Forms and profile distribution of soil Fe in the Sanjiang Plain of Northeast China as affected by land uses

Purpose Since the mid-1950s, the wetlands in Sanjiang Plain of Northeast China have experienced greater changes in land use under which the mobility of soil Fe could be changed giving definite effects on the biomass production of adjacent regions. The aim of this work was to investigate the effects of land use change on the characteristics of soil Fe vertical distribution with a focus on evaluating the effects of cultivation on the soil Fe mobility in Sanjiang Plain. Materials and methods Twelve sites between two upper reaches of Amur River, i.e., Naoli River and Nongjiang River in the Sanjiang Plain, were selected as sampling sites, covering natural wetland, lowland rice field, and upland soybean field. Samples of different land use type soil were collected at the depths of 0-10, 10-20, 20-40, 40-60, 60-90, and 90-120 cm with their organic C, pH, total Fe (Fet), free Fe oxides (Fed), amorphous Fe oxides (Feo), Fe(II), and water-soluble Fe (Few) determined. Results and discussion After the conversion of wetland into lowland rice field and upland soybean field, the organic carbon content in 0-10-cm soil layer decreased by 25.7% (P < 0.05) and 48.0% (P < 0.05), respectively, and the pH value at the soil depths below 40 cm in lowland rice field and upland soybean field was higher than that in natural wetland. Fe oxides concentration profiles suggested that a significant amount of Fe in wetland soil was moved downward to deeper layers while part of the Fe in farmland soils was deposited in subsurface layer where a good aeration occurred preventing its further leaching loss. Cultivation promoted the production of soil Fed and retarded the formation of Feo, Fe(II), and Few with the sequence of soil Feo/Fed ratio, Fe(II), and Few being wetland > lowland rice field > upland soybean field. Conclusions The results supported the ideas that, in the Sanjiang Plain, the conversion of wetland into farmland, especially into upland, could change the soil Fe vertical distribution giving potential effects on the mobility of soil Fe. A quantitative study on the dissolved iron discharge from different land use type would be made to quantify the flux of dissolved iron from the Sanjiang Plain to the Amur River and the Sea of Okhotsk based on the high-resolution geographical distribution maps of land surfaces in the alluvial plain.