Characteristics Of Soil Dissolved Organic Matter In The Drawdown Zone Of The Three Gorges Reservoir And Its Impact On Greenhouse Gas Emission

Soil dissolved organic matter（DOM）is an important component of natural organic matter in terrestrial ecosystems.DOM in the riparian zone,coastal zone,and hydro-fluctuation zone enter the water body with the process of surface runoff and flooding release,which is one of the important sources of DOM in the water body.Soil DOM is active and plays an important role in the global carbon cycle.As one of the world’s largest land-water alternating zone,the environmental quality and ecological security of the riparian zone in the Three Gorges Reservoir area have attracted more and more attention.Understanding the source,composition,structure,and spatial characteristics of soil DOM in the riparian zone of the Three Gorges Reservoir area will help to further clarify the impact of human activities such as water conservancy and hydropower projects on the ecosystem of the riparian zone.The riparian zone is also a typical ecologically fragile area.Understanding the structure and source of soil DOM is of great significance for the study of material circulation between aquatic and terrestrial ecosystems and the protection of the ecological environment in the riparian zone.Global warming caused by greenhouse gases is one of the important environmental problems facing mankind.The wetland in the riparian zone is an organic carbon reservoir in the aquatic and terrestrial ecosystems.The alternating anaerobic/aerobic environment,material,and energy cycles caused by water level changes make greenhouse gas emissions such as CO₂ and CH₄ in the riparian zone extremely complicated.Under the background of global warming,it is of great practical significance to study the response of CO₂ and CH₄ emissions to temperature changes in the Three Gorges Reservoir area for soil carbon cycle and climate change.In this study,based on ultraviolet-visible absorption spectroscopy and three-dimensional fluorescence spectroscopy,combined with parallel factor analysis,the composition,structure,source,and distribution characteristics of DOM in the surface soil of the wetland in the Three Gorges Reservoir area were preliminarily studied,to provide a scientific basis for the protection of the wetland ecosystem in the Three Gorges Reservoir area.Through incubation experiments,the release characteristics and temperature sensitivity of CO₂ and CH₄ in the riparian zone soils of the Three Gorges Reservoir area were scientifically evaluated,and the characteristics of soil DOM were further related to the temperature sensitivity of CO₂ and CH₄ release,to clarify the contribution of DOM in the process of soil greenhouse gas emission to ambient temperature change.The results showed that:（1）The source,composition,and structure of soil DOM in the riparian zone showed obvious spatial heterogeneity at the water level elevation,while the difference of DOM in the soil of the riparian zone between the mainstream and the tributary was only reflected in the molecular composition of DOM.The differences in soil DOM characteristics at different water level elevations/mainstream-tributaries were mainly caused by the differences in the concentrations of fluorescent components C1,C3,and C4 and the bioavailability of DOM.On the whole,DOM in the mainstream of the Three Gorges Reservoir area is mainly derived from endogenous sources,that is,it is mainly composed of microbial metabolic components,with high bioavailability and low humification.（2）The characteristics of DOM are important factors in regulating greenhouse gas emissions in the mainstream.The release rate of CO₂ and CH₄ from soil organic carbon decomposition under flooding conditions increased with the increase of incubation temperature.This process was sensitive to the increase in temperature(Q₁₀>1.0),but its response was not as severe as that of other terrestrial ecosystems.The aromaticity（SUVA254）and humification degree（HIX）of DOM can directly or indirectly affect the temperature sensitivity of CO₂ and CH₄ emissions by mediating a254 and a350.The lignin fraction of soil DOM was the strongest predictor of temperature sensitivity variation of CO₂ and CH₄ emissions.