of Pu after only 4 days of incubation, suggesting the development of a microbial population particularly capable of alteration of Pu solubility. Again, there was no change in the < 0.01 u fraction which amounted to approximately 10% of the Pu present in the < 0.45 u fraction. At least under the conditions of this study, the evidence strongly suggested that the solubility of Pu in soil was influenced by the activity of the soil microflora. The potential mechanisms effecting the change in solubility include mechanisms (1) and (2) described above, i.e. indirectly through the production of organic acids that may complex Pu or the alteration of the solution pH and/or Eh near the soil colloid (or Pu polymer) without measureable effects on the overall acil pH; or directly through the transformation of Pu. Microbial effects may have occurred at the colloidal level since detectable increases in solubility did not occur in the < 0.01 » filtrates. This appears to implicate the indirect mechanisms since direct changes generally occur at the molecular level and these would be reflected in changes in Pu concentration in the < 0.01 filtrates provided polymerization did not occur during extraction. Of course, a combination of the mechanisms is possible, i.e., alteration of affinity for organic ligands through a change in valence state. If the mechaniam of solubilization was indirect, the results might be applicable to other transuranic elements, e.g., from consideration of the aqueous chemistry described in a previous section, a reduction in pH would be expected to increase the solubility of the other transuranic elements as well as Pu. Increased water solubility of Pu on incubation under optimum conditions for microbial activity may be expected to increase Pu uptake by plants provided the limiting factor is not discrimination at the root membrane. In order to determine if the increased solubility on incubation resulted in increased Pi uptake by plants, the soila, incubated as previously described, were planted to barley and cultured using a split-root technique which allowed measurement of the uptake, sites of deposition and chemical forms of Pu in plant shoots and roots (Wildung and Garland, 1974), The results were compared to the results of similar plant studies in which the soils had not been incubated. Prior incubation, which in microbial studies was shown to increase the solubility of Pu in soil, increased Pu uptake by shoots compared to the unincubated controls. The effect was greatly accentuated in the case of the soil-free roote and incubation increased the soil to plant concentra~ tion ratios by up to 37 times relative to the umincubated control, depending upon Pu soil concentration level. Thus, plant uptake measurements tend to verify measurements of increased Pu solubility in the < 0.45 uy fraction in the incubated soil. However, if, as previously discussed, Pu in particulate form (i.e., > 0,01 UW) wag not available to plants, and if increases in the concentration of Pu in the 0.01 te 0.45 wy fraction resulting from microbial activity were not due to procedural artifacts arising from polymerization, the colloidal particles may be serving as a reservoir of plant—available Pu manifested only when the equilibrium concentration of Pu in solution is reduced by plant uptake, 146 Effect on the Soil Microflora Soil organisms may be expected to be present at highest levels in the immediate vicinity of soil colloids (Alexander, 1961). From the aqueous chemistry of the transuranics and on the basis of recent information on transuranic chemistry in soil (previous section), the transuranic elements in soil are likely to be associated mainly with colloids. Thus, soil microorganisms may be exposed to relatively high concentrations even when total transuranic seil concentration is low. It is therefore necessary to determine the toxicity of the transuranic elements to soil Microorganisms, as microorganisms exhibiting resistance to the chemical effects of the transuranics may have the highest potential for participating in alteration of transuranic form. However, the transuranic series does not contain stable isotopes and organisms chemically resistant to these elements must exhibit a degree of radfation resistance which is dependent, in large part, upon the radiochemistry of the isotope. Resistance to the chemical effects of transuranics may occur by three general mechanisms including (1} tnability of the transuranics to produce a toxic effect on cell metabolism at the cytoplasmic or exocytoplasmic levels (2) tnability of organisms to transport the transuranics or (3) ability of the organisms to convert transuranics, by the direct and indirect mechanisms discussed in a previous section, to a form that is either less capable of entering It is che latter mechanism which is the cell or is not toxic to the cell, likely most important in alteration of transuranic form in soil. Effect on Microbial Types, Numbers and Activity. The effect of soil Pu concentration on the soil microflora has been measured as a function of changes in microbial types and numbers and soil respiration rate (Wildung et al., 1973, 1974a). A noncalcareous Ritzville silt loam (pH 6.7) was amended with 7?°Pu(NO3), at levels of 0.05, 0.5 and 10 wCi/g and with Subsamples of starch, N and water to provide optimal microbial activity. soil were periodically removed to determine the changes in types and ° During this period, soil respiration numbers of soil microflora with time. rate was monitored by continuous collection of sotl-evolved CU>. The growth curve of fungi (Fig. 7) was generally typical of the growth response for other classes of microorganisms. Total microbial numbers were compared at the end of logarithmic growth. The organisms generally Growth rates were reached this stage after 8 to 14 days of incubation. compared over the intervals of maximum microbial growth for each organism The results are summarized in Table 3}. at each Pu concentration. The Pu did not generally affect the rate of growth but decreased the total numbers of all classes of microorganisms at levels as low as 0.05 uci/e or 7 ug/g. The fungt were the exception, differing from the controls only Thus, the Pu did not at a Pu concentration of 10 uCi/g or 144 ug/g. affect maximum generation rate but rather affected the lag period or onset of the stationary phase, limiting microb{al numbers. The accumulative CO2 curve generally corresponded to the growth curve of In the case of the other classes of organisms, maximum logthe fungi. arithmic growth occurred before the rate of C02 evolution reached minimum 147