Spectroscopic Characteristics of Soil Humus Components Extracted With Acetone Hydrochloric Acid Mixture

The components of soil humic acid (HA) and humin (HU) were extracted by acetone hydrochloric acid mixture, their elemental components were analyzed, and their spectroscopic characteristics were analyzed by UV-Vis diffuse reflectance and infrared spectroscopy. Elemental analysis showed that HA had higher carbon, hydrogen, nitrogen, and sulfur contents while having lower oxygen content than HU. Atomic ratio analysis showed that HA had a higher condensation degree and more complex molecular structure than HU. The UV-Vis diffuse reflectance spectrum analysis showed that HA and HU had no obvious characteristic peak because of the complex composition of humic substances and the mutual interference of various functional groups. The UV absorbance decreased with the increase of wavelength, and HA contained more light absorbing organic components. The UV characteristic parameters of SUV254 and E-4/E-6 showed that HA had higher aromaticity and humification degree than HU. Infrared spectrum analysis shows that HA and HU have similar infrared spectra. However, the absorption intensity of characteristic absorption peaks of each research object is different. The vibration amplitude of HA in soil layer of 20 similar to 40 cm cultivated black soil and 18 similar to 37 cm uncultivated gray soil is larger, indicating that they had high contents of phenolic compounds, hydroxyl functional groups, aliphatic compounds, carboxyl groups, aldehydes, ketones, ethers, carbohydrate and amine compounds. The vibration amplitude of HU in soil layer of 10 similar to 20 cm cultivated black soil and 18 similar to 37 cm uncultivated gray soil is large, indicating that they contain more phenolic compounds, carboxylic acid, aliphatic hydrocarbons and sugars. In addition, intensity comparison of each absorption peak in the infrared spectrum showed that the content of phenolic compounds, hydroxyl functional groups and aliphatic compounds in HA and HU of black soil increased after cultivation. In contrast, the content of phenolic compounds, carboxyl groups and aliphatic compounds in HA and HU of gray soil decreased after cultivation. These results showed that cultivation has relatively little impact on black soil organic matter, and to a certain extent, increased soil organic matter content, but promoted the decomposition of gray soil organic matter. In conclusion, the acetone hydrochloric acid mixture extraction method provides new technical support for studying the biochemical and physiological activities of humic substances, which provide a theoretical basis for rational utilization of soil resources.