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A process-based model for 137 Cs transfer in forest surface environments was developed to assess the dynamic behavior of Fukushima-derived 137 Cs in a Japanese forest. The model simulation successfully reproduced the observed data from 3 year migration of 137 Cs in the organic and mineral soil layers at a contaminated forest near Fukushima. The migration of 137 Cs from the organic layer to the mineral soil was explained by the direct deposition pattern on the forest floor and the turnover of litter materials in the organic layer under certain ecological conditions. Long-term predictions indicated that more than 90% of the deposited 137 Cs would remain within the top 5 cm of the soil for up to 30 years after the accident, suggesting that the forest acts as an effective long-term reservoir of 137 Cs with limited transfer via the groundwater pathway. The model was also used to explore the potential impacts of soil organic matter (SOM) interactions on the mobility and bioavailability of 137 Cs in the soil–plant system. The simulation results for hypothetical organic soils with modified parameters of 137 Cs turnover revealed that the SOM-induced reduction of 137 Cs adsorption elevates the fraction of dissolved 137 Cs in the soil solution, thereby increasing the soil-to-plant transfer of 137 Cs without substantially altering the fractional distribution of 137 Cs in the soil. Slower fixation of 137 Cs on the flayed edge site of clay minerals and enhanced mobilization of the clay-fixed 137 Cs in organic-rich soils also appeared to elevate the soil-to-plant transfer of 137 Cs by increasing the fraction of the soil-adsorbed (exchangeable) 137 Cs. A substantial proportion (approximate 30%–60%) of 137 Cs in these organic-rich soils was transferred to layers deeper than 5 cm decades later. These results suggested that SOM influences the behavior of 137 Cs in forests over a prolonged period through alterations of adsorption and fixation in the soil. [ABSTRACT FROM AUTHOR]

In this study we investigated the effect of fine (ϕ < 0.05 mm) fraction, i.e., silt + clay (FF) content in soils, site moisture, metal (Al and Fe) of soil organic matter (SOM) and forest species composition on the spatial distribution of carbon (C) pools in forest soils at the landscape scale. We established 275 plots in regular 200 × 200 m grid in a forested area of 14.4 km 2 . Fieldwork included soil sampling of the organic horizon, mineral topsoil and subsoil down to 40 cm deep. We analysed the vertical and horizontal distribution of soil organic carbon (SOC) stocks, as well as the quantity of physically separated fractions including the free light (fLF), occluded light (oLF) and mineral associated fractions (MAF) in the mineral topsoil (A, AE) horizons. Distribution of C in soils was predominantly affected by the variation in the FF content. In soils richer in the FF more SOC was accumulated in mineral horizons and less in the organic horizons. Accumulation of SOC in mineral soil was also positively affected by the degree of saturation of SOM with Al and Fe. The increasing share of beech influenced the distribution of C stock in soil profiles by reducing the depth of O horizon and increasing C stored in mineral soil. The content of FF was positively correlated with the content of C in MAF and fLF fractions. The content of oLF and MAF fractions was also positively influenced by a higher degree of metal saturation, particularly Al. Our results confirmed that Al plays an important role in the stabilization of SOM inside aggregates (C oLF ) and as in C MAF fractions. We also found a significant, positive effect of beech on the C fLF and fir on the C oLF content. [ABSTRACT FROM AUTHOR]