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.

为了研究放射性铯（¹³⁷ Cs）如何通过与有机物的相互作用保留在O层中，我们在福岛发生后的8年中收集了日本雪松（Cryptomeria japonica）和konara橡树（Quercus serrata）在福岛的森林中的O层样品。第一核电站事故。为了评估有机物的降解性和¹³⁷ Cs保留容量，我们使用有机溶剂和硫酸进行了化学序贯萃取，收集了以下馏分：有机溶剂萃取物（组分1），酸溶性碳水化合物（组分3）和酸不溶物残留物（部分4）。在所有采样年和采样点中，^共137个馏分1、3和4中的Cs含量占总¹³⁷ Cs的比例分别为0.0-23.6％，18.4-42.9％和44.8-76.0％。通常，认为¹³⁷ Cs与有机物发生静电结合并且相对移动，使其易于通过硫酸处理而提取。但是，我们在馏分4中观察到相对较高的¹³⁷ Cs比例，表明¹³⁷ Cs的强保留性和在O层中的固定性。木质素或单宁等复杂有机物可能会导致这种保留。我们还注意到^137的某些部分铯也可能被O层中的粘土矿物所保留。尽管第1部分和第3部分中的有机物被认为比第4部分中的有机物分解更快，但在观察期内，¹³⁷ Cs的比例和¹³⁷ Cs含量（总和各部分）的净减少率几乎保持恒定。该结果暗示有机物的分解和结合的¹³⁷ Cs的释放可以部分地通过冠层中¹³⁷ Cs的额外输入和土壤微生物的¹³⁷ Cs回收得到部分补偿。我们的研究强调的有机物于O地平线的潜在作用的暂时贮存器¹³⁷ Cs和驾驶员¹³⁷森林生态系统中的CS循环。