Effect of wetland reclaimation on dissolved iron species

Baixing Yan⁽¹⁾，Xiaofeng Pan⁽¹⁾, Muneoki Yoh^(2)
(1) Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130012, China
⁽²⁾ Tokyo University of Agriculture and Technology, 3-5-8, Saiwai-cho, Fuchu, Tokyo, 183-0054, Japan

Abstract

With wetland reclamation in Sanjiang Plain since mid-1950s, both of wetland and agriculture ecosystem had experienced greatly changes which had been resulting in the changes either in macro-environment or micro-environment involving decreases of rainfall, soil moisture and soil organic matter, increases of soil temperature, Eh and pH. This research investigated the concentrations and chemical forms of dissolved iron in agricultural waters and wetland waters, including groundwater, paddy waters, canal waters, marshy waters, marshy streams and main stream with Cross-Flow Filtration(CFF) method. The purpose is to reveal the species of dissolved iron under wetland reclamation. The results show that the concentrations of dissolved iron in marshy waters and streams were higher than agricultural waters. The higher concentrations of total dissolved iron were observed in marshy rivers, Yalv R. (1.359 mg/L) and Bielahong R.(0.807 mg/L) resulting from higher DOC contents. Wetland waters and marshy rivers play an important role of the iron source for iron supplement to main rivers. Low molecular weight(LMW, <0.01μm) iron was the major fraction of dissolved iron both in marshy waters and streams, and 71% of LMW iron was in organic form in wetland. Moreover, wetland plants played an important role in controlling the rate and mobility of iron in surface water and soil by rhizosphere oxidation and uptake. There were noticeable differences in the concentrations and species of iron in the different vegetation covering area in marsh land. The concentration of dissolved iron in Calamagrostis angustifolia meadow were higher than that in Carex lasiocarpa marsh implying particulate iron would more readily release from soils and sediment in the environment. The concentrations of high molecular weight(0.05-0.7μm) and medium molecular weight iron (colloid iron, 0.01-0.05μm) increased in paddy waters and canal waters. The highest concentrations of ferrous and LMW were observed in groundwater because of reductive conditions. Ferrous account for 80.45% of total dissolved iron in groundwater. However, Ferrous in the paddy waters and canal waters were only 0.147 mg/L and 0.176 mg/L, respectively. The concentrations of HMW and MMW iron in paddy waters and canal waters were a little higher than that of in groundwater. Concentrations of HMW and MMW iron in river waters vary from 0.017 to 0.197 and 0.025 to 0.063 mg/L, respectively. The average concentration of ferrous in the marshy river is 0.090 mg/L, about two times higher than that of in the main rivers (Songhua R., Amur R. and Ussuri R.). The concentrations of total dissolved iron decreased 62.5% from groundwater to canal waters due to the formation of precipitate. Since 1964, the concentration of dissolved iron was decreased sharply with wetland reclamation. Hence, wetland reclamation in Sanjiang Plain reduces the output of dissolved iron, especially LMW iron and influences finally iron transportation and transformation to Amur River and adjacent Okhotsk Sea. Therefore, it is imperative to take care of wetland ecosystem in all the basin of the Amur River.

Keywords: wetland reclaimation; iron species; Sanjiang Plain; Cross-Flow Filtration(CFF) method