Responses of Soil Humus Composition and Humic Acid Structural Characteristics to the Addition of Different Types of Biochar in Phaeozems

Biochar addition can effectively increase the soil organic carbon (SOC) content and improve soil quality. However, the formation mechanisms and structural changes of soil humic substances under biochar addition, especially under different types of biochar, are poorly understood. In this study, we compared the effects of different sources of biochar on the humus composition and humic acid (HA) structural characteristics of the Phaeozem region of Northeast China. Four treatments, including RB (rice husk biochar), CB (corn stalk biochar), TB (tobacco stalk biochar), and CK (no biochar application), were explored. SOC components were analysed, and HA structures were determined through Fourier transform infrared spectroscopy (FTIR) and 13C nuclear magnetic resonance spectroscopy (13C-NMR). Compared with the CK treatment, RB and TB increased the fulvic acid (FA) content by 32.67%, and CB significantly increased it by 56.33%. The FTIR and NMR spectra showed that CB strengthened the aliphatic characteristics of soil HA, while the aromatic structure proportion decreased. The CB treatment significantly reduced the ratio of alkyl C/O-alkyl C (A/O-A) and hydrophobic C/hydrophilic C (HB/HI), and TB increased the HB/HI ratio by 9.82%. CB simplified the HA structure and improved soil HA activity. TB was superior to the other biochar types (RB and CB) in terms of soil carbon sequestration. Obvious differences in the effects of adding different biochar types on the humus composition and HA structural characteristics were presented.