Adsorption And Desorption Dynamics Of Organic Carbon In Farmland Soil Based On Column Experiment

Soil organic matter（SOM）is an important component of farmland soil,and its dynamics directly affect the physical and chemical properties and fertility of soil.Agricultural irrigation can cause the adsorption and desorption of SOM,and different types of groundwater have different effects on the dynamic changes of SOM in the process of irrigation.Therefore,based on the column experiment,the dynamic of SOM adsorption and desorption in irrigation process were studied.Based on two column experiments of desorption and adsorption,the dissolved organic matter（DOM）in leachate was analyzed and characterized by ultraviolet visible spectrum and three-dimensional fluorescence spectrum,and the mechanism of organic matter migration under irrigation conditions was explained from the perspective of components.The main conclusions are as follows:（1）The leaching processes of soil organic matter include the rapid decline stage and stable leaching stage of organic matter.In the early stage of rapid decline,the organic matter was small molecular weight,low aromaticity and recently generated weak humus characteristic biogenic organic matter.while in the stable leaching stage,organic matter was mainly terrigenous organic matter with high aromaticity and humification.（2）Cation exchange occurred when the soil was leached by Na⁺and Ca²⁺solution,which promoted the desorption of protein-like components and inhibited the desorption of humic-like components.The concentration of organic matter desorbed by sodium salt and pure water was similar,which was significantly higher than that of organic matter desorbed by calcium salt leaching.Ca²⁺can promote the adsorption of terrigenous organic matter with high aromaticity,high molecular weight and strong humus characteristics.Therefore,the organic matter desorbed by calcium salt is mainly from newly generated autogenous and biological sources,while the organic matter desorbed by sodium salt and pure water is mainly from terrigenous sources with strong humus characteristics.（3）The results of PARAFAC showed that the leaching organic matter of Zhengding soil can be divided into four organic components C1-C4,and the leaching organic matter of Xianxian soil can be divided into C1-C5 five organic components.Both Zhending soil and Xianxian soil contained protein-like components and humic-like components.In the early stage of pure water leaching,humic-like component was the main component of organic matter.In the stage of salt solution stable leaching,tryptophan-like component C3 and humic-like component C1 were the main components of organic matter in Zhengding soil and Xianxian soil.Salt can promote the release of C4（C5 in Xianxian soil）.The presence of Ca²⁺can inhibit the desorption of humic-like components,especially humic-like component C1 and humic-like component C4 in Xianxian soil.Na⁺and Ca²⁺can promote the desorption of protein like components,and calcium salt has a stronger promoting effect than sodium ions,especially C3.The leaching of humic acid like component C2 had little relationship with cation concentration and cation species.（4）Both the equilibrium convection dispersion equation（CDE）model and the two-region model can well fit the penetration curve of Br^-.The effect of continuous leaching process on organic matter exchange was obtained by model fitting:with the decrease of dispersion coefficient D,the exchange capacity of organic matter between soil particles became worse;Distribution coefficientβincreases,the proportion of water content in movable area increases;The organic matter in the soil micro pores gradually flowed out with the increase of mass exchange coefficientω,andωin the soil column leached by sodium salt in the later stage is larger than it in the soil column leached by calcium salt.The type of salt solution can affect the desorption characteristics of organic matter by changing the physical structure of soil column.（5）The adsorption experiment verified that the ability of sodium salt to promote the adsorption of organic matter was significantly less than that of calcium salt.The effect of calcium salts on the adsorption of organic matter with high aromaticity,high molecular weight and high humification degree was related to the humification degree of organic matter.The promotion of adsorption of organic matter from different sources is related to the degree of humification and Aromaticity.In a certain range of salt concentration（4m M-10m M）,high concentration salt solution can promote the adsorption of highly aromatic terrigenous organic matter components,but the effect is weak.