Abstract: The main forms of iron in soil are oxides, the transformations of which are affected by land use change. We collected soil samples at 0-40 cm layers of four land types from returning farmland to wetland with different restoration years (3, 6, 11, and 16 years) as well as a farmland and a natural wetland in the Yellow River Delta. We analyzed soil physical and chemical properties and the contents of iron oxides in different forms, and explored the effects of restoration years on the variation of the contents of iron oxides in different forms. The results showed that the contents of soil water, organic matter, total nitrogen, and total phosphorus at 0-40 cm layers and clay content at 0-10 cm layer increased with increasing restoration years, and that there were no significant changes in soil pH and bulk density. The contents of total iron (Fe_T) ranged from 23.68 to 26.60 g·kg^-1, showing no significant change in the lands returned from farmland to wetland in the initial six years and a decrease after six years. The contents of free iron oxide (Fe_d) ranged from 4.97 to 5.81 g·kg^-1, which were similar to the change of Fe_T contents. The contents of complex iron oxide (Fe_p) and amorphous iron oxide (Fe_o) ranged from 0.03 to 0.21 g·kg^-1 and from 0.54 to 0.76 g·kg^-1, respectively. Both Fe_p and Fe_o showed a trend of decreasing first and then increa-sing with increasing restoration years. The contents of Fe_T and various iron oxides were correlated with soil physicochemical properties in varying degrees. Our results indicate that returning farmland to wetland could significantly affect soil physicochemical properties and the contents and distribution of iron oxides in the Yellow River Delta.

Key words: returning farmland to wetland, wetland soil, iron oxide, soil physicochemical property, Yellow River Delta.