Enhancing soil labile organic matter through oilseed rape-rice rotation and straw returning in paddy-upland systems

Background and aims: Soil labile organic matter (LOM) plays a vital role in enhancing soil productivity and fertility. However, the annual variability of LOM content and structure under diverse crop rotation and fertilization practices remains insufficiently investigated. Methods: This 5-year field study investigates the impact of different fertilization treatments-no fertilization (CK), conventional fertilization (CF), recommended fertilization (RF), and recommended fertilization with straw returning (RF + S)-on the content and structure of LOM under oilseed rape-rice (OR) and wheat-rice (WR) cropping systems. Results: Our results showed that the effect of rotation type on soil LOM was most pronounced during the upland season, while fertilization effects were significant across the entire annual paddy-upland rotation system. During the upland season, the OR rotation, excluding the CF treatment, resulted in significantly higher increases in soil easily oxidizable organic carbon (EOC), particulate organic carbon (POC), and particulate nitrogen (PN) compared to the WR rotation with same fertilization treatments. The RF and RF + S treatments significantly enhanced soil LOM contents in both rotations. Excitation Emission Matrix (EEM) and UV-Visible absorption analyses indicated that the OR rotation had a higher degree of humification, a more aromatic structure, and greater levels of colored dissolved organic matter (DOM) than the WR rotation. Conclusions: In summary, the OR rotation, especially with straw returning, is an effective strategy for enhancing both the content and structural stability of LOM. This approach promotes improved soil quality and offers a sustainable solution for long-term fertility management in paddy-upland rotations. Highlights: Rotation and fertilization practices jointly affect soil LOM content and structure. Oilseed rape in paddy-upland rotation improves LOM content and composition. The LOM difference between OR and WR rotations mainly appears in the upland season.