Impact of land-use changes in iron transport

Muneoki Yoh (1), Baixin Yan (2), Takeo Ohnishi (3) and Hideaki Shibata (4)
(1) Tokyo University of Agriculture and Technology, 3-5-8, Saiwai-cho, Fuchu, Tokyo, 183-0054, Japan
(2) Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130012, China
(3) Research Institute for Humanity and Nature, 457-4 Motoyama, Kamigamo, Kita-ku, Kyoto, 603-8047, Japan
(4) Hokkaido Univ., 250 Tokuda, Nayoro, Hokkaido, 096-0071, Japan


Iron is fundamentally insoluble during ordinary weathering process in land, but could be transported by river after solubilization under certain terrestrial situations such as wetlands, where Fe is reduced to produce soluble Fe2+ and more stable organic Fe complex form.  Lowland area is widely distributed along the Amur Basin.  Sanjiang plain in China side is a typical lowland with an elevation as low as 40 to 60m, surrounded by Songhua River, Usuri River and Amur River.  Most of this vast area is nowadays converted to upland fields and paddy fields.  We have investigated the impact of such land-use changes in iron transport.  Upland fields, formed after reclamation by drainage, proved to have an oxidized condition with high Eh, where little iron solubilization takes place.  Paddy fields had lower dissolved iron concentrations in both surface and soil interstitial waters throughout a year and a shorter period of reductive condition due to agricultural water managements.  Controlled water drainage and negligible water penetration into soil may result in reduced iron transport from paddy fields, though utilization of ground water with high dissolved iron concentrations for irrigation could have a reverse effect.  Presence of peat layer observed in most terrains suggests a predominance of wetlands in Sanjiang plain.  Current ground water tables were found to be greatly lowered in most regions, which is possibly due to a reclamation by water drainage.  Such alterations in land condition from wetlands likely result in a decrease in river transport of dissolved iron as observed in a major branch river for these decades.

Keywords: land use change, dissolved iron, wetlands, Sanjiang plain, ground water table