Characteristics Of Dissolved Organic Matter And Migration And Release Of Labile Cadmium In Typical Water Level Fluctuation Zone Of Three Gorges Reservoir

Cadmium（Cd）is one of the main heavy metal pollutants in the water level fluctuation zone（WLFZ）soils after the impoundment of the Three Gorges Reservoir（TGR）,which has attracted extensive attention of scholars at home and abroad.However,the existing reports mostly focused on the concentration and occurrence extractions of heavy metals,ignoring the migration and release of Cd between the soil solid and liquid phases,and cannot objectively reflect the migration and effective state of Cd under the field in situ conditions.At the same time,the research about the influence of dissolved organic matter（DOM）on the release behavior and resupply process of Cd in the soil solid and liquid interphase under the anti-seasonal regulation and storage mode of the TGR is still limited.This paper collected the soil samples from the WLFZ along Xiangxi River in October 2019 and October 2020,and determined the soil physical and chemical property,the content and occurrence form of Cd.The UV-Visible absorption spectrum,three-dimensional fluorescence spectroscopy and fluorescence regional integration（FRI）were conducted to analyze the spectral characteristics of DOM.The diffusive gradients in thin films technology（DGT）and DGT-induced fluxes in soils（DIFS）model were used to reveal the labile-Cd（DGT-Cd）and its release kinetics,and the effects of DOM on its release kinetics and recharge process were discussed.The main conclusions are as follows:（1）The three-dimensional fluorescence peaks of soil DOM in the study area were mainly four peaks:A,C,B and T,and the A and C peaks showed high fluorescence intensity;DOM was dominated by fulvic acid with the low degree humification and strong bioavailability.The source of DOM was dominated by land sources and contained endogenously generated inputs.（2）The average contents of Cd in soils in 2019 and 2020 were（0.474±0.301）mg·kg^-1and（0.249±0.058）mg·kg^-1respectively,which were higher than the background value of soil heavy metals in the Three Gorges Reservoir area.The non-residual components of Cd were the dominant fraction in the soils.The distribution characteristics of content of Cd in soil liquid phase was similar to that in soil solid phase and the occurrence of Cd.（3）The content of DGT-Cd in study area soils ranged from 0.02 to 0.18μg·L^-1,and the average content in 2020 was lower than in 2019.The fitting results of DIFS model showed that all samples belong to"partially continuous type"in terms of dynamic supply of labile-Cd.The R value is lower than 0.25 and the adsorption rate of Cd in soils was higher than the desorption rate,which indicated that the recharge capacity of soil solid relative to Cd and the potential release risk of labile-Cd in soil in the WLFZ along Xiangxi River were low.Besides,the analysis results of DOM characteristics and DGT-Cd release parameters showed that low molecular weight DOM and fulvic acid can potentially inhibit the release of labile-Cd in soils.