Group Report 4: Iron dynamics in terrestrial ecosystems in the Amur River basin

Our focus is to understand the spatial and temporal patterns and processes of biogeochemical iron transport from terrestrial ecosystem to river with special attention to the human impacts such as forest fire, land-use change and agricultural activities. Field monitoring of iron dynamics including stream water, soil water, and groundwater has been conducted from the upstream to downstream basin of Amur river in northeastern China and far-eastern Russia under tight collaboration with Chinese and Russian scientists and institutes. We found that the major source of dissolved iron from terrestrial system to river was mainly natural wetland which largely covered near the middle to lower region of Amur river basin. In upstream forested basin, dissolved iron in soil was mainly transported with dissolved organic carbon (DOC) rather than the forms of Fe(II) and Fe(III). The swamp forest and riparian zone near the stream channel was the important source area of iron due to the wet and anaerobic conditions which increase the DOC concentration and dissolved iron in soil and groundwater. The forest fire, one of the major human disturbances in the upstream mountain, changes the quantity and qualities of the organic matter in the surface soil, resulting to the decrease of the iron transport from the burned forest to the stream. The downstream areas with gentle topography are largely covered by natural wetland especially surrounding the middle and lower part of Amur river. The spatial distribution of iron concentration in stream water indicated that the stream water at the lower elevation and the gentle watershed contained much higher iron concentrations than the upper and steep basin. DOC was the important carrier of dissolved iron in soil water and stream water in these lower wetland as well as in the upstream region. The land-use change from wetland to farmland (paddy field and cropland) caused significant changes in soil chemistry, redox potential (Eh) and soil water quality. The drainage during the crop production system increase the Eh (indicating changes from anaerobic to aerobic condition in soil), resulting the decrease of dissolved iron in soil water due to the land-use change. The development of the irrigation system has significantly decreased the groundwater table over the last several decades, possibly contributing to the decrease in iron concentration in river water in the Sanjiang plain. The irrigation of groundwater with high dissolved iron resulted in the accumulation of amorphous iron oxides in the surface soil of the paddy filed, which was retained and not mobile from the soil. Our results indicated that the natural anaerobic environment in wetland is important for iron mobilization from terrestrial system to Amur river and that the human impact such as forest fire and land reclamation tended to make these irons immobile through mainly oxidation in the ground surface