Forest type effects on the retention of radiocesium in organic layers of forest ecosystems affected by the Fukushima nuclear accident

Abstract

The Fukushima Daiichi nuclear power plant disaster caused serious radiocesium (137Cs) contamination of forest ecosystems over a wide area. Forest-floor organic layers play a key role in controlling the overall bioavailability of 137Cs in forest ecosystems; however, there is still an insufficient understanding of how forest types influence the retention capability of 137Cs in organic layers in Japanese forest ecosystems. Here we conducted plot-scale investigations on the retention of 137Cs in organic layers at two contrasting forest sites in Fukushima. In a deciduous broad-leaved forest, approximately 80% of the deposited 137Cs migrated to mineral soil located below the organic layers within two years after the accident, with an ecological half-life of approximately one year. Conversely, in an evergreen coniferous forest, more than half of the deposited 137Cs remained in the organic layers, with an ecological half-life of 2.1 years. The observed retention behavior can be well explained by the tree phenology and accumulation of 137Cs associated with litter materials with different degrees of degradation in the organic layers. Spatial and temporal patterns of gamma-ray dose rates depended on the retention capability. Our results demonstrate that enhanced radiation risks last longer in evergreen coniferous forests than in deciduous broad-leaved forests.

Effectiveness of decontamination by litter removal in Japanese forest ecosystems affected by the Fukushima nuclear accident

“…Since these levels are markedly higher than the global 137 Cs fallout from atmospheric nuclear weapons testing (estimated at 2-4 kBq m −2 ) 58,63 , the 137 Cs observed in the present study was considered to originate primarily from the Fukushima NPP accident. This has been confirmed by 134 Cs/ 137 Cs activity ratios of litter and soil samples collected at these sites since the accident 25,31,56 .…”

“…Deciduous broadleaved forests (49.1% of the forested area) and evergreen coniferous forests (47.7%) were the dominant forest types in the area heavily contaminated with 137 Cs from the Fukushima NPP accident 6 . These forests greatly differ in their tree phenology and the initial contamination process (e.g., initial interception of 137 Cs by the tree canopy and subsequent input to the forest floor via litterfall and throughfall 55 ); it is therefore likely that the behavior of 137 Cs in forest ecosystems after atmospheric fallout differs depending on the forest type 25,36,52 . An investigation conducted in an evergreen coniferous forest affected by the Fukushima NPP accident showed a slower migration of 137 Cs from the litter layer to the mineral soil compared with deciduous broadleaved forest 25 .…”

“…These forests greatly differ in their tree phenology and the initial contamination process (e.g., initial interception of 137 Cs by the tree canopy and subsequent input to the forest floor via litterfall and throughfall 55 ); it is therefore likely that the behavior of 137 Cs in forest ecosystems after atmospheric fallout differs depending on the forest type 25,36,52 . An investigation conducted in an evergreen coniferous forest affected by the Fukushima NPP accident showed a slower migration of 137 Cs from the litter layer to the mineral soil compared with deciduous broadleaved forest 25 . Taking into account the different 137 Cs retention behavior in the litter layers, a limited but positive effect of litter-removal decontamination may be expected for evergreen coniferous forests in Japan.…”

“…Three replicate litter samples were collected manually from an area of 900 cm 2 (30 × 30 cm), each being randomly selected within each of the untreated and decontaminated areas. Three replicate topsoil (0-5 cm) samples were then collected using a cylindrical soil sampler (5 cm in diameter and 5 cm in depth) from the soil surface where the litter layer was collected 25 . The depth range (0-5 cm) of the soil sampling was chosen on the basis of studies showing that most of the total 137 Cs inventory is located within the litter and the upper 5 cm of soil for several years after deposition 31,53 .…”

“…Notwithstanding reports of successful reduction of the local external radiation level in European forest ecosystems by litter removal 4,20,21 , its effects on 137 Cs recycling in forest ecosystems remain poorly understood 3 . It is reasonable to assume that the 137 Cs behavior, and thus the decontamination efficiency, depends on the nature of forest ecosystems, which varies widely according to climate (temperature and precipitation), tree species composition, soil properties, and topography 7,15,[22][23][24][25] . Because these climatological and ecological factors differ greatly between European and Japanese forest ecosystems, the findings from the post-Chernobyl studies for European forest ecosystems may not be entirely applicable in the case of the Fukushima NPP accident.…”