Temporal changes of the total iron

free iron

chelated iron

amorphous iron

free degree

complex(chelating) degree and activation degree of soil iron oxides in the topsoil of the paddies after reclamation in the Sanjiang Plain were studied.The results showed that total iron in the topsoil of the paddies increased exponentially with time since reclamation. Free iron showed a fluctuating upward trend with the wave troughs appeared in the 3rd year and the 11th year

and the wave peaks in the 5th year and the 14th year. Chelated iron increased during the first eight years after reclamation

reached 1562.41 mg/ kg at the 8th year

and then decreased. Amorphous iron decreased exponentially with time. Free degree of soil iron oxides first increased then decreased

and reached a maximum value of 0.46 at the 8th year.Complex degree increased continuously to a maximum value of 0. 18 during the first 8 years

with a slower increasing rate from the 5th to 8th year;and decreased gradually after the eighth year. Activation degree decreased exponentially with time.Different variation trends of free degree

complex degree and activation degree of soil iron oxides between paddies and soybean fields were caused by water flooding in paddies and differences in farming methods between the two systems. Compared with the natural wetland

rice cultivation increased contents of free iron

chelated iron and amorphous iron as well as free degree of soil iron oxides

but decreased contents of complex degree and activation degree of soil iron oxides. However

complex degree and activation degree of soil iron oxides in paddies were greater than those in soybean cultivation stage.