Carbon sequestration by application of organic materials and biochar in soil is an important strategy to increase soil organic carbon (SOC), but the stability of SOC, particularly humic substances (HS) vary with the types of organic material. In this study, cotton straw and its derived compost and biochar were added with equivalent carbon content to soil and incubated for 180 days. The structural characteristics of humic acid (HA), fulvic acid (FA) and humin (Hu) were investigated using solid-state