Characteristics of soil aggregate organic carbon accumulation and its influencing mechanism in double cropping paddy fields in southern China

The multi-cropping approach of double-cropping rice based on planting green manure in winter is important for improving the quality of cultivated land and ensuring national food security. However, at the aggregate scale, our understanding of soil organic carbon (SOC) accumulation characteristics and sequestration mechanisms in croplands under multiple cropping conditions is very limited. Therefore, this study set up four multi-cropping patterns: Chinese milk vetch-rice-rice (MV-R-R), rapeseed-rice-rice (RP-R-R), ryegrass-rice-rice (RG-R-R), and winter fallow-rice-rice (WF-R-R) as the control. Microbial community, microbial residual carbon, and aggregate bound carbon of each aggregate grain size were measured to explore the aggregate retention mechanism of SOC. The results showed that the multi-cropping approach increased the content of super-large aggregates (>2 mm), enhanced aggregate stability, and increased organic carbon and aggregate-bound organic carbon in the 0–20 cm soil. The aggregate organic carbon content decreased with decreasing aggregate size. Microbial residual carbon analysis demonstrated that the multi-cropping pattern increased microbial residual carbon content, especially the fungal residual carbon under the grain size <0.25 mm. In addition, the fungal residual carbon was significantly correlated with SOC (P<0.001). Alpha diversity of microorganisms in large aggregates (0.25–2 mm) was the highest, which subsequently decreased with a reduction in particle size. Random forest model analysis showed that aggregate size was the most important factor affecting SOC content. Therefore, this study emphasizes the importance of the physical protection of aggregates for SOC sequestration.