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. 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 13C nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. The results showed that biochar treatment increased the aryl C of HA, FA, and Hu by 1.38%, 1.68%, and 10.46% compared to straw treatment and increased the aryl C of HA, FA, and Hu by 1.46%, 1.99% and 2.01% compared to compost treatment. The O-alkyl C of HA was 10.59% and 10.65% in high biochar/straw and biochar/compost ratios respectively, while it was 9.81% and 9.61% in low biochar/straw and biochar/compost ratios. In addition, the O-alkyl C of FA was 62.83% and 58.48% in high ratios of biochar/straw and biochar/compost, respectively, while it was 55.85% and 55.94% in low ratios of biochar/straw and biochar/compost. These results suggest that biochar is advantageous for aryl C formation of FA and Hu due to its high aryl C content, whereas straw or compost is advantageous for alkyl C formation of HA. The stability of aryl C and O-alkyl C of HA, FA, and Hu can be improved in soils by incorporating biochar in combination with straw or compost.

Organic materials are essential to increase soil organic carbon (SOC). However, it is unclear whether C sequestration is primarily affected by the form of organic materials or soil types. Wheat straw, tobacco straw, and their derived biochars were added to acidic soil, saline soil and calcareous soil in the same C concentration and incubated for 30, 90, and 180 days, respectively. The contents of humic substances (HS), the structural characteristic of SOC, and enzyme activities were investigated. The results revealed that both biochar-C and crop straw-C were mainly sequestered in humin (HU) across all soil types. Moreover, humic acid (HA) levels increased in straw treatments but not in biochar treatments. The cluster analysis and principal components analysis showed that HU had a significantly positive correlation with SOC. The aryl C of SOC increased in biochar treatments, while phenolic C and O-alkyl C of SOC increased in straw treatments. A positive correlation was found between aromatic C and SOC concentration, as well as between polyphenol oxidase activity and O-alkyl C. The formation of O-alkyl C was affected by polyphenol oxidase activity. These findings suggest that the form of organic materials and microbial activity, rather than soil types, influence the formation of HS and functional groups of SOC. Soil amended with biochar can sequester more recalcitrant C while also increasing the hydrophobicity of SOC.

Carbon sequestration of organic materials and biochar in soil is a vital way to increase soil organic carbon (SOC), but the stability of SOC especially the humic substances (HS) is different with these amendments. In this study, cotton straw, compost derived from cotton straw (compost) and biochar derived from cotton straw (biochar) were added with equivalent carbon content into soil and incubated for 180 days. The structural characteristics of humic acid (HA), fulvic acid (FA) and humin (Hu) was studied by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The results showed that the aryl C of Hu and FA increased by biochar treatment or biochar incorporated straw or compost and decreased with low ratio of biochar incorporated straw or compost. In addition, phenolic C of Hu and HA increased by straw treatment or biochar incorporated straw and increased with low ratio of biochar incorporated straw or compost. However, O-alkyl C of Hu and HA increased by compost treatment or biochar incorporated compost and increased with low ratio of biochar incorporated straw or compost. These results suggest that the stability of FA amended with straw or compost can be enhanced by incorporating biochar through increasing aryl functional groups, but the stability of HA amended with straw or compost can be enhanced by incorporating biochar through increasing alkyl groups. Biochar was in favor of aryl C formation of FA and Hu, while straw or compost was beneficial to alkyl C formation of HA and Hu.