Objectives Iron oxides in soil facilitate organic carbon sequestration through adsorption or coprecipitation and serve as critical cementing agents in aggregate formation. This study investigated the effects of carbon input on the content and stability of iron oxide-associated organic carbon within soil aggregates.

Methods Based on the long-term positioning experiment of double-cropping rice in Qiyang Red Soil Experimental Station in Hunan Province, undisturbed soil samples were collected in treatment plots of the CK (no straw return), RS1 (only returning early rice straw to field), and RS2 (returning both early and late rice straw to field) continuously for 11 years. The dry-wet sieve method was used to separate soil aggregates into macro- aggregate (>2 mm), small- aggregate (0.25～2 mm), micro- aggregate (0.053～0.25 mm), and clay-silt particle (<0.053 mm). Organic carbon content and iron-bound organic carbon (OC_Fe) were analyzed across aggregates. OC_Fe was further partitioned into three fractions: complexed iron-bound organic carbon (OC_PP), amorphous iron oxide-bound organic carbon (OC_HH), and crystalline iron oxide-bound organic carbon (OC_DH). Their proportions in OC_Fe and aromaticity were quantified.

Results The proportions of OC_PP, OC_HH and OC_DH in each particle size aggregate were 16.0%−22.4%, 1.6%−3.0%, and 0.5%−1.8%, respectively, indicating that the iron-bound organic carbon in the aggregate is mainly OC_PP. Compared to CK, RS1 and RS2 increased OC_Fe content in macro-aggregates by 21.3% and 36.2%, respectively. RS1 significantly enhanced OC_Fe in micro-aggregates but reduced it in clay-silt particles. Both RS1 and RS2 elevated OC_PP in macro-aggregates and OC_DH in clay-silt particles, with linear correlations between these two forms of iron-bound organic carbon fractions and aggregate SOC content, suggesting straw return altered iron-bound organic carbon distribution. Additionally, straw returning increased the aromaticity of OC_HH in micro-aggregates and OC_DH in clay-silt particles.

Conclusions Straw returning promoted the accumulation of OC_PP in macro-aggregates and OC_DH in micro-aggregates and clay-silt particles, while enhancing the aromaticity of OC_DH in clay-silt fractions. These shifts in iron-bound organic carbon fractions and their stabilization within aggregates likely contribute to increased soil organic carbon sequestration under straw returning, highlighting a key mechanism for SOC preservation in paddy soils.