Abstract
Studying changes in soil humus composition and humic acid (HA) structural characteristics caused by agronomic practices provide insights into the pathways of soil organic carbon (C) stabilisation dynamics. This five-year field study evaluated the effects of straw returning modes on humus composition and HA structure. Treatments included (i) corn straw returned on the soil surface (NTS), (ii) corn straw incorporated into soil within 0–10 cm (MTS), (iii) corn straw incorporated into soil within 0–20 cm (CTS) and (iv) no corn straw applied (CT). Soil HA was characterised by Fourier transform infrared (FTIR) and fluorescence spectroscopies. The results demonstrated that corn straw returning improved humus C fractions in this order NTS > MTS > CTS > CT in 0–20 cm depth. The FTIR and fluorescence results demonstrated that corn straw returning enhanced aliphatic, hydroxyl, methoxyl and carboxyl groups and simplified HA molecular structure, indicating regenerated and newly formed HA. Among all treatments, NTS was more conducive in simplifying HA molecular structure and enhancing aliphatic and hydrophobic C. Hydrophobicity in aliphatic C is the driving force in the stabilisation of soil C, which is important for sustainable agriculture. Therefore, we conclude that NTS is the better practice to turn arable lands into a sink for C.
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