Molecular composition and structure of organic matter in density fractions of soils amended with corn straw for five years

Abstract

Corn straw is an important source of carbon (C), and when applied to soil, it alters the accumulation and distribution of organic C. However, the mechanistic pathways by which newly added C is stored and stabilized in soil remain a subject of interest and debate among scholars. In this study, we investigated the chemistry of organic matter in different soil density fractions of Haplic Cambisol (sandy clay loam) separated from no-till (NTS), minimum tilled (MTS), and conventional tilled (CTS) fields amended with 8900 kg ha−1 year−1 of corn straw. After five years of corn (Zea mays L.) monocropping, soils were collected from the 0–20 and 20–40 cm depths, and processed to obtain their light fraction, occluded particulate, and heavy fraction organic matter(LFOM, oPOM, and HFOM, respectively). The results showed that compared with CT (conventional tillage without corn straw return), corn straw-amended soils (i.e., NTS, MTS, and CTS) increased soil organic C content by 11.55–16.58%. Thermogravimetric and Fourier transform infrared analyses demonstrated that the HFOM was characterized by a greater proportion of easily biodegradable substances, which may be due to the deposition of microbially processed materials on the surface of soil minerals. The LFOM and oPOM fractions were distinguished by their greater phenolic, aromatic C, and thermally stable compounds. Compared with CT, the NTS and MTS fields showed higher abundances of hydrophobic, aliphatic, and thermally unstable organic compounds, which increased soil C content and stability in the HFOM. Therefore, NTS and MTS may be ideal practices to increase soil organic C content in Haplic Cambisol.