The Research On Soil Microbial Respiration And Its Driving Mechanism In Four Subtropical Forests

Forests are important carbon pools in terrestrial ecosystems.China’s subtropical forest vegetation types are diverse and it plays an important role in carbon sequestration of global forest ecosystems.Soil microorganism is the main driver of soil carbon source-sink balance.Soil microbial respiration is the key to the carbon cycle process of underground ecosystem and the assessment of global carbon budget.It has an important impact on soil carbon emission from forest ecosystems.And it is the focus of systematic mechanism research of global carbon cycle and is of great significance to reasonably predict the global carbon balance pattern.However,the characteristics and driving mechanism of soil microbial respiration in subtropical forests are still unclear.In this paper,four typical subtropical forests-Camphor tree(innamomum camphora),Beautiful sweetgum(Liquidambar formosana),Masson pine(Pinus massoniana),and mixed Camphor tree and Masson pine forests were studied.The dynamic change of soil microbial respiration was monitored for two years.The contribution of soil microbial respiration to soil respiration was clarified,and the influence of soil temperature and humidity on soil microbial respiration was also explored.The soil microbial community structure was analyzed.And the relationship among vegetation,soil and microbial community structure was discussed.The potential influence path and key driving factors of vegetation,soil,microbial community structure on microbial respiration were explored.The research results provide data support for the research on the regulation mechanism of "source" and "sink" of subtropical forest ecosystem under the background of global climate change,and provide a scientific basis for the government to formulate carbon peaking and carbon neutralization management strategies.The main results are as follows:(1)The soil microbial respiration and soil respiration of the four forests showed the seasonal variation of high in summer and low in winter.The annual average soil microbial respiration rates of Camphor tree forest,Masson pine forest,mixed forest and Beautiful sweetgum forest were 2.57 μmol·m-2·s-1.1.51 μmol·m-2·s-1/1.73 μmol·m-2·s-1、1.57 μmol·m2·s-1,and the annual average soil microbial respiration rate of Camphor tree forest was significantly higher than that of the other three stands.The contribution of soil microbial respiration to soil respiration was 79.09%～92.24%.Soil temperature and moisture explained 51.1%～72.9%and 24.8%～47.1%of soil microbial respiration variation,and the interaction of soil temperature and moisture could explain 66.7%～84.9%of soil microbial respiration variation.(2)Among the four forests,plant community Margalef richness index(DM),Shannon Wiener diversity index(H’),Simpson dominance index(DS)and Pielou evenness index(E)of mixed Camphor tree and Masson pine forest were the highest.The soil water content of Masson pine forest and Camphor tree forest was significantly higher than that of Beautiful sweetgum forest and mixed forest.The soil pH in four forests was less than 4.50,which had been seriously acidified,and the pH of Masson pine forest and Camphor tree forest was higher than that of mixed forest and Beautiful sweetgum forest.The soil carbon and nitrogen content and their stoichiometric ratio,sucrase,urease and catalase activities in Beautiful sweetgum forest and mixed forest were significantly higher than those of Camphor tree forest and Masson pine forest.The total phosphorus content in 0～20cm of the four forest soils was lower than the national average value(0.78 g·kg-1).The total phosphorus content and phosphatase activity in the soil of Beautiful sweetgum forest were significantly higher than those of the other three forests.Soil physical and chemical properties had a great impact on soil enzyme activity,but different enzymes had different responses to soil physical and chemical properties.(3)The contents of soil microbial biomass carbon and nitrogen(MBC,MBN),microbial entropy(MBC:SOC)and the stoichiometric ratio of microbial biomass carbon,nitrogen and phosphorus(MBC:MBN,MBC:MBP,MBN:MBP)in Beautiful sweetgum forest were significantly higher than those of the other three stands(p<0.05).There was no significant difference in microbial biomass phosphorus(MBP)and MBN:TN among the four forest soils(p>0.05).MBP:TP was the lowest in Beautiful sweetgum forest,and it was significantly different from Camphor tree forest and mixed forest(p<0.05).Vegetation and soil factors had significant effects on soil microbial biomass carbon,nitrogen and phosphorus.(4)The total phospholipid fatty acids(PLFA)content of soil microorganisms showed that bacteria(77.70%～80.78%)>fungi(10.15%～12.32%)>actinomycetes(7.30%～10.46%)>protozoa(0.35%～0.84%)in four forests.The PLFA content of microbial groups in Beautiful sweetgum forest was significantly higher than that in the other three stands(p<0.05).The soil fungi/bacteria ratio(F:B)of Camphor tree forest and Masson pine forest was higher.The ratio of Gram-positive bacteria/Gram-negative bacteria(GP:GN)in Beautiful sweetgum forest was significantly higher than that in Camphor tree forest.Hierarchical partitioning analysis showed that the order of individual contribution of important factors affecting microbial community structure composition(PLFA)is soil water content(SWC)>Plant Shannon Wiener diversity index(H’)>MBC:SOC>pH>N:P>SOC.(5)A total of 26 phyla,72 classes,166 orders,233 families and 351 genera of bacterial communities were measured in four forest soils.Acidobacteria,Chloroflexi and Proteobacteria are the dominant phyla.A total of 10 phyla,36 classes,94 orders,208 families and 357 genera of fungal communities were measured.The dominant phyla were Basidiomycota and Ascomycota.The alpha diversity of bacteria in Camphor tree forest was the highest,while the alpha diversity of fungi in Beautiful sweetgum forest was the highest.Bacterial and fungal communities have obvious aggregation effects in different forest types.And there were significant differences in β-diversity of soil bacterial and fungal communities in different forest types.Hierarchical partitioning analysis showed that the individual contribution of important factors affecting bacterial community structure was as follows:Plant Shannon Wiener diversity index(H’)>soil water content(SWC)>soil organic carbon(SOC)>pH>MBC:SOC>N:P;and it was Plant Shannon Wiener diversity index(H’)>MBC:SOC>pH>soil water content(SWC)>MBN>N:P for fungal communities.(6)Shannon Wiener diversity index(H’)and Simpson dominance index(DS)of plant community,soil water content,pH,carbon nitrogen ratio,soil temperature,MBC:SOC,MBN and microbial community composition had significant direct effects on soil microbial respiration.Vegetation and soil basic properties can also indirectly affect microbial respiration by affecting microbial community structure.Soil carbon nitrogen ratio(C:N)and soil temperature(T)significantly affected microbial community structure,and then had a significant indirect impact on microbial respiration.Microbial entropy(MBC:SOC)and microbial biomass nitrogen(MBN)had the greatest impact on soil microbial respiration,followed by soil carbon nitrogen ratio(C:N),pH,microbial community composition,soil water content(SWC),soil temperature,Shannon Wiener diversity index(H’)and Simpson dominance index(DS)of plant community.