Vertical changes in the geochemical distributions of iron, manganese and heavy metals in wetland soil cores from cold temperate zones in northeastern China

Abstract

Pedogenic processes influence the geochemical distributions of elements in soils. Such knowledge, however, is lacking on wetland soil in the cold temperate zone of northeastern China. Two wetland soil cores were collected, sliced at 5 cm intervals, and analyzed for pH, soil organic matter (SOM), Al, Fe, Ti, Mn, Co, Cr, Cu, Ni, and Pb. The results showed that the wetland soil cores were characterized by acidic pH levels, SOM accumulations in the A horizon, eluviation of Fe and Mn from the A and E horizons, and their illuviation in the B horizon. The heavy metals, Co and Pb, were leached from the A and E horizons and subsequently accumulated in the B horizon during pedogenic processes, while Cr, Cu, and Ni might not have migrated significantly. The average Co and Pb concentrations in the B horizon were 47.6 and 70.4 mg kg−1, respectively, which were several times higher than those in the A and E horizons. In the B horizon, five heavy metals, especially Co and Pb, preferentially partitioned in the Fe/Mn oxide phase (e.g., 45.4%, 27.5%, 9.4%, 8.3%, and 7.3% for Co, Pb, Ni, Cr, and Cu, respectively), following the residual phase. In the A and E horizons, however, these metals preferentially partitioned into different mineral phases (e.g., the Fe/Mn oxide phase for Co, organic matter phase for Cr and Cu, Fe/Mn oxide and organic matter phases for Ni, and carbonate and Fe/Mn oxide phases for Pb) following the residual phase. Therefore, pedogenic processes influenced the vertical distributions of both the heavy metal concentrations and their chemical forms in the wetland soil of cold temperate zones. This work provides insights into the behavior of major and trace metals in the soil depth profiles of seasonally waterlogged and freezing-thawing wetland soils.