Objectives Iron oxides are active and wildly exist in paddy soil, and their forms are easy to conversed with environmental conditions such as water and organic matter content. Therefore, we simulated different water conditions and organic carbon contents in soil, and observed the conversion within iron oxide forms for better understanding of the chemical procession.

Methods A indoor incubation experiment was carried out using rice soil collected in Shenyang City, Liaoning Province. Three simulated water conditions included alternating dry and wet (AWD), flooding (F), and dry (FC); glucose used as active organic carbon source, was added (+C) or not added (−C) under each water condition to compose 6 treatments in total. All the treatment soils were incubated in the dark at 25℃ for 80 days. Soil samples were collected for determining iron reduction rate, the content of iron oxide in free, amorphous and complexed forms, and soil pH, oxidation-reduction potential (Eh).

Results At the end of incubation, F treatment was recorded the highest increase in amorphous iron content, and the increment were 1.16 g/kg under −C and 1.33 g/kg under +C addition; AWD and FC treatment increased free iron content by 20.57% and 15.90% under −C, respectively; AWD, F and FC treatment increased complexed iron by 68.66%, 16.09% and 17.89% under +C, respectively, compared to the initial soil. Amorphous and complexed iron contents were negatively correlated with soil Eh (P<0.01), and free iron content was positively correlated with Eh (P<0.05).

Conclusions The addition of active organic carbon is conducive to promote the complexation of iron oxides, and the complexation could be increased by 75% under AWD condition. Flooding condition reduces soil Eh, that promoted the transformation of free iron oxide to complexed and amorphous form, and thereby increases the activity of iron. Regardless of adding active organic carbon or not, the activation of iron under flooding condition is higher, reaching 0.46 and 0.47, respectively. Repeated drying and wetting stimulates transition of noncrystalline iron oxide to crystalline state, the process of drying after flooding significantly increases the iron reduction rate, and adding active organic carbon boosts the reaction, contributing to improve the availability of soil iron.