inte microbial tissues, and (4) release of metals on decomposition of organic residues. All of these mechanisms may be operational in transformatione of transuranic elements in soils. However, on the basia of present knowledge, it is not possible to draw conclusions as te their relative importance in affecting the long-term behavior of the transuranic elements. Since there ie a paucity of information available, rather than discuss these mechaniems individually they will be addressed around a framework of current information, limited principally to Pu. Microbial Alteration of Solubility in Soil 1500 ACCUMULATIVE CARBON DIOXIDE, mg To provide a preliminary assessment of the potential for microbial alteration of Pu solubility in soil under aerobic conditions, Wildung et al. (1973, 1974) measured soil reapiration rate (an index of soil microbial activity), microbial types and numbers and Pu water solubility in sterile (gamma irradiation) and nonsterile soils, which contained 10 Ci Pu/g of Carbon dioxide evolution waa utilized aa a soil [added as Pu(NO3),]. measure of soil respiration rate. To measure Pu solubility, the soil was subsampled at intervals during incubation over a 30 day period, and the subsamples (1g) suspended in 1 liter of distilled water. After a 4 hr equilibration period, an aliquot of the soil suspension was filtered through 5, 0.45 and 0.01 » millipore filters. The Pu in the 0.5 and 0.01 »p filtrates was designated water soluble although it was recognized that Pu Likely was present as fine colloids (previous section). which represented a Pu level leas than 0.2% of that applied, did not change significantly with treatment, o 0.0 a 100 fF STERILE TREATMENTS \ 0 PLUTONIUM WATER SOLUBILITY (< 0.45u) IN SOIL, % Changes in the soil respiration rate and Pu solubility during the initial 30 day incubation are shown in Fig. 6. The concentration of Pu in the 0.45 p filtrate during the incubation period ranged from approximately 0.5 to 1.5% of the Pu initially applied. Solubility was initially higher in the sterile soil than in the nonsaterile soil but was relatively constant with time in the sterile soil. The initial increase in solubility in the sterile soil was anticipated in view of the known increases in soluble organic matter which result from gamma {irradiation of soil. The Pu solubility (< 0.45 y) in the nonsterile soil, while initially lower, increased by a factor of 3 with incubation time to 14 days and remained significantly higher than the sterile soil during the incubation period. This increase generally followed the accumulative CO2 curve, and maximum solubility occurred at the end of logarithmic growth for all classes of organisms. The concentration of Pu in the 0.01 u filtrate, 1000 PLUTONIUM, uCi/g 1.0 0.5 0 Fig. 6, STERILE soll x 0 NONSTERILE SOIL ! ] 10 20 INCUBATION TIME, DAYS ' 30 Changes in soil respiration rate and solubility of appt i with time of soil incubation 1 . In an ancillary experiment, incubation was continued for 65 days until the COz evolution rate reached a constant level. The Pu-containing sterile soli was then inoculated with the Pu-treated nonsterile soil and the respiration rate and solubility of Pu in the inoculated soil measured for a period of 30 days, When the sterile soils were inoculated with nonaterile soil, COQz evolution increased at a much more rapid rate without a lag phase, and thia was accompanied by a factor of 2 increase in water solubility (< 0.45 yu) 144 145 (Wildung Qu akh.y ,