Increase in more stable iron-organic carbon associations by swine slurry application in paddy soils

Swine slurry (SS) application, recognized as a green and sustainable agricultural practice, has attracted greater attention. However, applying SS to farmland often leads to fluctuations in soil redox conditions and changes in soil properties. The biotic and abiotic transformations of iron (Fe) phases and associated organic carbon (OC) induced by SS remain unclear, especially in iron-rich paddy soils. Herein, SS was infused into paddy soil and conducted three cycles of redox fluctuation incubation. The findings demonstrated that SS slowed the transition from low-crystalline to highly crystalline Fe phases, particularly hindering the accumulation of haematite. SS reduced the formation potential of hydroxyl radicals (·OH) by influencing ligand-complexed Fe(Ⅱ) and dissolved phenols (DP). It was notable that SS enhanced the content of more stable complexes, which may be formed by the combination of highly crystalline Fe phases and aliphatic OC. Concurrently, the protection of aromatic OC by low-crystalline Fe phases increased. The increase of these two complexes is key to significantly improving the stable SOC pool. Moreover, SS promoted the enrichment of Clostridium_sensu_stricto_1 , causing the soil more inclined to use easily decomposable OC (such as carbohydrates) as electron donors for the reduction of low-crystalline Fe phases. In summary, SS application influences Fe and OC cycling in paddy soil through both biotic and abiotic pathways, while also presenting considerable potential for enriching the stable soil organic carbon (SOC) pool over the long term. • Swine slurry slows low-crystalline Fe phases aging and inhibits haematite accumulation. • The formation potential of ·OH in swine slurry application soil is reduced. • Swine slurry holds significant potential for enriching the stable SOC pool. • Clostridium_sensu_stricto_1 enrichment promotes reduction of low-crystalline Fe phases in swine slurry application soil.