Despite the presence of minerals that allow Cs fixation in soils, 137Cs remains available to crops for several years after its deposition, particularly in pasture soils. Larger amounts of organic matter derived from herbage residues are accumulated in pasture soils than in tilled farmland soils. As the above-ground part of herbage crops initially received airborne 137Cs during the accident at Fukushima Daiich nuclear power plant (FDNPP), the organic matter originated from the contaminated herbage should play an important role in the fate of 137Cs in soils. To evaluate the role of organic matter on 137Cs distribution between potentially mobile and immobile fractions, we compared the distribution of 137Cs and stable 133Cs, which are differently associated with organic matter, by sequential extraction and density fractionation. Soil samples were collected 8 years after the accident from Andosols in pasture fields located about 160 km southwest of FDNPP. More than 90% of 137Cs was not extracted even after oxidative digestion of organic matter, suggesting that most 137Cs was strongly associated with soil minerals. Density fractionation results showed that the 137Cs/133Cs ratio was highest in the density fraction of 1.6-1.8 g cm-3, in which organic matter -including fragmented and decomposed plant detritus -was associated with minerals. Mineral-free organic matter, mostly composed of fresh plant detritus (<1.6 g cm-3), had a higher 137Cs/133Cs ratio than that of crops harvested in the same year of soil sampling. Thus, the transfer of 137Cs from soil to plants decreased with cultivation cycles. Our results demonstrate that plant-available 137Cs in pasture soil decreased with aging time, not only through increased 137Cs fixation in mineral-dominated fractions but also through its physical sequestration in aggregates.