Iron-driven organic carbon sequestration in arid saline-alkali soils: Mechanisms of Fe-C coupling via reactive iron oxides and microaggregate stabilization

• PFS exhibits a synergistic effect on carbon sequestration and soil health. • Fe-C co-precipitation is identified as the core stabilization mechanism. • Microaggregates are the hotspot for iron-carbon interaction. • PFS plays a dual role in carbon storage and salinity reduction. Reactive iron oxides (particularly poorly crystalline phases) act as “rust sinks” by facilitating soil organic carbon (SOC) accumulation through mechanisms such as ligand exchange, co-precipitation, and aggregate formation, owing to their high specific surface area. However, how exogenous iron input regulates Fe-C coupling and SOC sequestration in arid saline-alkali ecosystems remains poorly understood. A field experiment included five treatments: CK (chemical fertilizer alone), M (CK + organic manure), MG (M + desulfurization gypsum), MF (M + ferrous sulfate), and MS (M + polymeric ferric sulfate). Partial least squares path modeling (PLS-PM) indicated that SOC sequestration was directly and positively associated with particulate organic carbon (POC) and iron-bound organic carbon (Fe-OC), while dissolved organic carbon (DOC) had a direct negative effect. The MS treatment significantly increased reactive iron oxides, promoting POC accumulation and co-precipitation with DOC to form more stable Fe-OC, thereby reducing the specific carbon mineralization rate, especially within microaggregates. Additionally, the MS treatment effectively reduced soil pH and exchangeable sodium percentage, contributing to a more stable carbon pool. This study proposes an iron-carbon coupling strategy for improving SOC sequestration in arid saline-alkali soils, offering both theoretical and practical insights for regional carbon management.