Abstract:

Soil humic substance (HS) was extracted from different soil types with sodium pyrophosphate (Na₄P₂O₇) solution and its spectral and thermogravimetric characteristics were studied. The effects of reclamation on the chemical composition and molecular structure characteristics of HS were studied by scanning electron microscopy, UV-visible diffuse reflectance spectroscopy, infrared spectroscopy and thermogravimetric analysis. The results showed that the appearance of HS was uneven, and the surface was irregular with some fine particles and large specific surface area. The influence of reclamation on the appearance of black soil HS was relatively small, while on gray soil HS was relatively large. Due to the complexity of HS chemical composition and the interaction of its functional groups, no obvious characteristic absorption peak was observed in UV-Vis diffuse reflectance spectrum. UV characteristic parameters showed that the values of SUV₂₅₄ and E₄/E₆ was inconsistent, suggesting that multiple methods were required to study the chemical composition and structural characteristics of soilHS. The infrared spectroscopy results indicated that HS from uncultivated gray soil (AY+AEL), cultivated gray soil (PY1), and cultivated black soil (PU1) contained high contents of phenols, carboxylic acids, oxygen-containing functional groups, aliphatic and aromatic structures. Reclamation had different impacts on black soil and gray soil. After reclamation, the contents of phenolic compounds, carboxylic acids, hydroxyl groups, quinones, aliphatic and aromatic compounds in black soil HS increased, but the contents of aliphatic compounds and alkanes in gray soil HS decreased. Thermogravimetric analysis indicated that reclamation increased the content of aliphatic structure in black soil HS, but decreased the contents of aliphatic and aromatic structure in gray soil HS, which was basically consistent with the results of infrared spectroscopy. The impact of reclamation on black soil was relatively small, while on gray soil was relatively large. Under reclamation conditions, black soil organic carbon was not easy to lost, which to some extent promoted the accumulation of soil organic matter. On the other hand, organic carbon in gray soil was oxidized and decomposed into compounds with relatively simple structures under reclamation. This study provided a theoretical basis for research on HS biochemistry, and development and utilization of soil resources rationally.

Key words: humic substances, reclamation, microscopic morphology, chemical composition, molecular structure, thermogravimetric analysis, physiological activity, soil management, organic carbon pool, biostimulants, tillage methods, stability of soil organic carbon