Research On Spectral Fingerprint Characteristics Of Soil Dissolved Organic Matter In Hetao Irrigation Area

Dissolved Organic Matter(DOM)refers to organic matter that is easily dissolved in water,acid and alkaline solutions and can pass through 0.45 nm filtration membrane.As a natural ligand and sorption carrier in the soil,DOM is a very active chemical substance,and its functional groups have strong complexing adsorption capacity,which plays an important role in the soil formation process and soil organic matter decomposition process.The pH,nitrogen,phosphorus and ions in the soil also affect the composition,structural characteristics and spatial distribution of DOM.Soil under agricultural plant types was collected in Hetao irrigated area: corn fields(CF),seed melon fields(SM),sunflower(SF),abandoned farmland(AF)and soil under non-agricultural plant types: bare saline-alkali soil(BS),phragmites communis(PC),tamarix ramosissima(TR),populus alba(PA)and suaeda glauca(SG).Four soil layers of 0-20 cm,20-40 cm,40-60 cm and 60-80 cm were selected for each sampling point.Application of synchronous fluorescence spectrum(SFS),ultraviolet-visible spectra(UV-Vis)and Fourier transform infrared spectrum(FTIR)the DOM in soil samples were characterized,and combining with two-dimensional correlation(2D-COS)and self-organizing map stoichiometric method and the multivariate statistics,to identify the composition,structure characteristics of DOM and humification degree in the soil,and build a structural equation model(SEM)tried to expound the soil physical and chemical environmental factors and soil humification degree of response mechanism.The results showed that :(1)based on DOM ultraviolet spectrum analysis,the degree of polymerization of benzene ring C skeleton in the DOM of Hetao irrigation area was similar.The degree of molecular condensation of DOM is that agricultural soil is smaller than non-agricultural soil.The substituents of aromatic rings in agricultural soils were mainly nonpolar functional groups such as fat chains and esters.The proportion of lignin,quinine and other organic matter in the soil was non-agricultural soil > agricultural soil.Agricultural soil DOM has a large proportion of non-humic substances and a low degree of humic substances.And with the change of soil depth,the degree of humification of different soil layers in the same soil varies greatly.Based on the 2D-COS analysis method of SFS and FTIR,it was found that the positive and negative correlations between DOM components and functional groups in soil were significantly different under different vegetation conditions.(2)five DOM components were identified by SFS binding factor analysis,including tyrosine like,tryptophan-like,microbial metabolites,fulvic-like and humic-like.With the increase of soil depth,the change trend of fulvic-like and humic-like in soil under different plant types was positively correlated,while the correlation between fulvic-like and other components was different.(3)through three-dimensional fluorescence spectrum combined with self-organizing map,five components were obtained: tyrosine-like,tryptophan-like,UV fulvic-like,visible fulvic-like,and humic-like.It was found that the relative abundance of protein-like substances in agricultural soil CF1,SM1,SF1 and AF1 was lower(13%~19.5%),while the relative abundance of fulvic-like was higher(66.9%~75.4%).The relative abundance of protein-like substances of SG1-2,PA1-2 and TR1-2 in non-agricultural soil is very large(68.1~76.4%).In addition,SG,PA and TR decreased the abundance of protein-like substances and increased the abundance of fulvic-like as the soil layer deepened.DOM in soil is mainly composed of exogenous organic matter,with weak contribution from autogenous source,low bioavailability and high overall humification.(4)the SEM model was constructed to obtain the comprehensive weight results of the influence of environmental factors on the degree of soil humification.Environmental factors such as soil ions and physical and chemical indexes can directly or indirectly affect the degree of soil humification,among which the weight of physical and chemical indexes on the degree of soil humification is greater(59.12%)than the weight of ions on the degree of soil humification(40.88%),while the total weight of indirect influence(83.02%)is greater than the total weight of direct influence(16.98%).Among the indirect effects,humus(35.85%)and functional group composition(36.48%)mainly affected the degree of soil humification.