Following nuclear releases to the environment, 137 Cs (half-life 30 years) is a long-term contaminant of many ecosystems, including forests. We recently sampled soils under pine forests in temperate and tropical climates to test the hypothesis that migration of 137 Cs, 50 years after nuclear weapons fallout, is coupled with organic matter (OM) accumulation in these soils. Depth profiles of 137 Cs, naturally-occurring 210 Pb and weapons-derived 241 Am were measured. After 50 years, migration of 137 Cs into the temperate and tropical soils is limited to half-depths of 7-8 cm and 2-3 cm, respectively. At both locations, most 137 Cs is associated with OM that accumulated from the early to mid-1960s. Illite, which immobilises radiocaesium, was undetectable by X-ray diffraction in the layer of peak 137 Cs accumulation in the temperate forest soil, but apparent in the zone of peak concentration in the tropical soil. The data indicate that long-term (50 year) fate of 137 Cs in organicrich, temperate forest soil is coupled with OM accumulation; fixation of 137 Cs by illite is more important in the tropical forest soil where OM is rapidly decomposed. Models of long-term radiocaesium migration in forest soils should explicitly account for the role of OM, especially when considering forests under contrasting climatic regimes.
Surface soil and above-ground parts of Pinus kesiya (Three-needled pine) were collected at Wat Chan pine forest to study cycling of 137Cs. Cs-137 derived from nuclear weapons tests up to 1963 was found to have accumulated at a shallow depth (<5 cm) in the forest soil. Calculated total deposition of 137Cs was in the range of previous studies and could be used as a reference for nuclear test deposition of 137Cs fallout in northern Thailand and nearby locations. Potassium in soil does not appear to be a good indicator for long-term radiocaesium cycling. Finally, ‘available’ stable caesium (133Cs) in the soil was found to be an appropriate indicator for studying uptake of 137Cs by Pinus kesiya at Wat Chan.
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