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.

Keywords: Fukushima Dai-ichi Nuclear Power Plant accident; O horizon; forest soil; radiocesium (137Cs); soil organic matter.