Conservation tillage for 17 years alters the molecular composition of organic matter in soil profile.

Conservation tillage is considered as a potential measure to mitigate climate change by sequestering soil organic matter (SOM), however its stabilization mechanisms remain elusive. In this study, we revealed the molecular composition of SOM in soil profile (~50 cm depth) from a 17-yr tillage experiment in North China. The soils were collected from 0-10, 10-20, 20-30 and 30-50 cm layers under conventional tillage (CT), and conservation tillage such as rotary tillage (RT) and no-tillage (NT). The sequential solvent extraction and CuO oxidation methods were used to quantify free lipids and lignin-derived phenols. The results showed that NT (cf. CT) increased labile compounds (i.e., carbohydrates) and plant-derived SOM (i.e., long-chain (≥C20) aliphatic lipids and steroids) in the 0-10 and 30-50 cm layers. The RT (cf. CT) increased the total free lipids by 72-133% in the sublayers (>10 cm). The RT (cf. CT and NT) resulted in higher preservation of plant-derived (≥C20 aliphatic lipids and steroids) and microbial-derived compounds (<C20 aliphatic lipids and trehalose) SOM in deep soils (i.e., ≥10 cm). Conservation tillage suppressed lignin degradation, as reflected by 32-137% higher total lignin-derived phenols under RT and NT than CT in the 0-10 and 30-50 cm layers. The NMR revealed higher aliphatic to aromatic C ratio under NT and RT in the whole soil profile, suggesting more aliphatic lipids accumulated. Conservation tillage increased SOC stocks by 10-14% in 0-10 cm layer but not in deeper profiles. These results suggest that conservation tillage have increased plant-derived lipids and lignin accumulation. Our study highlights that conservation tillage (particularly the RT) after 17 years alters SOM molecular compositions and degradation processes in the soil profile. These findings have implications for improving our understanding of C stabilization mechanisms in agroecosystems.