Lead Distribution in Plant Residues Amended Calcareous Soils as a Function of Incubation Time

We investigated the effect on lead (Pb) fractionation of the addition of plant residues and incubation time in three metal-amended calcareous soils of Iran. Four amended treatments with plant residues (helianthus, potato, rape, and wheat) were established at the rate of 2% and incubated for four weeks at 25°C and constant moisture. Lead was added to amended-soils at the rate of 200 mg kg^?1 as chloride. A control without amendments but with Pb (control) was also set up. The samples were incubated for 3, 72, 168, 336, 672, 1008, and 1344 h at 25°C and constant moisture. After incubation, the soils were sequentially fractionated into water-soluble plus exchangeable (EXCH), organically bound (OM), inorganically bound (CARB), and residual (RES) forms. There were changes in the proportional distribution of Pb in all three studied soils during 1344 h of incubation with spiked Pb. In general, the proportions of Pb associated with the most weakly bound fraction (EXCH) tended to decrease, with corresponding increases in the other three more strongly binding fractions during the incubation. Application of plant residues increased Pb significantly in EXCH fractions in studied soils. Among the plant residues, application of potato residue gave the highest increase in EXCH fractions in two soils, which may be related to its lower C/N ratio. The higher proportions of EXCH fraction of spiked and amended soils in these calcareous soils indicate the higher potential of downward leaching and runoff transport, especially at the early stage of pollution.