Research On Extraction Optimization And Application Of Soil Dissolved Organic Matter Based On Spectral Analysis

Dissolved organic matter（DOM）is an important part of soil and aquatic ecosystems.In particular,soil DOM has a significant impact on various environmental processes such as microbial activity,mineral weathering,soil quality,and transport of contaminants in soil.Currently,water or salt solutions are often used to extract DOM from soils under certain conditions,but the results obtained from different extraction conditions vary widely,which seriously restricts intercomparison among different studies.Additionally,it is a necessary prerequisite for conducting soil DOM research to use appropriate characterization techniques.Recently,the Ultraviolet-Visible absorption spectrometry（UV-Vis）and the Three-Dimensional Fluorescence-Excitation Emission Matrix Spectroscopy-Parallel Factor Analysis（3D-EEMs-PARAFAC）have been widely used for the characterization research of DOM in different ecosystems due to its advantages of high sensitivity,good selectivity and less damage to samples.Therefore,it is of great significance to compare and apply soil DOM based on spectral technology to understand the formation and fate of DOM in soil and its response to the changes of environmental conditions.In this study,10 representative arable soils were selected nationwide,and the laws of different physical and chemical leaching conditions on soil DOM properties were studied by UV-Vis and EEM-PARAFAC.Moreover,the technique was also applied to the study of soil DOM of different land-use types in typical loess region（Yan’an area）.It provided a theoretical reference for the efficient extraction and evaluation study of soil DOM and a scientific basis for the improvement of soil quality and the management,protection and restoration of the soil ecological environment.The main conclusions are as follows:（1）Suitable soil DOM extraction conditions were:1 mm of step sizing,10:1 of water-soil ratio,1 h of leaching time,40°C of leaching temperature,p H=7,0.05 M Na H₂PO₄·2H₂O（focus on DOC variation）or H₂O（focus on overall DOM properties）.The step sizing of 1 mm was helpful to express the diversity and complexity of DOM components.The water-soil ratio of10:1 could ensure high DOC content and the expression of terrestrial and hydrophilic protein-like DOM components.The leaching time of 1 h was beneficial to the release of stable DOM components.The leaching temperature of 40°C could ensure high DOC content and the expression of hydrophilic organic components.The leaching solution with p H close to neutral7 was beneficial to reflect the DOM characteristics of the soil under the actual p H environment.The H₂O was more comprehensive in expressing the component types of DOM and the Na H₂PO₄·2H₂O was beneficial to the leaching of DOC,but may affect the expression of protein-like components.The moderate concentration of 0.05 M Na H₂PO₄·2H₂O was conducive to the stable leaching and objective expression of DOM components.（2）Physical conditions tend to significantly affect the leaching of endogenous DOM components.Compared with other physical conditions,the leaching temperature had a greater impact on the leaching of DOM.The higher temperatures（>40°C）tend to reduce the degree of humification and the content of aromatic components of DOM,especially in alkaline soils.There were similar effects of step sizing and water-soil ratio on DOM,and the smaller step sizing and larger water-soil ratio could promote the release of hydrophilic protein DOM components.Chemical conditions had more significant effects on DOM leaching than physical conditions,and there were similar effects of types and concentrations of extractants on DOM.（3）The highest and lowest DOC contents were found separately in 30-yr forestland(203.33 mg kg^-1)and cropland(83.70 mg kg^-1)in typical loess areas.After the forestland has been planted for more than 20 years,the DOC contents increased significantly(97.53 to 201.25mg kg^-1),and the DOC contents of grassland and forestland decreased continuously in the 1 m soil profile,while that of cropland increased first and then decreased.This showed that land-use type and anthropogenic activities would significantly affect the horizontal and vertical distribution characteristics of soil DOM,and the ecological improvement could effectively improve soil quality only after a longer period of time（about 20 years）.（4）Two humic acid-like components（C1 and C3）and one protein-like components（C2）were identified from the soil samples,and there was an interaction mechanism between the two DOM components.The humic acid-like components gradually decreased with the increase of soil depth,while the opposite was true for protein-like components,and this situation was particularly obvious in cropland soil.The surface soil contained a large number of DOM components with large molecules,high aromatic structure and high degree of humification.This was the result of the comprehensive effect of the transformation of land-use types and anthropogenic activities affecting soil humification and mineralization process.