Studies On The Transformation Process And Molecular Chemical Diversity Of Litter-DOM In Six Tree Species From The Mid-subtropical Region

Forest ecosystems are the largest carbon(C)reservoirs in terrestrial ecosystems.As an important part of C cycle in forest ecosystem,soluble organic matter(DOM)is of great scientific significance to reveal the changes of forest C budget,source,and sink.DOM is composed of tens of thousands of molecules,mainly C,H,O,N,P,and S.Plant litter is one of the main sources of DOM in forest ecosystems.Studying the mechanisms of forest plant litter-DOM ecological processes can increase the understanding of the dynamics of C,N,and P in forest soils.The litter-DOM transition process is dominated by soil microorganisms,and the interaction between soil bacterial and fungal communities and DOM is an important process in the global carbon cycle.In this paper,six tree species,Evergreen broad-leaved tree species Elaeocarpus decipiens,Castanopsis carlesii,Schima superba,Evergreen conifers Cunninghamia lanceolata and Pinus massoniana,deciduous broad-leaved trees Liquidambar formosana,were studied in the common garden of the Fujian Sanming Forests Ecosystem Nation Observation and Research Station,in spring(April),summer(August),autumn(November),Molecular chemical diversity composition of litter DOM in tree layer and understory,DOM in soil,and soil microbial community diversity in understory.This paper aims to explore the relationship between litterDOM and soil DOM,and between soil DOM and microbial community diversity,in an attempt to provide a scientific reference for the transition mechanism of the litterDOM process between forest plants and soil.The main results of this study are as follows:(1)The different tree species and different seasons of litter DOM have different chemical composition.DOM of litter of Castanopsis carlesii understory in spring and Elaeocarpus decipiens tree layer in summer contained refractory compounds with high aromatic indices.Moreover,the DOM of Elaeocarpus decipiens litter has a higher N and P content,indicating that it could provide more N and P elements to the soil.DOM of litter of Castanopsis carlesii understory in spring and Elaeocarpus decipiens tree layer in summer contained refractory compounds with high aromatic index.Moreover,the DOM of Elaeocarpus decipiens litter has a higher N and P content,indicating that it could provide more N and P elements to the soil.With high protein and lipid content and low tannin content,conifers’ DOM is susceptible to degradation by microorganisms.In addition,low levels of N and P were found in the understory litter DOM,which may be related to the species and nutrient turnover of the understory plants.This suggests that the DOM composition of the litter is significantly affected by seasonality,metabolism,and tree species.(2)The differences in tree species had a significant effect on the chemical composition of DOM in the soil.The soil DOM of conifer species has more protein and less tannin,while the soil DOM of broadleaf species is more specific and difficult to degrade.Among them,the soil DOM of the Schima superba contains more nutrients rich in N,S,and P,which can provide more nutrients to the soil.Soil DOM chemical composition and nutrient content also vary from season to season.In spring,soil DOM nutrients at the lowest level mostly contain non-degradable substances,mostly DOM from plants;In summer,soil DOM nutrients at the highest levels mostly contain easily degradable compounds,most of which are by-products produced by microorganisms.In autumn,the degradation ability of microorganisms decreases and a large number of plants fall leaves,so DOM from plants is retained.There is also DOM produced by microorganisms.Living habits and adaptive responses of soil microbial communities have a significant impact on the degree of litter-DOM conversion.In addition,the selection of appropriate tree species for mixing according to the different nutrients released by the soil DOM of different tree species in the same time period can alleviate the nutrient constraints of the plant growth process and provide a new reference for forest tree species cycle management.