Nuclear weapons fallout 137Cs in temperate and tropical pine forest soils, 50 years post-deposition

Following nuclear releases to the environment, Cs-137 (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 Cs-137, 50 years after nuclear weapons fallout, is coupled with organic matter (OM) accumulation in these soils. Depth profiles of Cs-137, naturally-occurring Pb-210 and weapons-derived Am-241 were measured. After 50 years, migration of Cs-137 into the temperate and tropical soils is limited to half-depths of 7-8 cm and 2-3 cm, respectively. At both locations, most Cs-137 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 Cs-137 accumulation in the temperate forest soil, but apparent in the zone of peak concentration in the tropical soil. Data indicate that long-term(50 year) fate of Cs-137 in organic-rich, temperate forest soil is coupled with OM accumulation; fixation of Cs-137 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. (C) 2019 Elsevier B.V. All rights reserved.