Study On Vertical Variation Of Organic Matters Characteristics Along The Soil Profiles In The Typical Area

Organic matter is an important component of the soil, playing a significant role in organic pollutants migration. In the vadose zone under the topsoil, the contents, composition and structure of the organic matter are strikingly different from the topsoil. However, they play a key role in pollutants absorption. So it is significant to study the vertical variation of organic matters characteristics along the soil profiles,which are the basic research of organic pollutants migration and transformation in the vadose zone.In this study, a fine profile sampling was selected in the typical agricultural area in southeast of Beijing. Physical-chemical parameters of the samples such as clay content, total amount of clay minerals, cation exchange capacity (CEC) and total organic compounds (TOC) were analyzed. HA/TOC, HA/FA and light absorbance of alkali extractions methods were used to determine the soil humification degree. In addition, humic acids and fulvic acids were studied by elemental composition, scanning electron microscopy (SEM) and FTIR to understand changing along the depth of functional groups. Results show that:Correlation analysis between humification and physical-chemical parameters indicates that they have good relationship with each other at 0.01level, except TOC and CEC at 0.05 levels. Likewise, humification indexes HA/TOC, HA/FA and light absorbance of alkali extractions have good relationships either. So, light absorbance can be used as indicators of soil humification degree in loose sediments vadose zone.Element analysis of extracted organic matter reveals that C/H value of humic acids is higher than that of fulvic acids. At the same time, O/C value of humic acids is lower than that of fulvic acids. Aromatization degree of HA is higher than FA and the degree of oxidation of HA is lower than FA, which explain why the structure of HA is much dense than that of FA. SEM results also confirmed the conclusion.Results of FTIR analysis proved that stronger CEC and lower clay content will lead to dramatic change in FA functional groups.Comprehensive study on TOC content, humification degree and humic acids composition and structure (elemental composition, molecular size, composition and content of functional groups) proved that they have obvious correlation with clay content, total clay minerals and CEC. So we can conclude that soil composition, especially clay content, is an important factor to TOC content, humification degree and humic acids composition in the soil vertical profile and it has a higher effect than soil original organic matter and deposition age.