Straw return promotes soil organic carbon sequestration through aggregate protection and chemical bonding mediated by iron oxides

Straw serves as a major source of soil organic matter, yet the specific pathways and mechanisms by which straw incorporation regulates different forms of iron oxide, and thereby their influence on organic carbon accumulation, remain poorly understood in paddy soil. To address this gap, a long-term field experiment was established in 2012 in a double-cropping rice system. Soil samples were collected after 11 consecutive years from four treatments: NPK (NPK fertilization, winter fallow without straw return), NPKS (NPK fertilization, winter fallow with straw return), NPKGM (NPK fertilization, winter green manure [Chinese milk vetch] without straw return), and NPKGMS (NPK fertilization, winter green manure with straw return). We systematically analyzed soil organic carbon (SOC), iron oxides, aggregates, and iron-bound organic carbon. Results showed that SOC content increased significantly by 13.6% in NPKS, 5.3% in NPKGM, and 22.7% in NPKGMS relative to the NPK treatment. Straw return (NPKS and NPKGMS) promoted the formation of > 2 mm aggregates and increased the mean weight diameter by 57.2% and 73.1% compared to NPK and NPKGM, respectively. Straw return significantly elevated soil iron oxide content, with the complexed iron predominantly accumulating in macro-aggregates (> 2 mm) and amorphous iron oxides in micro-aggregates (< 0.25 mm), thereby enhancing the stability of aggregates. Crystalline iron oxides showed inconsistent responses to straw returns across aggregate fractions and were not significantly correlated with aggregate stability. Total iron-bound organic carbon (OCT) accounted for 21.2%–26.7% of total SOC and was dominated by the complexed iron-bound carbon. Straw return significantly increased OCT content, with NPKS and NPKGMS showing increases of 41.0% and 30.9% relative to NPK and NPKGM, respectively. Characterization of the different forms of iron-bound organic carbon using specific ultraviolet absorbance (SUVA) revealed that straw return enhanced the chemical stability of both complexed and amorphous iron-bound carbon, as indicated by higher SUVA254, SUVA260, and SUVA280 values, supporting greater accumulation and persistence of organic carbon. In summary, straw return enhances SOC primarily through two iron-mediated pathways: (i) physical protection via improved aggregate formation and stability; and (ii) chemical stabilization through stronger binding between iron oxides and organic carbon.