Study On The Composition And Characteristics Of Dissolved Organic Matter(DOM) In Soil Under Different Land Uses By Using Fractionation,Spectral And Chromatographic Techniques

Dissolved organic matter（DOM）,one of the most important and active components in soil,strongly participates in a variety of critical environmental and ecological processes.Land use is the key factor influencing DOM characteristics,dynamics,and fate in ecosystems.In this study,a rapid batch procedure,ultraviolet-visible absorbance spectroscopy（UV-Vis）and excitation-emission matrices of fluorescence spectroscopy in combination with parallel factor analysis（EEMs-PARAFAC）,were applied to characterize DOM extracted from soils under three different land use regimes across China,including dry land,paddy field,and forest land.High performance liquid chromatography-size exclusion chromatography（HPSEC）was used to obtain the accurate information of apparent molecular weight（AMW）of DOM and its component from the batch procedure.The results of various characterization techniques（UV-Vis,EEMs-PARAFAC and the rapid batch procedure）were compared to reveal the differences in the amount,composition and spectral characteristics of soil DOM under different land use types and the possible reasons.Meanwhile,the accuracy and application value of common DOM characterization techniques were evaluated.The main results and conclusions are as follows:（1）The soil DOC（dissolved organic carbon）concentrations under different land uses were 10.43±0.85 mg C·L^-1,10.65±0.73 mg C·L^-1,13.34±1.58 mg C·L^-1 for dry land,forest and paddy soil,respectively,but there was no significant difference.A significant positive correlation between DOC concentrations and soil organic matter（SOM）content was found,indicating that DOM production mainly depends on SOM content.In addition,DOC concentrations were generally higher in tropical and subtropical regions,especially in agricultural soils,probably the climate features,such as high temperature and rainy conditions through all year round,which enhance soil microbial activity and promote the generation of DOM.（2）DOM spectral characteristics varied significantly by land use.Three fluorescence components were identified in soil DOM from different land uses by EEMs-PARAFAC,including high molecular weight UVC humic-like substances（C1）,low molecular weight autochthonous humic-like substances（C2）,and protein-like substances（C3）.Paddy soil DOM had less portion of hydrophobic compounds(lower SUVA₂₆₀),lower degree of aromaticity(lower SUVA₂₅₄)and humification（lower HIX）but higher proportion of microbial derived protein-like components（C3）than that from forest soils and dry land soils,whereas dry land soil DOM had more autochthonous humic-like substances（C2）.Besides,DOM spectral characteristics also varied significantly by the latitude of sampling sites,suggesting that climate may play an important role in soil DOM content,component and spectral characteristics.In general,soil DOM from tropical and subtropical regions（Guangdong and Guangxi）had low degree of humification and obvious autochthonous origin,indicating that soil activity is strong in these regions and DOM is mainly derved from microbial source.（3）The results of batch procedure showed that soil DOM was mainly composed of Hy（69.7±1.5%）and FA（25.4±1.2%）,then FA was the main active component of DOM.FA%of DOM in dry land（28.6±2.0%）and forest soil（24.1±2.6%）were higher than that in paddy soil（23.4±1.6%）,and were generally higher than that in tropical and subtropical regions（Guizhou,Guangdong and Guangxi）.In addition,DOM in paddy soil contained more Hy（72.7%±2.0%）,followed by forest soil（69.8%±3.4%）and dry land soil（66.6%±2.5%）,but there was no obvious difference between it.（4）The results of HPSEC showed that the weight-averaged molecular weight（M_w）of DOM in paddy soil（2913±79 Da）was significantly lower than that in dry land soil（3177±87 Da）and forest soil（3246±112 Da）.The same trends also can be found in number-averaged molecular weight（M_n）that M_n of DOM in paddy soil（2027±67 Da）was significantly lower than that in dry land soil（2372±84 Da）and forest soil（2328±80 Da）.It reflected that DOM in paddy soil usually characterized by low molecular weight.In general,the molecular weight of soil DOM decreased with the latitude of sampling sites,which was consistent with the results of spectral parameters,suggesting that climate has an important influence on soil DOM properties.（5）UV-Vis and EEMs-PARAFAC are the common spectral techniques to characterize soil DOM.The results of our study showed that the spectral characteristics obtained from the two techniques in good agreement with the accurate molecular weight information of DOM which obtained by HPSEC.However,the DOM composition information which obtained by the batch procedure was not correlated well with the results of both spectral and chromatographic techniques.Furthermore,Hy component（small molecular compound）defined by the batch procedure still contained a certain amount of macromolecular substances,and the molecular weight was close to that of FA,which may be FA component.Therefore,batch procedure needs to be further optimized in soil DOM characterization.