Oilseed rape-rice rotation with recommended fertilization and straw returning enhances soil organic carbon sequestration through influencing macroaggregates and molecular complexity

The sequestration of soil organic carbon (SOC) is influenced by agricultural management practices. However, the mechanisms that underlie the impacts of diverse paddy-upland rotation systems and fertilization strategies on SOC sequestration remain to be elucidated. Within this study, we investigated the variations in SOC sequestration under oilseed rape-rice (OR) and wheat-rice (WR) rotations with various fertilization regimes (CK, without fertilization; EF, excessive fertilization; RF, recommended fertilization; and RF+S, recommended fertilization with straw returning) through a five-year field experiment. Crop C inputs, soil aggregate-associated organic C, SOC functional groups, and their importance in SOC sequestration were examined. The results revealed that both SOC stock and sequestration rate were significantly enhanced by the OR rotation and straw returning. Specifically, SOC stock and sequestration rates were 7.7–14.3% and 28.6–83.7% higher in the OR rotation compared to the WR rotation under the same fertilization treatment. In addition, SOC stocks and sequestration rates were 6.7–13.3% and 24.4–77.6% higher in the RF+S treatment compared to the RF treatment. However, excessive fertilization (EF) did not further promote SOC sequestration. The OR rotation also showed higher values of aggregate mean weight diameter (MWD) and large-macroaggregate C compared to the WR rotation, with increases of 7.7–14.1% and 4.7–33.8%, respectively. The nuclear magnetic resonance studies showed that the OR rotation exhibited an elevated abundance of aromatic C and a more intricate molecular configuration compared to the WR rotation. Furthermore, crop C input increased significantly with fertilization, and crop C input was the highest in the RF+S treatment. The aggregate MWD and large-macroaggregate C also significantly increased after fertilization. Partial least squares path modeling revealed that crop rotation affected SOC sequestration mainly through its effect on large-macroaggregate C and SOC molecular structure complexity, whereas fertilization mainly affected SOC sequestration though its effects on crop C inputs and large-macroaggregate C. We conclude that OR rotation combined with straw returning is an effective measure for enhancing SOC sequestration in paddy-upland rotation systems, contributing to mitigating climate change in agricultural systems.