10:00 AM - 10:15 AM
[MAG34-05] The effects of fine tree roots on downward migration of Cs-137 in a forest soil
Keywords:downward migration mechanisms of Cs-137, Translocation, Forest soil, Fine tree root
Abstract
Since the Fukushima Daiichi Nuclear Power Plant accident, various studies have been conducted on the dynamics of Cs-137 in forest soils, and it has become clear that the migration of Cs-137 from litter layer to mineral soil layers is faster than in the contaminated forests by the Chernobyl accident, and that there is a slight downward migration of Cs-137 from surface soil to deeper soil. However, the mechanism of downward migration is still poorly understood. Especially in deeper areas, the advection-diffusion model alone tends to underestimate, therefore other migration mechanisms such as colloidal transport and bioturbation has been pointed out, one of which is fine tree roots. Like leaves, fine tree roots grow, die, and abscise turnover in a relatively short period. Therefore, fine roots not only absorb nutrients from the soil, but also play an important role as a source of nutrients by exuding carbon compounds produced through photosynthesis, supplying carbon, nitrogen and other nutrients through the decomposition of fine roots themselves. If Cs-137 is translocated from above-ground or upper roots to lower roots as well as carbon, it may contribute to the downward migration of Cs-137. However, the studies of Cs-137 in fine tree roots, including this contribution to downward migration are limited. Therefore, the purpose of this study was to investigate the depth distribution of Cs-137 in tree roots, and to estimate the translocation and evaluate its contribution to downward migration through field culture experiments of fine tree root.
The study site was a cedar forest in Namie town, Fukushima Prefecture, where the initial deposition was about 4700 kBq/m2. Soil and root (<2 mm) samplings were conducted four times in 2022. In addition, a fine root culture experiment using commercially available soil with little Cs-137 (0.018 Bq/g) was conducted on five cedar trees at the same site during a four-month from April to August 2022. The soil was placed about 2.5 cm thick in a Tupperware container (20 cm x 30 cm x 10 cm deep), and three fine roots from which new roots (white roots) had been removed were placed in the container after being thoroughly washed, and then covered with an additional 2.5 cm of soil. As a control, five containers with local soil (186 Bq/g) were placed on the same trees. After an cultivation, only the new roots were collected and the Cs-137 concentrations were measured.
The depth distribution of Cs-137 concentration decreased exponentially with depth in both soil and fine roots. However, the decreasing trend was smaller for the fine roots, and the Cs-137 concentration in the fine roots was higher than those in the soil below 15 cm. Therefore, the concentration ratio of Cs-137 in fine roots/soil ranged from 0.15 in the 0-5 cm surface layer to about 26 (maximum 125) in the deeper layer below 15 cm. On the other hand, the inventory ratio of Cs-137 in fine roots/soil show no clear decreasing trend with depth (about 0.19% in whole), reflecting the lower biomass of the fine roots in the deeper layers.
As for the culture experiment, no significant difference was observed between the new roots collected at the time of installation, which were approximately 12.6 ± 8.4 Bq/g in the Cs-137-free experimental soil and 9.1±10.2 Bq/g in the local experimental medium. On the other hand, new roots recovered after incubation were approximately2.7±1.3 Bq/g in the experimental medium and 7.6±3.5 Bq/g in the local experimental medium, which were significantly lower in the experimental medium. Therefore, we assumed that about 30% of the Cs-137 concentration in the fine roots of trees was derived from translocation, indicating that the amount of Cs-137 supplied to the soil by the turnover of fine tree roots accounts for 0.6 to 1.3% of the Cs-137 in the soil at the time of the survey (11 years after the accident).
Since the Fukushima Daiichi Nuclear Power Plant accident, various studies have been conducted on the dynamics of Cs-137 in forest soils, and it has become clear that the migration of Cs-137 from litter layer to mineral soil layers is faster than in the contaminated forests by the Chernobyl accident, and that there is a slight downward migration of Cs-137 from surface soil to deeper soil. However, the mechanism of downward migration is still poorly understood. Especially in deeper areas, the advection-diffusion model alone tends to underestimate, therefore other migration mechanisms such as colloidal transport and bioturbation has been pointed out, one of which is fine tree roots. Like leaves, fine tree roots grow, die, and abscise turnover in a relatively short period. Therefore, fine roots not only absorb nutrients from the soil, but also play an important role as a source of nutrients by exuding carbon compounds produced through photosynthesis, supplying carbon, nitrogen and other nutrients through the decomposition of fine roots themselves. If Cs-137 is translocated from above-ground or upper roots to lower roots as well as carbon, it may contribute to the downward migration of Cs-137. However, the studies of Cs-137 in fine tree roots, including this contribution to downward migration are limited. Therefore, the purpose of this study was to investigate the depth distribution of Cs-137 in tree roots, and to estimate the translocation and evaluate its contribution to downward migration through field culture experiments of fine tree root.
The study site was a cedar forest in Namie town, Fukushima Prefecture, where the initial deposition was about 4700 kBq/m2. Soil and root (<2 mm) samplings were conducted four times in 2022. In addition, a fine root culture experiment using commercially available soil with little Cs-137 (0.018 Bq/g) was conducted on five cedar trees at the same site during a four-month from April to August 2022. The soil was placed about 2.5 cm thick in a Tupperware container (20 cm x 30 cm x 10 cm deep), and three fine roots from which new roots (white roots) had been removed were placed in the container after being thoroughly washed, and then covered with an additional 2.5 cm of soil. As a control, five containers with local soil (186 Bq/g) were placed on the same trees. After an cultivation, only the new roots were collected and the Cs-137 concentrations were measured.
The depth distribution of Cs-137 concentration decreased exponentially with depth in both soil and fine roots. However, the decreasing trend was smaller for the fine roots, and the Cs-137 concentration in the fine roots was higher than those in the soil below 15 cm. Therefore, the concentration ratio of Cs-137 in fine roots/soil ranged from 0.15 in the 0-5 cm surface layer to about 26 (maximum 125) in the deeper layer below 15 cm. On the other hand, the inventory ratio of Cs-137 in fine roots/soil show no clear decreasing trend with depth (about 0.19% in whole), reflecting the lower biomass of the fine roots in the deeper layers.
As for the culture experiment, no significant difference was observed between the new roots collected at the time of installation, which were approximately 12.6 ± 8.4 Bq/g in the Cs-137-free experimental soil and 9.1±10.2 Bq/g in the local experimental medium. On the other hand, new roots recovered after incubation were approximately2.7±1.3 Bq/g in the experimental medium and 7.6±3.5 Bq/g in the local experimental medium, which were significantly lower in the experimental medium. Therefore, we assumed that about 30% of the Cs-137 concentration in the fine roots of trees was derived from translocation, indicating that the amount of Cs-137 supplied to the soil by the turnover of fine tree roots accounts for 0.6 to 1.3% of the Cs-137 in the soil at the time of the survey (11 years after the accident).