Sequential chemical extraction followed themethod described by Li
and Thornton (2001), developed from that of Tessier et al. (1979). The
extraction was carried out progressively on an initial soil weight of
1.0 g. The sequence of fractions is nominally designated as ‘Exchangeable’
(F1), ‘Carbonate’ (F2), ‘Fe/Mn oxides’ (F3), ‘Organic matter and
Sulphides’ (F4) and ‘Residual’ (F5). After each successive extraction,
the supernatant solutionwas separated by centrifugation and filtration The final extraction step (F5)was calculated by difference fromthe total
metal content. The concentrations of metals in the filtered fractions
were measured by ICP-MS.