The accident at the Fukushima Daiichi nuclear power plant caused serious radiocesium (137Cs) contamination of forest ecosystems located in mountainous and hilly regions with steep terrain. To understand topographic effects on the redistribution and accumulation of 137Cs on forest floor, we investigated the distribution of Fukushima-derived 137Cs in forest-floor litter layers on a steep hillslope in a Japanese deciduous forest in August 2013 (29 months after the accident). Both leaf-litter materials and litter-associated 137Cs were accumulated in large amounts at the bottom of the hillslope. At the bottom, a significant fraction (65%) of the 137Cs inventory was observed to be associated with newly shed and less degraded leaf-litter materials, with estimated mean ages of 0.5–1.5 years, added via litterfall after the accident. Newly emerged leaves were contaminated with Fukushima-derived 137Cs in May 2011 (two months after the accident) and 137Cs concentration in them decreased with time. However, the concentrations were still two orders of magnitude higher than the pre-accident level in 2013 and 2014. These observations are the first to show that 137Cs redistribution on a forested hillslope is strongly controlled by biologically mediated processes and continues to supply 137Cs to the bottom via litterfall at a reduced rate.

To gain a better understanding of the role of charred plant materials, which were produced during the burning of vegetation by human activity and wildfires, in the formation of humic and fluvic acids in Japanese volcanic ash soils, the quantitative contribution of charred and buried plant fragments to their acids in whole soils were investigated using three volcanic ash soil samples. Charred fragments were the main components in the fraction of less than specific gravity 1.6 g cm −3 (< 1.6 fraction), which was isolated after HCl-HF treatment of the soil samples. The percentage contribution of organic C content in the < 1.6 fractions to that of the whole soils ranged from 13.9 to 32.0%. All humic acids obtained from the < 1.6 fractions and whole soils were classified into Type A, which is characterized by a high degree of darkening and the presence of a graphite-like structure. However, the color coefficient (∆logK) and relative color intensity (RF) values of the humic acids in the < 1.6 fractions were different from those in whole soils. In all soils studied, the amounts of NaOH-extractable humic (OH-HA) and fulvic acids (OH-FA) were much greater than those of Na 4 P 2 O 7 -extractable humic (SPP-HA) and fulvic acids (SPP-FA), respectively, and the amounts of humic acids were substantially greater than those of fulvic acids. The proportion of the quantitative contribution of humic and fulvic acids in the < 1.6 fractions to those in the whole soils ranged from 12.0 to 43.8% for OH-HA, from 3.80 to 9.56% for OH-FA, and from 2.92 to 22.3% for SPP-HA, respectively. The proportion was very small for SPP-FA. It was assumed that in Japanese volcanic ash soils, parts of charred plant materials are subjected to oxidative degradation over a long period of time after burning, and are converted to fulvic acids and, particularly, humic acids.

No abstract

No abstract

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium ( 137 Cs) contamination of the soil in multiple terrestrial ecosystems. Soil is a complex system where minerals, organic matter, and microorganisms interact with each other; therefore, an improved understanding of the interactions of 137 Cs with these soil constituents is key to accurately assessing the environmental consequences of the accident. Soil samples were collected from field, orchard, and forest sites in July 2011, separated into three soil fractions with different mineral–organic interaction characteristics using a density fractionation method, and then analyzed for 137 Cs content, mineral composition, and organic matter content. The results show that 20–71% of the 137 Cs was retained in association with relatively mineral-free, particulate organic matter (POM)-dominant fractions in the orchard and forest surface soil layers. Given the physicochemical and mineralogical properties and the 137 Cs extractability of the soils, 137 Cs incorporation into the complex structure of POM is likely the main mechanism for 137 Cs retention in the surface soil layers. Therefore, our results suggest that a significant fraction of 137 Cs is not immediately immobilized by clay minerals and remains potentially mobile and bioavailable in surface layers of organic-rich soils.