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Little is known about the polychlorinated biphenyl (PCB) pollution of tropical environments, although part of the contribution to global PCB contamination originates in the tropics. The objective of this study was to compare PCB concentrations and patterns in urban soils of Bangkok with data from the temperate zone to detect indications for possible differences in sources and fate of PCBs. The sum of congeners 1, 8, 20, 28, 52, 101, 118, 138, 153, 180, 206, and 209 (=Σ12PCBs) ranged from 0·19 to 11·4 µg/kg, which was lower than values reported for urban soils in the temperate zone. On average, the hexa-chlorinated PCBs 138 (mean of Σ12 PCBs± s.d., 22·7 ± 8·1%) and 153 (19·9 ± 9·0%) were most abundant. The Bangkok soils contained higher percentages of lower chlorinated PCBs (≤ 4 Cl) and correspondingly lower percentages of higher chlorinated PCBs (>4 Cl) than soils in 3 temperate cities. The differences may be explained by different sources, a lower age of contamination and therefore lower volatilisation, or anaerobic dechlorination during water-stagnant periods. The comparison of the PCB profiles in Bangkok soils with those of widely used commercial PCB mixtures indicated that the major PCB sources of Bangkok soils were the highly chlorinated Clophen A60, Aroclor 1260, or similar mixtures dominated by hexa-chlorinated PCBs.

Sulfur, besides phosphorus, is crucial for the nutrition of plants on tropical soils. Its availability is closely related to the turnover of soil organic matter. To get a better insight into transformation of soil S forms during the decomposition of organic matter, we studied inorganic and organic S pools in bulk samples and alkaline extracts of soils under different land uses representative of the tropical highlands of northern Thailand. Samples were taken from a cabbage cultivation, a Pinus reforestation, a secondary forest, and a primary forest. Total S ranged from 483 549 mg/kg in the subsoil to 1909 376 mg/kg in the organic layers, which is relatively high for tropical soils. The major S component in soil was organic S, comprising 75–99% of total S. Organic S was significantly correlated with total S, organic C, and total N, indicating that there is a close relationship between C, N, and S cycling in soil. C-bonded S was the predominant form in the topsoils (35–99% of total S) but its presence decreased with soil depth. The maximum concentrations of ester SO4-S were found in the A horizons (128 49 mg/kg), whereas the concentrations of inorganic SO4-S were small in all horizons. Compared with the forest site, the cabbage cultivation site was strongly depleted in S. C-bonded S was more depleted than ester SO4-S. A comparison of the S forms in NaOH extracts with S forms in bulk soil and C forms as indicated by 13C-NMR spectroscopy showed (i) that the extracts were very representative of soil organic S fractions and (ii) that ester SO4-S was mainly associated with O-substituted aliphatic C. In contrast, C-bonded S seemed to be connected to more-or-less all C binding types. transformation of soil organic matter, sulfate.