Redox fluctuations induced the formation of hydroxyl free radicals and transformation of Fe solid phases in two paddy soils

Abstract Redox fluctuation in paddy soils could lead to the formation of reactive oxygen species (ROS) as well as the Fe phases transformation, which would have great impacts on soil organic carbon (SOC) retention. However, detailed knowledge on this important geochemical process is still limited. In this study, a 11-week anoxic incubation experiment was conducted to reveal the mechanisms of ROS production by wet chemical methods and the changes in Fe phases by 57 Fe Mössbauer spectra (MBS) in two paddy soils (DS and XT). The results showed that significant amounts of • OH was produced with the anoxically incubated soil slurries exposed to air oxidation. Moreover, the formation of • OH mainly resulted from the oxygenation of surface-bound Fe(II) instead of aqueous Fe(II), and the intermediates O 2 •‑ and H 2 O 2 was confirmed to involve in the production of • OH. Due to contain more amorphous and crystalline Fe oxides, the production of Fe(II) and • OH in DS was larger than those in XT. An alternate increase and decrease trend was observed for the • OH production during the redox fluctuations. The MBS results showed a noticeable decrease in the content of crystalline goethite after 11-week anoxic incubation, but the content of amorphous ferrihydrite in both DS and XT kept unchanged or was increased, which resulted from the re-oxidation of Fe(II). By intensifying the oxic condition, Fe(II)-catalyzed transformation of amorphous ferrihydrite to crystalline goethite was only observed in XT, an indicative of the difference in soil mineral matrix and organic matter between the two soils. During the 11-week anoxic incubation, the concentration of dissolved organic carbon (DOC) gradually increased to its maximum at the end of 2 w, followed by a gradual decrease as anoxic incubation prolonged. This finding suggested the released DOC could be re-adsorbed onto the solid phases, possibly the newly formed amorphous ferrihydrite. The results above enhanced our understanding on the role of Fe phases in regulating the mineralization and conservation of SOC.