Effect of moisture regime on the redistribution of heavy metals in paddy soil

Sequential extraction procedure was applied to assess the dynamics of solid-phase transformation of added Cu, Pb, Cd, and Hg in atypical Chinese paddy soil incubated under three moisture regimes (75% field capacity, wetting-drying cycle, and flooding). The heavymetals spiked in the soil were time-dependently transferred from the easily extractable fraction (the exchangeable fraction) into lesslabile fractions (Fe-Mn oxide- and organic matter-bound fractions), and thus reduced lability of the metals. No significant changeswere found for the carbonate-bound and residual fractions of the heavy metals in the soil during the whole incubation. Change rateof the mobility factor (MF), a proportion of weakly bound fractions (exchangeable and carbonate-bound) in the total metal of soil,reflected the transformation rate of metal speciation from the labile fractions toward stable fractions. It was found that soil moistureregime did not change the direction and pathways of transformation of metal speciation, but it significantly a ected the transformationrate. In general, the paddy soil under flooding regime had higher metal reactivity compared with 75% field capacity and wetting-dryingcycle regimes, resulting in the more complete movement of metals toward stable fractions. This might be related to the increased pH,precipitation of the metals with sulfides and higher concentration of amorphous Fe oxides under submerged condition.