Chemical Structure Characterization And Biodegradability Of DOM In Soil Of Different Stands Under The Background Of Forest Conversion In Subtropical Region

DOM is a very active and important chemical component in forest ecosystems.It is also the component most easily used by microorganisms.And it is an important material and energy source for microbial metabolism and plays an extremely important role in the global carbon cycle process.Forest conversion will directly affect the balance of the forest ecosystem carbon pool,resulting in changes in soil dissolved organic matter(DOM)content,structure composition and its degradation in the soil,thus affecting the soil carbon cycle process.Therefore,in this paper,Fujian Sanming Forest ecosystem and Global change Research Station was established in the middle subtropical region to study broad-leaved forest(secondary broadleaved forests of Castanopsis carlesii(SF)?human-assisted naturally regenerated forests of Castanopsis carlesii(AR)and coniferous forest(plantation of Pinus massoniana(PM)?plantation of Cunninghamia lanceolata(CF))transformed from Castanopsis lanceolata forest(NF).The water extraction method was used in the experiment,and the DOM concentration and spectral structure characteristics of the soil in different forests were analyzed in combination with UV spectroscopy,fluorescence spectroscopy,and infrared spectroscopy.At the same time,we carried out the soil DOC culture experiment to simulate the degradation process in the soil,and used a double exponential decay model to fit to study the biodegradability of soil DOM.In order to better understand the true biological activity of the DOM,this is also an important part of the dynamic simulation of the C cycle.Research indicates:(1)After forest conversion,the content of DOM in soil of each stand changed.The content of DOC and DON in the topsoil of broad-leaved forests is greater than that of coniferous forests.This is mainly due to the comprehensive factors such as the amount of litter,soil organic matter,and soil microorganisms.The DOC and DON concentrations in different soil layers of the forests vary greatly.This is mainly due to the fact that the litter and plant roots are mainly concentrated in the topsoil and the humus formed after decomposition is accumulated in the topsoil.(2)The spectral results showed that:after forest conversion,the aromatization index and humification index of broad-leaved forests were significantly higher than that of coniferous forests,indicating that they were rich in nutrients,strong in microbial activity,rapidly decomposed and transformed soil organic matter,and accumulated more microbial metabolites.The source of DOM in soil is mainly the combination of exogenous and microbial sources,including the input of surface litter and underground roots,as well as internal microbial intervention and human activities.The three-dimensional fluorescence results showed that the DOM structure of the soil was mainly lytic microbial metabolites and humic acid-like substances.The three-dimensional fluorescence results combined with parallel factor analysis showed that the humic acid-like substances,protein-like substances,and fulvic acid-like substances were mainly included,indicating soil The DOM structure is very complex and is mainly affected by the input of microorganisms and litter.The relative absorption of DOM at the 3700-3000 cm-1,1680-1640 cm-1,1150-1000 cm-1,and 690-500 cm-1 soils was highest in the NF of rice bran,indicating the soil DOM structure of this stand.It is very complex and contains both simple and straight alkanols and complex carboxylic acids and aromatics.(3)The degradation rate of soil DOC in broad-leaved forests was significantly higher than that of coniferous forests in surface soils,indicating that the fraction of DOC in broad-leaved forest soils was more likely to be decomposed by microorganisms.The value of aromatization index and humification increased during soil biodegradation,indicating that with the progress of degradation,microorganisms preferentially use volatile components such as carbohydrates,and organic components of aromatic substances and humus accumulate more in the soil.The soil DOM is stable.The linear fitting of the residual degradation rate of soil DOC after cultivation with the initial spectral index of soil DOM solution shows that the soil spectral index can explain the biodegradability of soil DOC to some extent.