Changes in soil humus composition and humic acid structural characteristics under different corn straw returning modes

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. Additional keywords: corn straw retention, fluorescence EEM, FTIR spectra, humic acid structure. Received 25 January 2020, accepted 27 March 2020, published online 21 May 2020
Introduction It is generally accepted that removal of crop residues and conventional tillage have a profound effect on soil organic carbon (SOC) quantity and quality. Warren Raffa et al. (2015) [...]