Humification of exogenous soil organic matter (ESOM) remodels the organic compositions and microbial communities of soil, thus exerting potential impacts on the biogeochemical transformation of iron (hydr)oxides and associated trace metals. Here, we conducted a 70-day incubation experiment of straw and arsenic (As)-bearing ferrihydrite in paddy soil to investigate how aerobic humification of ESOM influenced the repartitioning of As. Results showed that straw humification was characterized by rapid degradation of OM in the initial 14 d (i.e., degradation stage) and subsequent slow maturation of OM till the end (i.e., maturation stage). During the degradation stage, a considerable As of 13.1 mg·L^-1 was released into the aqueous phase, which was re-immobilized to the solid phase in the maturation stage. Meanwhile, the solid-phase As and Fe were converted from low-crystalline structural species to more stable ones, with a subtle crystalline phase transformation, throughout the whole humification period. The generation of highly unsaturated and phenolic compounds, along with the enriched Enterobacter and Sphingomonas, induced the reduction of ferrihydrite (~3.1% of total Fe) and As(V), leading to the As release during the degradation stage. In the maturation stage, the formation of carboxylic-rich alicyclic molecules facilitated the re-immobilization of aqueous As. Throughout the straw humification process, the formation of organo-mineral complexes between OM and ferrihydrite via C-O-Fe bond contributed to the stabilization of solid-phase As and Fe. Collectively, this work shed light on the crucial role of ESOM humification in driving iron (hydr)oxides transformation and associated As redistribution, advancing our understanding of the coupled biogeochemical behaviors of C, Fe, and As in soil.

Arsenic, Exogenous soil organic matter, Ferrihydrite, Humification, Organo-mineral complex