Affecting Factors And Fixation Mechanism Of Soil Organic Carbon In Wetlands

An increase in the amount of greenhouse gases in the atmosphere caused by human activities can lead to global climate change such as global warming and increasing extremes climate.In order to cope with global climate change,China has proposed "carbon peaking and carbon neutrality goals" of "carbon peaking" by 2030 and "carbon neutrality" by 2060.At present,there are two main ways to mitigate global climate change:"increase carbon sinks" and"reduce carbon emissions".In order to achieve the "carbon peaking and carbon neutrality goals",it is of strategic significance to increase the content and stability of soil organic carbon in ecosystems.As an important part of the terrestrial carbon pool,wetland ecosystems only account for 5-8%of the global land area,but their soil organic carbon storage accounts for 30%of the global soil organic carbon pool,which is the ecosystem with highest density of soil organic carbon.Through the study of the influencing factors and fixation mechanism of wetland soil organic carbon,it can not only reveal the potential and stable mechanism of wetland ecosystem carbon sequestration,but also contribute to wetland carbon sequestration and wetland management.It is of great significance to achieve the "carbon peaking and carbon neutrality goals".Focusing on wetland soil organic carbon,this study has carried out research work in four aspects:the analysis of the influencing factors of wetland soil organic carbon at a small scale(Chapter 3);an analysis of the influencing factors of wetland soil organic carbon at a large scale(Chapter 4);a new method for traceability of wetland soil organic carbon and its application(Chapter 5);and a study on the mechanism of wetland soil organic carbon fixation(Chapter 6).Among them,the third and fourth chapters are the research on the external factors affecting the stability of wetland soil organic carbon.First,small-scale research was carried out to obtain the influencing factors of soil organic carbon in a single wetland,and then the generality of the small-scale research results was verified by large-scale research.The fifth and sixth chapters study the internal influencing factors of wetland soil organic carbon stability.First,a new method system for traceability of organic carbon in wetland soil was proposed,and the system was used to trace the source of organic carbon.Then,based on the traceability results,the fixation mechanism of specific types of organic carbon in wetland soil was explored.The above studies were carried out in the four inflow rivers of the Sanhekou Reservoir in Shaanxi Province,the Yanghe Wetland in Hebei Province and the Yellow River Delta.Environmental scanning electron microscopy and energy dispersive X-ray spectroscopy,principal component analysis,16S rRNA sequencing,microbial function prediction,orthogonal matrix factorization,redundancy analysis,normalized stochasticity ratios,random forest models,X-ray diffraction,and quantum Chemical theoretical calculations and other methods.The main conclusions of this study are as follows:(1)A small-scale study on the influencing factors of soil organic carbon in Yanghe wetland showed that,in addition to microbial factors,the influence of other factors on the content of soil organic carbon in wetlands varies with the change of organic carbon components.The content of dissolved organic carbon(DOC)was significantly affected by sediment pH and vegetation height,the content of light fraction organic carbon(LFOC)was mainly affected by pH value of water body,and the content of heavy fraction organic carbon(HFOC)was less affected by the environment.In order to quantify the impact of microorganisms on various types of organic carbon,a new indicator,weighted abundance of organic carbon-related microbes(WARM),was proposed,and it was found that various types of soil organic carbon in wetlands were significantly affected by microorganisms.This study found the important role of microorganisms through the analysis of the influencing factors of wetland soil organic carbon at the scale of a single wetland,and proposed a new parameter WARM to quantify the impact of microorganisms on wetland soil organic carbon.