Spatiotemporal dynamics in soil iron affected by wetland conversion on the Sanjiang Plain

Since the 1950s, nearly 80% of wetlands in the Sanjiang Plain have been converted into paddy fields. The conversion might affect the solubility and mobility of soil iron, influencing the export of iron into the Amur River and the primary production of the Okhotsk Sea. However, information regarding long-term studies of the spatiotemporal dynamics of soil iron after cultivation is limited. In this study, six regions, including 18 plots in the Sanjiang Plain, were selected as sampling sites covering natural wetlands and paddy fields with planting ages of 2, 5, 11, 18, and 25 years after conversion from the wetland. Samples were collected at six different depths (0-10, 10-20, 20-40, 40-60, 60-90, and 90-120 cm) analyzed for water-soluble ferrous iron (Fe[II]), water-soluble iron (Fe-w), complex iron (Fe-p), amorphous iron oxides (Fe-o), free iron oxides (Fe-d), and total iron (Fe-t) and six soil physicochemical characteristics. Two years after the conversion of wetlands to rice fields led to an immediate decrease in Fe(II), Fe-w, Fe-p/Fe-d, and Fe-o/Fe-d, while the Fe-p and Fe-o contents decreased at 5 years. Both the concentrations and stocks of soil Fe-t increased gradually during the first 18 years. The findings in the Sanjiang Plain suggest that the function of wetlands after conversion as a source of iron might decrease with increasing time, with potential ecological effects on the neighboring marine environment. Recently initiated wetland restoration would protect the land ecosystems in the Sanjiang Plain and promote the future sustainability of the Amur Basin.