Mollisols have a high potential to mitigate climate change and play an important role in the global carbon (C) cycle due to their inherently high soil organic matter (SOM). However, little is known about the mechanism of C stabilization in Mollisols, especially subjected to different management effects. To trace C stabilization between aggregates and SOM density fractions in Mollisols, soil samples were collected from different experimental plots: controlled irrigation + rice (Oryza sativa L.) straw removal (CI), flooded irrigation + rice straw removal (FI), controlled irrigation + rice straw return (CI-SR), and flooded irrigation + rice straw return (FI-SR). Each soil sample was separated into three aggregate size classes (>250 μm, 53–250 μm, and <53 μm), then each class individually subjected to density fractionation to obtain free and occluded light fractions (fLF and oLF), as well as dense and mineral-heavy fractions (DF and MF). The overall C content and ¹³C abundance of fractions were measured to interpret C transfer and accumulation among aggregate and density fractions. The highest increase in the soil organic C (9.27–24.88%) was observed in CI-SR compared with the other treatments. Irrigation and straw return primarily affected C accumulation within macroaggregates and the MF, which were the dominant forms in the aggregates and density fractions, respectively. An enrichment in δ¹³C was found from macroaggregates to silt + clay, and from light to heavy fractions, indicating that Mollisols macroaggregates and light fractions acted as the source or initial store of plant residues, and that the C in the silt + clay class and heaviest fraction contained more microbially-transformed C than did the macroaggregates and light fractions. A detailed scheme of C transfer within aggregates and SOM fractions based on the δ¹³C natural abundance revealed the following general sequence: free light → occluded light → dense → mineral fractions, concurring with results reported at upland sites. There is a greater probability of C transfer between SOM density fractions under CI. This contrasted with the results that CI decreases the possibility of C exchange between aggregates than FI, which indicates differences in the C stabilization processes between no-flooded and flooded conditions of Mollisols. In addition, straw returning can reduce the possibility of C exchange both between and within aggregates, which plays a significant role in maintaining the stability of the Mollisols carbon cycle. The present study provides further detailed insights into the C stabilizing mechanisms in Mollisols which depend on management (straw, water status). This finding is conducive to the sustainable use and management of Mollisols toward maintaining or increasing C stock and in realizing the objective of C-neutral agriculture.

由于其固有的高土壤有机质 (SOM)，Mollisols 具有缓解气候变化的巨大潜力，并在全球碳 (C) 循环中发挥重要作用。然而，关于 Mollisols 中 C 稳定化的机制知之甚少，特别是受到不同管理效果的影响。为了追踪 Mollisols 中聚集体和 SOM 密度部分之间的 C 稳定性，从不同的实验地块收集土壤样品：控制灌溉 + 水稻（Oryza sativaL.) 除草 (CI)、漫灌 + 稻草去除 (FI)、控制灌溉 + 稻草还田 (CI-SR) 和漫灌 + 稻草还田 (FI-SR)。将每个土壤样品分成三个聚集体尺寸等级（>250 μm、53-250 μm 和 <53 μm），然后对每个等级单独进行密度分级以获得游离和封闭的轻组分（fLF 和 oLF），以及致密和富含矿物的部分（DF 和 MF）。总C含量和¹³测量组分的 C 丰度以解释聚集体和密度组分之间的 C 转移和积累。与其他处理相比，在 CI-SR 中观察到土壤有机碳的最高增加（9.27-24.88%）。灌溉和秸秆还田主要影响大聚集体和 MF 内的 C 积累，它们分别是聚集体和密度部分中的主要形式。^{δ 13}的富集C从大团聚体到粉砂+粘土，从轻到重，表明Mollisols大团聚体和轻质组分是植物残体的来源或初始储存，粉砂+粘土类和最重组分中的C含量较多微生物转化的 C 比大聚集体和轻组分。基于 δ ^{13的骨料和 SOM 馏分内 C 转移的详细方案}C 自然丰度揭示了以下一般顺序：自由光→遮挡光→密集→矿物部分，与高地报道的结果一致。在 CI 下，SOM 密度分数之间的 C 转移概率更大。这与 CI 比 FI 降低聚集体之间 C 交换的可能性的结果形成对比，这表明 Mollisols 的未淹没和淹没条件之间的 C 稳定过程存在差异。此外，秸秆还田可以降低团聚体之间和团聚体内部碳交换的可能性，这对维持Mollisols碳循环的稳定性起着重要作用。本研究提供了对依赖于管理（秸秆、水状态）的莫里索尔中 C 稳定机制的进一步详细见解。