The effect of reclamation on the distribution of heavy metals in saline–sodic soil of Songnen Plain, China

The spatial distribution of total concentrations and the chemical speciation of heavy metals (Cu, Zn, Pb and Hg) in saline–sodic soil at different periods of time since reclamation (rain-fed arable land converted into paddy field) were investigated. The fluxes of different input sources of heavy metals were also analyzed. The results showed that long-term reclamation caused a decrease in soil pH and significant Hg accumulation in saline–sodic surface soil. Increasing Hg concentrations were identified at sites that had been reclaimed for longer periods, especially because the Songhua River was polluted by Hg between the 1960s and 1982; therefore, irrigation was assumed to be the predominant contributor of Hg accumulation in soil. Sequential leaching experiment revealed that a greater proportion of Hg occurred in the residual fraction, while the Cu, Zn and Pb were easily bound to oxidizable fraction in saline–sodic soil. Assessed by the spatial distribution of heavy metal concentrations in reclaimed soil, reclamation of saline–sodic soil in the Songnen Plain appeared to decrease the mobility of heavy metals, and did not pose the risk of accumulation in soil.