Effects of land use and changes in cover on the transformation and transportation of iron: A case study of the Sanjiang Plain, Northeast China

Wetland reclamation has been ongoing in the Sanjiang Plain since the mid-1950s, which has resulted in major changes in wetlands and the agriculture ecosystem in the region that have influenced the iron output to the Sea of Okhotsk and limited the primary productivity in the North Pacific Ocean. This study was conducted to investigate the chemical forms of iron in different aquatic environments (agricultural water including groundwater, paddy water, and canal water; wetland water including marsh water, marsh streams, and main streams) with the cross-flow filtration method to reveal the transportation and transformation characteristics of iron in response to major land use changes. In addition, the factors affecting iron behavior in different water bodies were reviewed. In marsh water and streams, the concentrations of dissolved iron were higher due to the high organic matter contents and marsh water becoming the main iron source for river water. The conversion of dissolved iron into acid-labile iron occurred during the discharge of wetland water into marsh rivers. Iron primarily existed in both the > 0.7 and < 0.01 mu m size fractions, accounting for about 58.3% and 26.4% of the total dissolved iron, respectively. In agricultural irrigation systems, ferrous ion entered the paddy fields from groundwater, and a fraction of this ferrous iron was subsequently converted into high molecular weight and medium molecular weight iron (colloid iron) in paddy and canal water. However, the concentrations of total dissolved iron decreased by 62.5% from underground to the surface due to the formation of precipitates. Despite this, water discharge in agriculture is still an important iron source for rivers and has the potential to supplement iron due to its higher acid-labile iron concentrations. Land use and cover change and agricultural irrigation increased the iron content of surface soil, but reduced the output of iron in water systems. Overall, the concentration of total dissolved iron in water systems has been reduced to 42.6% by wetland reclamation.