The effects of 137Cs deposit forms on its ageing in soil have not yet been reported. Soluble and Solid 137Cs 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 137Cs availability over time. Results show that the extracted 137Cs 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 137Cs 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 137Cs contamination strongly influences its extractability and mobility in soil. Although the degradation of organic materials was apparent, its impact on the 137Cs extractability was found to be weak. Nevertheless, more Ac-available 137Cs was obtained from broadleaf organic material mixes than the coniferous counterparts, suggesting that the lignified nature of latter tend to retain more 137Cs. When extrapolated to a field context, more available 137Cs 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. © 2018 Elsevier B.V.