(2)Analysis of global data and Yanghe wetlands and Sanhekou Reservoir area shows that cation exchange capacity and sediment bulk density are important influencing factors of soil organic carbon in wetlands at the global,continental and Chinese scales.Based on this,a new estimation equation for soil organic carbon content in global wetlands was constructed.At the same time,since the interaction of various physical and chemical properties in wetland soil is similar to the interaction of microbial communities,wetland soil is regarded as a "soil community" of physical and chemical indicators,and the stochasticity of its assembly process is quantified.It was found that although the influencing factors of wetland soil organic carbon on a global scale were the same,their respective influencing mechanisms were different.However,after adding WARM to the stochastic calculation of "soil community",it was found that in wetlands with less human disturbance,microorganisms played a key role in the certainty of the mechanism of soil organic carbon.Through further application of the small-scale research results,this study found that when considering the influence of microorganisms,the law of soil organic carbon content in wetlands with less human disturbance is more universal.(3)In order to make up for the insufficiency of wetland soil organic carbon traceability methods at this stage,this study proposes a new method system for wetland soil organic carbon traceability based on environmental scanning electron microscopy,energy dispersive X-ray spectroscopy,microbial analysis and positive matrix factorization.And applied this method to the analysis of four inflow rivers in the Sanhekou Reservoir area.The results showed that there were differences in the sources of DOC,LFOC and HFOC.In this study area,LFOC mainly comes from plant residues and organic carbon adsorbed on it,and its components contain carbohydrates,cellulose and alkyl aromatic compounds;HFOC mainly comes from "earthcarbon" combination,and the main components may be high molecular weight carbohydrate;and there are three main microbial sources of DOC:synthesis of lipopolysaccharide,apoptosis and decomposition of carboxylic acid,and its components contain lipopolysaccharide,carbohydrates and small molecular organic acids.This study proposes a new method system of soil organic carbon traceability,which provides a new understanding of the microbial source of soil organic carbon in wetlands.This study provides a new perspective for the source analysis and composition analysis of soil organic carbon in wetlands.(4)The research on the fixation mechanism of soil organic carbon in the Yellow River Delta showed that the density of HFOC in wetland soil was accumulating with time,while the density of LFOC and DOC did not increase or decrease significantly.However,the accumulation of HFOC is not a simple accumulation process,which includes the loss of the original accumulated organic carbon and the accumulation of new input organic carbon.This is due to the "stimulation effect" of plant root exudates that promotes the formation of characteristic microbial communities that destroy the protection of organic carbon by inorganic particles(amorphous iron oxide),thereby depleting the original accumulated organic carbon.At the same time,root exudates can be adsorbed on the surface of the sediment and accumulated under the protection of the "iron shield" formed by crystalline iron oxide to form a stable "earthcarbon" aggregate.The mechanism is verified by the results of quantum chemical theoretical calculation and X-ray diffraction analysis,which confirms that the mechanism is feasible and exists in both theory and practice.This study revealed a new organic carbon accumulation mechanism in the wetland soil of the Yellow River Delta,emphasized the protective effect of crystalline iron oxide on soil organic carbon,and provided new insights into the dynamic balance and fixation mechanism of wetland soil organic carbon.To sum up,this study quantified the impact of microorganisms on wetland soil organic carbon content through correlation analysis,RDA analysis and microbial analysis.The influencing factors of soil organic carbon content and distribution in wetlands,and on this basis,the estimation formula of global wetland soil organic carbon content is proposed;an organic carbon traceability method system including morphological and elemental analysis,microbial analysis,microbial functional gene prediction and PMF of soil organic carbon in wetlands is proposed.As a supplement to the existing wetland soil organic carbon traceability methods,it can be used in wetlands.Traceability and composition analysis of soil organic carbon;taking the Yellow River Delta wetland as an example,the mechanism of soil organic carbon fixation in wetlands under the influence of plants is revealed,and it is found that root exudates will cause the loss of the original accumulated organic carbon,and will also serve as a new organic carbon source with the protection of "iron shield".This study provides a scientific basis for wetland ecosystem management and carbon increase by analyzing the external and internal influencing factors of wetland soil organic carbon stability and uncover the affecting factors and stability mechanism of soil organic in wetlands,which is of great significance to the realization of the "carbon peaking and carbon neutrality goals".