ASSOCIATION OF FALLOUT RADIOCESIUM WITH SOIL CONSTITUENTS - EFFECT OFSTERILIZATION OF FOREST SOILS BY FUMIGATION WITH CHLOROFORM

The effect of soil sterilization by chloroform fumigation on the relea se of fallout radiocesium incorporated in the fungal biomass of the or ganic layer of two forest soils was investigated by applying a sequent ial extraction procedure for radiocesium. The amount of the biomass in all soil samples was estimated by determination of the dissolved orga nic carbon (DOC) before and after fumigation, and qualitatively also b y the ergosterol test. The five fractions obtained by sequential extra ction (modified Tessier procedure) were: (I) easily exchangeable, (II) bound to oxides, (III) bound to organic matter, (IV) persistently bou nd, (V) residual. For the samples from the soil under spruce trees, no significant effects were apparent in any of these five fractions as a result of chloroform fumigation and qualitatively eating that the amo unt of radiocesium in the biomass of this soil was obviously negligibl y small compared with the radiocesium associated with other soil const ituents. The results obtained for the soil samples from the beech stan d, however, reveal that the destruction of the biomass by chloroform f umigation modified considerably the extent of the association (i.e., b inding) of radiocesium with the various other soil constituents (espec ially the clay minerals). As a result of this rapid redistribution of radiocesium released by the fungal biomass, it is not possible, in gen eral, to attribute the observed increase of radiocesium in fraction I (easily exchangeable) after soil sterilization quantitatively to radio cesium released by the biomass. A reliable method to determine the amo unt of radiocesium incorporated in the fungal biomass of the soil samp les which also contain clay minerals has, therefore, still to be devel oped.