Extractable zinc and manganese as related to applied cadmium in contrasting Indian soils

Soil chemical interaction of applied cadmium (Cd) and native zinc (Zn) and manganese (Mn) was studied in four contrasting Indian soil types, namely, an ultisol, an alfisol, an entisol and a vertisol with and without sewage sludge amendment. The soils after incubation periods up to 16 weeks were sequentially extracted with 0.5 M KNO3, deionised water, 0.5 M NaOH, 0.05 M Na(2)EDTA and 4 M HNO3; these extracted fractions represent the exchangeable, adsorbed, organic-bound, carbonate and sulfide/residual forms of Zn and Mn, respectively. Results indicated that in both Cd-treated and Cd-enriched sludge treated soils very small amounts of plant-available Zn could be extracted and the sulfide/residual fraction was the predominant Zn fraction extracted throughout the incubation period of 16 weeks. High amounts of organic-bound Zn present in sewage sludge were transformed into sulfide/residual or bound forms during the incubation period. Data on the extraction of Mn from different Cd- and Cd-enriched sludge treated soils revealed that high amounts of exchangeable Mn were extracted in ultisol and alfisol throughout the period of 16 weeks of incubation, indicating its presence in plant-available forms for a longer period of time in these two soil types. In the case of entisol and vertisol, the extraction pattern was very much different and only very small amounts of plant-available Mn could be extracted, possibly due to the higher pH, cation exchange capacity and silt/clay loam texture of these soils.