Effects Of Logs On The Characteristics Of Soil Microbial Community

Coarse woody debris,mainly decaying logs,account for about 20-30% of forest woody biomass,playing crucial roles in forest regeneration,soil fertility maintenance,material circulation,and biodiversity conservation.Previous studies have demonstrated that the organic matter decomposition of logs and nutrient cycling are mainly achieved through the biochemical degradation process of soil microorganism,which are closely related to the characteristics of logs(such as types and decay classes)and decomposing environment.Decaying logs and their concomitant gaps can directly affect soil microbial diversity and community structure by changing microenvironment,and indirectly affect it by changing the epiphyte and the process of log decomposition,but these processes lack systemic attention.Therefore,we conducted an experiment incubating decaying logs in a subalpine coniferous forest on the eastern QinghaiTibet Plateau,China.Five classes(I-V)of decaying Minjiang fir(Abies faxoniana)logs were incubated on the forest floor under gap center,gap edge and closed canopy in 2013,and soil physicochemical properties and microbial community structure and diversity in soils underneath decaying logs were studied in the early,middle and late growing seasons during2019 across different forest gap positions.The main findings were showed as follows:(1)Soil temperature and p H value increased with the increase of the log decay classes,and were highest under the gap center and lowest under the closed canopy,respectively,while the opposite trend was found for soil water content.The contents of soil organic carbon(C),total nitrogen(N)and total phosphorus(P),C/N ratio and microbial biomass also increased with the process of log decomposition,and were highest for decay classes IV or V,while the N/P ratio and the ratio of microbial biomass showed an opposite trend.Soil C,N and P contents and microbial biomass underneath highly decayed logs were significantly lower under the gap center than under the closed canopy.(2)A total of 6505 operational taxonomic units(OTUs)were detected on the region of the fungal ITS2 gene by Illumina Novaseq sequencing analysis,which were subordinate to 16 phyla,51 classes,146 orders,329 families,and 751 genera.The Alpha diversity indices for soil fungal communities were highest for decay classes III and IV and showed lowest under the closed canopy.Basidiomycota and Ascomycota were the most dominant soil fungal phyla.The relative abundance of Basidiomycota increased during the process of log decomposition,and showed lowest under the gap center and highest under the closed canopy.The relative abundance of Ascomycota decreased during the process of log decomposition,and which did not differ significantly among gap positions.At the genus level,Inocybe,Lactarius,Russula,Fusarium,Piloderma,Ilyonectria,Mortierella and Sebacina were the most abundant genera.The relative abundance of Inocybe decreased during the process of log decomposition,and that of Lactarius and Russula were highest for decay classes V.The relative abundances of Lactarius,Russula,Piloderma,and Sebacina were lowest under the gap center,while that of Fusarium and Ilyonectria were lowest under the closed canopy.(3)A total of 11291 operational taxonomic units(OTUs)were detected on the region of the bacterial 16 S V3-V4 gene by Illumina Novaseq sequencing analysis,which were subordinate to 49 phyla,58 classes,135 orders,271 families,and 703 genera.The Alpha diversity indices for soil bacterial communities were highest for decay classes III and IV and showed lowest under the closed canopy.Proteobacteria,Acidobacteria and Actinobacteria were the most dominant soil bacterial phyla.Proteobacteria and Actinobacteria in abundances were lower under highly decayed logs,while the opposite trend was found in Acidobacteria.Additionally,the relative abundance of Proteobacteria was highest under the gap edge,that of Acidobacteria was highest under the closed canopy,while that of Actinobacteria was highest under the gap center.At the genus level,Bradyrhizobium,Candidatus＿Solibacter,Bryobacter,Acidothermus,Acidibacter,Granulicella,Roseiarcus,and Haliangium were the most abundant genera.The relative abundances of Bradyrhizobium,Bryobacter,Acidibacter and Roseiarcus were higher under highly decayed logs.The effect of gap positions on dominant genera of bacteria varied with log decay classes.(4)Analysis of similarity(ANOSIM)and permutation multivariate analysis of variance,nonmetric multidimensional scaling analysis(NMDS)and principal coordinate analysis(PCo A)based on Bray-Curtis(PERMANOVA)showed that soil fungal and bacterial community characteristics varied significantly with log decay classes and gap positions.Redundancy analysis(RDA)showed that Shannon index and Simpson index in soil fungi were significantly positively correlated with soil total organic carbon,total nitrogen and total phosphorus contents,and Chao1 index and ACE index in soil fungi were significantly positively correlated with the ratio of microbial biomass nitrogen to microbial biomass phosphorus.Basidiomycota and Ascomycota in abundance were significantly positively correlated with soil temperature and p H value,respectively.Inocybe in abundance was significantly positively correlated with the N/P ratio and p H value,and that of Piloderma and Cortinarius were significantly positively correlated with soil water content.Additionally,Shannon index in soil bacteria was significantly positively correlated with soil total organic carbon,total nitrogen and total phosphorus contents;while Chao1 index and ACE index in soil bacteria were significantly positively correlated with soil temperature.Proteobacteria,Acidobacteria and Actinobacteria in abundance were significantly positively correlated with the N/P ratio,microbial biomass carbon,and the ratio of microbial biomass nitrogen to microbial biomass phosphorus,respectively.Bradyrhizobium and Granulicella in abundance were significantly positively correlated with soil temperature,and Haliangium in abundance was significantly positively correlated with the ratio of microbial biomass nitrogen to microbial biomass phosphorus.In conclusion,decaying logs increased microbial biomass and microbial diversity of soil in subalpine coniferous forests,and slow decomposition process of logs provided a favorable environment and habitat for soil microbial growth and reproduction.Additionally,forest gaps reduced soil microbial biomass and increased microbial diversity by changing soil temperature,water content and p H value,and the effect was stronger in highly decayed logs.These research results provide a reference for further understanding of the material cycle process and productivity maintenance mechanism of subalpine forest ecosystems.