Stable isotopes reveal the formation diversity of humic substances derived from different cotton straw-based materials.

Humic substances (HS) are essential in environment processes and carbon (C) sequestration in soils. In this study, organic materials such as cotton straw and its derived compost and biochar were added to the soil on a C-equivalent basis and incubated for 30 and 180 days in order to investigate the different forms of plant biomass derived C sequestration in HS. The C distribution in humic acid (HA), fulvic acid (FA), and humin (Hu) derived from organic materials was investigated using the C isotope method, while the catalase, sucrose, and β-glucosidase activities were also determined. The results showed that C3 distribution of Hu derived from straw, compost and biochar increased from 40.94% to 67.12%, 74.47% and 80.75%, respectively. In addition, the increase of C3 distribution of HA or FA derived from straw, compost and biochar were 4.69%, 10.09% and 1.49%, respectively. There were significantly positive correlations between catalase, sucrase and β-glucosidase activities and C3 derived HA and FA. The principal component analysis showed that catalase, sucrase and β-glucosidase were explained mainly by the first principal component indicating a significant correlation. These findings suggest that straw, compost and biochar are mainly sequestrated in Hu. Comparatively, the straw and compost are more likely to contribute to the formation of HA and FA in soil, but biochar favors the Hu, which helps in soil C sequestration. The formation of HA and FA derived from organic materials was supported by catalase, sucrase and β-glucosidase activities.