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

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 density fractions of Haplic Cambisol (sandy clay loam) in a field experiment with corn straw at 8 900 kg ha^-1 year^-1 under no tillage (NT), minimum tillage (MT), and conventional tillage (CT). 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 the organic matter in light fraction (LFOM), occluded particulate (oPOM), and heavy fraction (HFOM) in the order. The results showed that compared with conventional tillage without corn straw return (CT₀), corn straw return (i.e., NT, MT, and CT) 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 were distinguished by greater phenolic, aromatic C, and thermally stable compounds. Compared with CT₀, the NT and MT fields showed higher abundances of hydrophobic, aliphatic, and thermally unstable organic compounds, which increased soil C content and stability in the HFOM. Therefore, NT and MT may be ideal practices to increase soil organic C content.