Structural Changes In Humic Acid During the Degeneration Process of a Steppe Soil.

The degradation of grassland causes changes in the internal properties of soil, including the structure and characteristics of humic acids (HAs).  In this study, HAs were extracted from steppe soils at different degeneration stages.  Elemental analysis and ¹³C nuclear magnetic resonance spectroscopy (¹³C NMR) were used to study the structural changes in soil HAs.  Structural changes in HA were further investigated by factor analysis.  The results showed, with degeneration increase, that the aromatic C content decreased from 27.1% to 20.5% indicating a decline in the degree of aromatization, while AlOC (the aliphatic C connected with oxygen atoms) content increased from 5.2% to 10.7% indicating an increase in oxidization. With the same process of degeneration increase, the AlNC (the aliphatic C connected with nitrogen atoms) content declined from 42.5% to 37.0%, and the AlHC (the aliphatic C without substitute) content increased from 28.9% to 30.9%. These results indicated during the degradation, the aromatic cores of HAs were broken down, and resulted in decrease of the aromaticity and stability of HAs.  During the recovery process, the contents of aromatic carbon and carbohydrates increased in soil HAs, associating with decrease in AlOC content and the hydrophobicity and increases in AlNC content and the acidity of HAs. The factor analysis grouped the areas with non- and less-degradation into the same category and the areas under severe-degradation and under recover process into another category based on the types of carbon in soil HAs. The results indicated that the aromatic carbon, the core structural groups of HAs, was not easy to reconstructed, even though after 15 years of protection from grazing.