Host: The Association of Japanese Geographers
By investigating in 2005-2008 and contrasting historical data, the iron biogeochemical cycle and outflux were studied in Sanjiang Plain, where land-use has been changing greatly since last fifty years although was formerly the largest concentrative distribution area of freshwater marshes in China. Land-use change has been inducing visible effect on hydrological cycles over there. Comparing with 1950, annual mean temperature has been warming 1.4-2.3, annual mean precipitation decreased 90mm, which lead to the fall of peak discharge and mean discharge of main rivers. Ground water level lowered from 3-4m in 1950's to 8-10m after 2000 due to large-scale lift irrigation from aquifer. And, marsh and groundwater are the important iron sources of Amur River. Thus, Sanjiang Plain is the representative area to understand the influence of land-use change on dynamics of transportation and transformation mechanism of iron biogeochemical cycle. The result show that basic cation and anion in water of main rivers have been changing since 1960's. Bicarbonate concentration increased from 67mg/L in 1960's to near 100mg/L in the year of 2005, chlorine and nitrate also increased at a certain extent. But sulfate, sodium and potass reduced obviously among the period both in Songhua River and Naoli R. Ferric and complexed iron were the main forms of dissolved iron, 73%-82% in river water in 2007. Dissolved iron concentrations in marshy rivers were obviously higher than in Amur River, Songhua River and Wusuli River, and the concentration in flood period was clearly higher than in other seasons. Ferric and ferrous concentrations in Songhua River(Jiamusi) decreased from 0.57mg/L, 0.43mg/L in 1960's to 0.28 mg/L, 0.17 mg/L in the year of 2006. The same variety can be discovered in Naoli River (Fig.1). Ferric is the domination form both in river and marsh, and ferrous is the main fraction of dissolved iron in groundwater(the ratio of ferrous to ferric in groundwater is 4:1.). In the standing waters of paddy, marsh and groundwater, the concentration ASPI and low molecular weight iron were higher. The distribution of iron species was in the trend of ASPI>complexed iron>colloid> ferrous in marsh, ASPI>ferrous>complexed iron>colloid iron in standing water of paddy, ASPI>ferrous>colloid>complexed iron in groundwater, complexed iron>ASPI>colloid>ferrous in river. DOC can not reflect the real concentration of organic matters binding with iron, and iron does not correlate with DOC significantly. Eh is more important than pH with dissolved iron concentration. Outflow from wetlands to rivers reduce and the role of marsh as iron source weaken affirmatively. Thus, Marsh reclamation decreased iron import to rivers. Dissolved iron concentration is very high in groundwater. Ferrous concentration in standing water of paddy is also appreciably higher and is the contributor of agricultural drainage and rivers. So, Paddy dilation increase iron export. The outflux of dissolved iron after reclamation increased at spot scale, but decreased at the regional scale. Marsh reclamation decreased the dissolved iron export to rivers. The current estimated outflux of iron forms is listed in Table 1. Therefore, land-use change in Sanjiang Plain has been influencing on water chemistry of main rivers, and chemical flux of Amur River to Okhotsk Sea.
