Effects Of Season And Slope Aspect On Soil Microbial Community Characteristics In Different Climatic Zones In The Upper Reaches Of Minjiang River

Soil microorganisms connect surface and subsurface systems,are important regulators that drive biogeochemical cycles and maintain ecosystem function,and often serve as key indicator species reflecting the stability of soil ecosystems.Season and slope orientation,as important environmental factors,may influence microbial community characteristics by changing temperature and moisture.Most studies in the past have focused on the effects of season or slope orientation on soil microorganisms at a small scale,while the response mechanisms of soil microbial community characteristics to both seasonal and slope orientation factors in different climatic zones at the watershed scale remain unexplored.Therefore,in this study,four climatic zones M1(subtropical humid monsoon climate zone),M2(semi-arid river valley climate zone),M3(plateau monsoon climate zone),and M4(highland mountain climate zone)in the upper reaches of Minjiang River were used to analyze the effects of season(summer and winter)and slope orientation(sunny and shady slopes)on soil microbial community characteristics,soil chemical properties,and enzyme activity based on indoor experiments and high-throughput sequencing technology.We further investigated the correlation between soil microbial community composition and environmental factors in order to identify the main factors affecting the change of soil microbial community composition.The results of the study were as follows:(1)Seasonal changes had significant effects on soil organic carbon,total nitrogen,total phosphorus,total potassium,carbon nitrogen ratio(C:N)and carbon phosphorus ratio(C:P)(P<0.05),while slope aspect changes only had significant effects on soil C:N(P<0.05).The interaction of season and slope direction had significant effects on total nitrogen,total potassium,C:N,N/P ratio(N:P),cellulase,phosphatase and catalase(P<0.05).(2)The diversity and structure of soil fungi and bacteria in the four climatic zones of the upper Minjiang River differed significantly,with fungal α-diversity being highest in the highland monsoon climate zone and bacterial α-diversity being lowest in the subtropical humid monsoon climate zone.(3)The effects of seasonal changes on soil microbial α-diversity differed in different climatic zones.The fungal Chao1 index and PD-tree index were significantly higher in winter than in summer on the shady slopes of M2 and M3 regions and on the sunny slopes of M4 region,and significantly higher in summer than in winter on the shady slopes of M1region;the bacterial PD-tree index was significantly higher in summer than in winter in M3 region,and the Chao1 index,PD-tree index and Shannon index were significantly higher in winter than in summer in M4 region.The PD-tree index,PD-tree index and Shannon index were significantly higher in winter than in summer.The results of the non-metric multidimensional scale analysis(NMDS)showed that the stress function value(stress)<0.1,the fungal community structure was different between summer and winter,and the bacterial community structure was also significantly different between winter and summer(except for the M2 area).(4)The effect of slope change on soil microbial α-diversity differed in different climatic zones.The Shannon and Simpson indices of fungi in summer were significantly higher on shady than on positive slopes in the M4 region;the Chao1 index of bacteria in winter was significantly higher on shady than on positive slopes in the M3 region,and the Chao1,PD-tree and Shannon indices in summer were significantly higher on shady than on positive slopes in the M4 region.The results of non-metric multidimensional scale analysis(NMDS)showed that the stress function value(stress)< 0.1,the fungal community structure in M3 and M4 regions differed between positive and negative slopes,and the fungal community structure in M1 and M2 regions also differed significantly between negative and positive slopes in winter;the bacterial community structure was more similar between negative and positive slopes in summer,and had some differences between negative and positive slopes in winter(except M4 region)..(5)The dominant phylum composition of soil fungal and bacterial communities was similar under different climatic zones,but the relative abundance differed,and the dominant genus composition varied greatly;the dominant phylum and dominant genus composition of soil fungal and bacterial communities were similar under different seasons and slopes,but the relative abundance differed.Ascomycota,Basidiomycota and Mortierellomycota were the main phylum in the fungal community,and Mortierella was the dominant genus in M1 and M4.Penicillium was the dominant genus in M2 and M3.The main phylum of bacterial community were Proteobacteria,Acidobacteria and Actinobacteria,and the dominant genus in M1 region was Candidatus＿Solibacter.The dominant genus in M2 and M3 was RB41,and the dominant genus in M4 was Candidatus＿Udaeobacter.(6)Spearman correlation analysis showed that soil fungi Chao1 index was significantly negatively correlated with soil C:N,Shannon index was significantly positively correlated with N:P,Mortierella was significantly negatively correlated with soil pH,Inocybe was significantly positively correlated with phosphatase.Chao1 index,PD-tree index,Simpson index and Shannon index of soil bacteria were significantly positively correlated with total phosphorus,total potassium,pH,C:P and urease,while RB41 was significantly positively correlated with total phosphorus,pH,sucrose and catalase.Candidatus＿Solibacter was negatively correlated with total potassium,pH and catalase.Redundancy analysis(RDA)showed that the cumulative explanatory capacity of the first two axes of RDA ranking of soil fungi was 47.58% in summer and 51.47% in winter,respectively.The cumulative explanatory capacity of the first two axes of bacterial RDA sequencing was 71.45% in summer and 64.86% in winter.The sequencing results showed that soil pH and sucrase were important factors affecting the composition of fungal and bacterial communities.In conclusion,there are significant differences in soil microbial community characteristics in different climatic zones,and the seasonal and slope orientation affect the microbial community composition and diversity to a certain extent.Soil pH and sucrase are important factors affecting the composition of fungal and bacterial communities,and bacteria are more sensitive to the external environment.These results provide reference for diversity conservation and sustainable development in the upper reaches of the Minjiang River.