Seasonal variation of heavy metal accumulation in soils was investigated from March 2002 to December 2003 in a constructed wetland in Kaminokuni, Hokkaido, Japan. Precipitation of iron (oxyhydro)oxides in the wetland is the principal reaction and also governs the immobilization of trace heavy metals. Immobilized forms of heavy metals were related with microbial metabolism and reactions of anaerobic microorganisms, which are active in wetland plant root zones in summer seasons. Mn(III,IV) -reducing and Fe(III)-reducing microorganisms enhanced the reductive dissolution of manganese and iron (oxyhydro)oxides, which resulted in the release of Mn(II) and Fe(II) and other bound heavy metals. Heterotrophic microorganisms such as sulfate-reducers decomposed organic matters to lower molecular weight acids and bicarbonates leading to increase in alkalinity, formation of zinc carbonates and complexation with Cu(II). Populations of sulfate-reducing bacteria in the wetland soil varied seasonally with 1~2 orders of magnitude and, depending on the depth, increased especially around root zones of wetland vegetations.
