Conservation tillage facilitated soil carbon sequestration through diversified carbon conversions

The organic carbon content of soil (SOC) is a key factor for soil health and in climate change. Both tillage practice and straw returns influence the SOC pool. However, their comprehensive impacts on SOC sequestration remain largely unknown. We quantified SOC characteristics, including different labile fractions of soil carbons, and chemical compositions of SOC in cropland soil under long-term (1985–2019) wheat-maize rotation with different tillage and straw return practices. No-tillage (NT) increased the plow layer SOC stocks significantly, as compared with the conventional tillage, and both contents of easily oxidizable organic carbon (EOC) and particulate organic carbon (POC) were enlarged by straw return practices. In particular, NT resulted in considerably larger contribution of labile C (O-alkyl C) and recalcitrant C (such as aromatic C) to SOC sequestration than conventional tillage, suggesting that more diversified C conversion was responsible for enhanced SOC sequestration. Additionally, NT decreased the ratio of alkyl C to O-alkyl C as well as the ratio of lignin to polysaccharides (SOC degradation indices), indicating a lower degree of SOC decomposition. The present study offers new insights into the functional roles of long-term NT and straw return practices on SOC turnover, which are crucial for improving agricultural soil quality and carbon management strategies.