The Research On Fine Root Structure And Pyrolysis Characteristics Of Different Tree Species In Subtropical Forest

Although 65%of soil carbon comes from plant roots,we still lack a comprehensive understanding of the complexity of root structure.At the same time,the root system also has significant heterogeneity within the branch structure.This heterogeneity of root morphology,physiological and chemical structure is also reflected in the degree of decomposition of different root levels of fine roots,which ultimately affects the effect of forest carbon sequestration.At present,most studies on chemical properties of roots are limited to the determination of element content,while higher resolution and deeper chemical structure analysis are still few.Integrating the structural characteristics of compounds at the root molecular level into the framework of plant functional traits can link the fine-tuning of chemical adaptation to species-level survival strategies and biogeochemical cycles at the ecosystem level.Therefore,in this paper,six representative tree species in the subtropical region were selected,and the morphological,anatomical,chemical composition and fungal infection character parameters of multiple root levels of different tree species were compared.The structural characteristics of fine roots and their changes with tree species and root levels were deeply analyzed at the molecular level through Fourier infrared transform spectroscopy,X-ray diffraction,thermal mass analysis and other methods.The correlation between chemical structure characteristics and other functional characteristics of roots was also discussed,which laid a data foundation for better predicting root decomposition dynamics and improving the accuracy of estimation of forest carbon sequestration ability.At the same time,plant roots are potential forestry biological resources with large biomass and high lignification degree.In this study,pyrolytic transformation characteristics of roots of different tree species and root grade samples of the same tree species were studied by thermogravimetric analysis technology,and their energy conversion potential was preliminarily measured.The main research results are as follows:(1)Functional traits(including morphological traits,chemical traits,anatomical traits,and fungal infestation of fine roots)of six representative tree species in subtropical secondary forest were measured and variance analysis was performed.The results showed that functional traits of fine roots varied significantly with different root order(p<0.01).In all species,the diameter and tissue density increased while the specific root length and specific surface area decreased,the content of C increased while the content of N decreased,the diameter of the middle column and the thickness of the cortex also increased to varying degrees,and the infection rate of arbuscular mycorrhiza showed a downward trend.(2)The differences of molecular chemical structure characteristics between different root levels of representative tree species,especially between fine roots and coarse roots,were compared.The results of Fourier transform spectroscopy showed that with the increase of root level,the relative intensity of lignin characteristic absorption peak decreased significantly compared with that of hemicellulose and cellulose,indicating that the relative content of lignin in fine roots of different tree species was higher than that in coarse roots.This is consistent with the results of X-ray diffraction analysis.The XRD diffraction peak pattern of fine and coarse roots of different tree species is similar,and the diffraction peak appears around 2θ=22°,but the diffraction peak intensity of coarse roots of different tree species is significantly higher than that of fine roots,that is,crystallinity and crystallization zone of coarse roots are increased in different degrees compared with fine roots.This indicated that the cellulose content in coarse roots was higher than that in fine roots of different tree species.By py-GC/MS analysis,the derived compounds of lignin structural units at different root levels of different tree species and their attribution were obtained.The results showed that the types of derived compounds at different root levels of the same tree species were the same,but the content was different.There are some differences in the types of derived compounds among different species,among which the fine roots of Chinese fir and Masson pine have no lilac base(S)structural units.(3)Thermogravimetric analysis was used to compare the pyrolysis characteristics of different root levels of representative tree species.The results showed that there were certain differences in the pyrolysis reaction characteristics between fine roots and coarse roots of the same tree species,and the differences between 1-3 fine roots and 4 and 5 coarse roots were small.Except for Southern Suanzao,the pyrolysis weight loss rate and maximum pyrolysis weight loss rate of fine roots of different tree species increased slightly with the increase of root class,but the temperature to reach the maximum pyrolysis weight loss rate did not change basically.There was no significant correlation between the chemical characters(C content,N content,C/N ratio)and morphological characters(diameter,specific root length,root specific surface area,and root tissue density)of fine roots,but there was significant correlation between the anatomical characters(cortical thickness and mesocolumn diameter)and mycorrhizal fungal infection.The types and contents of lignin in root basic structural units were significantly correlated with the morphological characters of fine roots,but not with the chemical characters,anatomical characters and infection characters.The results of PCA analysis showed that the functional traits of root were affected by root tissue density,diameter and several dimensions longer than the root.