Soil Carbon Mineralization and Aggregate Distribution in Various Tillage Practices of Rice–Wheat Cropping System: A Field and Laboratory Study

Different tillage and residue management practices can strongly impact soil structure stability and soil organic carbon (SOC) sequestration. However, the detailed information about the aggregate stability, SOC protection, and mineralization within aggregates are still lacking. Using aggregate fractionation with laboratory incubation, we investigated aggregate-associated SOC, soil structural stability, and SOC mineralization in rice–wheat rotation under different tillage treatments: CT0 (puddled rice, conventional wheat − residue); CTR (puddled rice, conventional wheat + residue); NT0 (direct rice seeding, zero-tilled wheat − residue); and NTR (direct rice seeding, zero-tilled wheat + residue). NTR significantly enhanced the large macro-aggregate fraction (> 2 mm) at the 0–45 cm soil layer and macro-aggregate-associated SOC at the 0–15 cm soil layer. However, CTR enhanced the macro-aggregate-associated SOC at the 15–30 cm layer. Notably, the mean weight diameter (~8%) and geometric mean diameter (~24%) were higher under NTR than those under other treatments, and the effect was more pronounced in 30–45 cm layer. The highest average cumulative carbon mineralization C m (~9%) was observed in macro-aggregates (> 2 mm) than micro-aggregates (< 2 mm). With regard to tillage systems, the C m was higher under NTR compared to other treatments. However, C m at the 15–30 cm layer was higher (~22%) under CTR than that in other treatments. Notably, a positive relationship was found between total carbon input and soil aggregation. Specifically, carbon input of NT0, NTR, and CTR increased > 2 mm aggregates at 0–15 cm, while carbon input of CTR increased > 2 mm at 15–30 cm soil depth. Overall, no tillage with residue return (NTR) could enhance the soil macro-aggregation and associated SOC accumulation by decreasing SOC mineralization in rice–wheat double cropping system.