Chemical sequential extraction of O horizon samples from Fukushima forests: Assessment for degradability and radiocesium retention capacity of organic matters

To investigate how radiocesium (137Cs) is retained in the O horizon via interactions with organic matter, we collected O horizon samples in Japanese cedar (Cryptomeria japonica) and konara oak (Quercus serrata) forest sites in Fukushima during the 8 years following the Fukushima Dai-ichi Nuclear Power Plant accident. To assess degradability and 137Cs retention capacity of organic matter, we conducted chemical sequential extraction with organic solvent and sulfuric acid, collecting the following fractions: organic solvent extractives (Fraction 1), acid-soluble carbohydrates (Fraction 3), and acid-insoluble residue (Fraction 4). In all samples, across sampling years and sites, 137Cs content in Fractions 1, 3, and 4, as a proportion of the total 137Cs content, was 0.0–23.6%, 18.4–42.9%, and 44.8–76.0%, respectively. Generally, 137Cs is considered to be electrostatically bound to organic matter and relatively mobile, making it easily extractable by sulfuric acid treatment. However, we observed a relatively high proportion of 137Cs in Fraction 4, suggesting strong retention of 137Cs and their immobility in the O horizon. Complex organic matter such as lignin or tannin may contribute this retention. We also noted that some part of 137Cs may be also retained by clay minerals in the O horizon. Although organic matter in Fractions 1 and 3 is considered to decompose faster than that in Fraction 4, over the observation period the 137Cs proportion and net rate of decrease in 137Cs content (in total and in each fraction) remained nearly constant. This result implies that decomposition of organic matter and the consequent release of bound 137Cs may be partly compensated by additional input of 137Cs from the canopy and 137Cs recycling by soil microorganisms. Our study highlights the potential role of organic matter in the O horizon as a temporary reservoir of 137Cs and a driver of the 137Cs cycle in forest ecosystems. •We collected O horizon samples from Fukushima forests from 2011 to 2019.•We conducted chemical sequential extraction with organic solvent and sulfuric acid.•Some organic matter in the O horizon showed strong 137Cs retention capacity.•Additional canopy input of 137Cs and 137Cs recycling by soil microbes may occur.•Organic matter may serve as a temporary reservoir of 137Cs, enhancing 137Cs cycling.