Iron species and output flux in the agricultural irrigation-drainage system in the Sanjiang Plain
三江平原水田灌溉-排水过程中铁形态变化及输出贡献

After the conversion of wetland to paddy land, agricultural drainage became an important iron source to the Amur River, which influences the iron content, species and transportation in the Amur valley. In this study, agricultural irrigation groundwater, paddy water, canal water and lateral seepage water were sampled to study iron transportation during the process of irrigation, drainage and lateral seepage in the Sanjiang Plain. The cross-flow filtration technique was employed to separate iron species according to their size fraction. The results showed that iron is primarily transported as low molecular weight and acid-labile iron forms in the process of irrigation and drainage. Ferrous ion accounted for 80.45% of total dissolved iron in groundwater. When groundwater was pumped to surface land, the conversion from Fe2+ to complexed iron occurred immediately. The complexed iron accounted for 75.50% of the iron in paddy water. After paddy water flowed into the canal, the complexed iron content decreased 13.58% but other iron species had no significant change. The Fe2+ concentration increased with the depth of soil in the lateral seepage water, and complexed and colloidal iron did conversely. However, after the lateral seepage water flowed from paddy field into canal, the concentration of complexed and colloidal iron increased. The output flux of total dissolved iron was 390 kg·a-1·km-2 and the output coefficient was 0.186 from agricultural drainage in the Sanjiang Plain. The amount of residual iron reached about 1460 kg·a-1·km-2 in the paddy fields during irrigation and 250 kg·a-1·km-2 in canal soil during drainage.