Abstract
Metals in soil are distributed among several geochemical forms, which influence its solubility, mobility and toxicity in soil and ground-water systems. In this study, the distribution of Fe and Mn in different soil fractions, their behavior and potential effects of soil contamination were investigated with respect to soil properties of Sundarban Mangrove Forest (SMF) wetlands. Soil samples (surface and subsurface) were collected from ten locations of SMF and were analyzed by the sequential extraction procedure to solubilize the Fe and Mn in exchangeable (Exch.), organically bound (OrgB), Mn-oxide bound (MnOB), amorphous Fe-oxide bound (AFeOB), crystalline Fe-oxide bound (CrFeOB) and residual (Res.) fractions. Results obtained from this study showed that pH of the soils was slightly to moderately alkaline in nature except Jamtola soil and there was a slight change of pH (6.12–8.09) in SMF soils. The textural class, organic matter, clay, CEC and Eh of Sundarban soils ranged from silt loam to clay loam, 2.15 ± 0.04% to 13.90 ± 0.11%, 6.63–32.81%, 18.68 ± 0.07 mV to 6.51 ± 0.21 mV and + 2.7 ± 2.11 mV to -98.6 ± 1.49mV, respectively. Moreover, the concentrations of total Fe and Mn varied significantly and ranged from 20,137.5 ± 232.5 µg g− 1 to 33,960 ± 1656.8 µg g− 1, 335.55 ± 2.5 µg g− 1 to 665.05 ± 35 µg g− 1, respectively. The highest contents of iron and manganese are found in Kotka area. In all the studied soils, Fe was associated with Res. fraction (55–73%) followed by CrFeOB, AFeOB and other fractions. The concentrations of Mn in the studied soils are primarily associated with Res. (29–58%) and MnOB (5–29%) fractions followed by CrFeOB > AFeOB > OrgB > Exch. sequence. Extremely high association of these two elements in residual fraction reveals no significant contamination. The findings may allow the evaluation of soil management techniques to limit mobility and plant availability of microelements and to minimize their transfer into the food chain.