(3)More than 55% of soil DOM comes from litter DOM.Using Mantel’s analysis of common molecules of litter DOM and soil DOM,it was shown that understory plants have a greater ability to enhance soil DOM in spring.In this study,the molecular chemical types of DOM lost and retained in the soil of litters were analyzed.It was found that different components of the litter DOM are lost and retained to varying degrees in the soil DOM,and that the proportions of carbohydrate,condensed hydrocarbons,lipids,and unsaturated hydrocarbons rich in N,P,and S in the litter DOM are all greater than those retained.This indicates that microorganisms prefer degraded components with more N,P,and S elements.The Elaeocarpus decipiens tree layer contains refractory compounds with high molecular weight and aromatization index,and the conifer species contain more P element,indicating that the broad-leaved tree species DOM is difficult to be mineralized and easy to be adsorbed by soil,which promotes soil carbon sequestration.Conifer species contain more lipids and proteins that are less stable,decompose more quickly,and are more mineralized than broad-leaved tree species,which can provide more P elements to the soil to support the P cycle of the forest ecosystem.There are clear seasonal differences in the fraction of litter DOM retained in the soil DOM.In spring,DOM contains many molecules and refractory compounds with high aromatization indices.N,P,and S of common DOM molecules reach their highest values in summer.Lipids,proteins,and unsaturated hydrocarbons are most abundant in autumn.Seasonal changes have a significant effect on the fraction of litter DOM that enters the soil DOM.The most important factor was the difference in microbial degradation ability of the litter among the six tree species.(4)The highest bacterial richness and diversity was found in spring for Pinus massoniana and in summer for Liquidambar formosana,with no difference found in autumn in soil.There was little difference in the community composition of soil DOM bacteria among the six tree species,with mostly Acidobacteriota and Proteobacteria.In contrast to spring and summer,soil bacteria are less active in autumn.This study found that Proteobacteriota can accelerate the degradation of condensed hydrocarbons and lignin and promote the production of amino sugars and carbohydrates.Furthermore,bacteria often cause negative correlations between proteins and tannins in DOM.During spring and summer,Myxococcota has a different effect on unsaturated hydrocarbons,which are closely related to the vegetative activities and life processes required by the flora.Thus,the requirements of bacterial adaptation and seasonal changes can lead to changes in the molecular chemical diversity of DOM during the litter-DOM process.The composition of fungi in the soil of six species varied greatly,and the richness and diversity of fungi in Castanopsis carlesii soil were the smallest in the three seasons.In spring,Streptophyta and Chytridiomycota promote protein production and accelerate tannin degradation,resulting in high protein and low tannin content in the DOM under coniferous forests.The Russulales fungus promotes the production of condensed hydrocarbons,and the Cantharellales fungus promotes the degradation of unsaturated hydrocarbons,resulting in the lowest unsaturated hydrocarbon content and the highest condensed hydrocarbon content in the DOM under the Castanopsis carlesii forest.Basidiomycota fungi have a higher abundance of DOM in the summer under Liquidambar formosana forests,which accelerates the degradation of amino sugars,while Cantharallales fungi promote the production of unsaturated hydrocarbons.In autumn,the abundance of the fungus Venturiales is higher in the soil beneath the forest,which results in a higher concentration of condensed hydrocarbons in the soil below the forest.The diversity of a fungal community is significantly affected by its own metabolic type and fitness,which affects the molecular chemical diversity of DOM.As a result,the fungal litter-DOM process is significantly affected by changes in fungal diversity,and changes in fungal communities and their adaptability are affected by seasonal variations.The soil microbial community and the compounds in the soil DOM,and that the microorganisms are selective adaptors to the substances they degrade.The degree of difficulty of the DOM decomposition determines the diversity of the microbial community.The molecular composition of the DOM affects the composition of the microbial community,and by utilizing unstable components to increase the stability of the soil DOM,microorganisms tend to convert unstable components into more stable ones,which affects the molecular chemical diversity of the DOM.Thus,the molecular chemical diversity of the DOM and the microbial community diversity are mutually influenced in the litter-DOM ecological process.These findings have important implications for further understanding of the contribution of litter DOM to soil.Conclusion: The main factors affecting litter-DOM ecological processes and DOM molecular chemical diversity are the adaptive degradation of soil microorganisms under the forest,and differences in tree species and seasons that cause differences in DOM molecular chemical diversity.The interplay between DOM molecular chemical diversity and microbial population diversity ultimately influences litter-DOM ecological processes and DOM molecular chemical diversity.