoretical basis for the restoration and sustainable management of larch plantation forests.