The effects of Cs-137 deposit forms on its ageing in soil have not yet been reported. Soluble and Solid Cs-137 input forms were mixed with the mineral soils collected under Fukushima's coniferous and broadleaf forests, incubated under controlled laboratory, and examined the evolution of Cs-137 availability over time. Results show that the extracted Cs-137 fraction with water was less than 1% for the soluble input form and below detection limit for the solid input forms. Likewise, with an acetate reagent, the extracted Cs-137 fraction ranged from 46 to 56% for the soluble input and from 2 to 15% for the solid input, implying that the nature of the Cs-137 contamination strongly influences its extractability and mobility in soil. Although the degradation of organic materials was apparent, its impact on the Cs-137 extractability was found to be weak. Nevertheless, more Ac-available Cs-137 was obtained from broadleaf organic material mixes than the coniferous counterparts, suggesting that the lignified nature of latter tend to retain more Cs-137. When extrapolated to a field context, more available Cs-137 fraction may be expected from wet-derived contaminated forest soils than contaminated via solid-derived inputs. Such information could be helpful for radioecological management schemes in contaminated forest environments.