Decadal decline in exchangeable ¹³⁷Cs in a Cryptomeria japonica forest soil estimated from storage-corrected...

Exchangeable ¹³⁷Cs is an operationally defined fraction that represents a potentially mobile component of radiocesium in forest soils. Its long-term dynamics are important for understanding radiocesium behavior after nuclear accidents. This study quantified storage-induced changes in ¹³⁷Cs extractability and estimated decadal changes in exchangeable ¹³⁷Cs in the forest soil of a Cryptomeria japonica plantation on Mount Tsukuba, Japan, following the Fukushima nuclear accident in 2011. Surface soil samples collected annually from 2012 to 2021 were air-dried and archived until batch extraction in 2024. Exchangeable ¹³⁷Cs was extracted using an optimized procedure with 1 M NH₄NO₃ for 48 h. Repeated extraction of aliquots from a single air-dried soil sample over storage durations of 0.2-10.5 years demonstrated decreasing extractability, described by an exponential function. This function was applied to the archived monitoring samples to estimate extraction yields at sampling, assuming the same storage response, although the estimates could not be independently validated against measurements made immediately after collection. The storage- and decay-corrected extraction yields suggest a decrease of approximately half during the decade following the Fukushima nuclear accident, whereas decay-corrected values alone exhibit no clear temporal trend. A similar decline was estimated after excluding samples requiring extrapolation. Despite this decline, the mean storage-corrected extraction yield for samples collected in 2021 was approximately 19%. These results suggest a progressive decline in exchangeable ¹³⁷Cs in forest soils following the Fukushima nuclear accident and highlight the importance of considering storage effects when estimating long-term trends from archived samples.