RELATIONSHIP OF MICROBIAL PROCESSES 315 



Prior incubation, which in microbial studies was shown to increase the solubihty of 

 plutonium in soil, increased plutonium and americium uptake by shoots compared with 

 the unincubated controls (Table 3). The effect was greatly accentuated in the case of the 

 soil-free roots, and incubation increased the soil-to-plant concentration ratios by up to 

 37 times relative to the unincubated control, depending on plutonium soil concentration 

 level. 



TABLE 3 Uptake of Plutonium and Americium in Barley as a Function 

 of Prior Incubation in a Ritzville Silt Loam Soil 



*(Microcuries of plutonium per gram of oven-dry plant tissue per microcuries 

 of plutonium per gram of oven-dry soil) x 10~^. Initial concentrations of ^"Pu 

 and ^'''Am were 0.5 ^Ci/g and 0.03 juCi/g, respectively, on a dry-weight basis. 

 Mean standard errors (n = 3) were ±39 and ±10% for plutonium and americium, 

 respectively. 



tSoil was incubated 30 days after amendment with carbon and nitrogen to 

 provide optimal microbial activity. 



Effect on the Soil Microflora 



Soil microorganisms may be exposed to relatively high transuranic-element concentra- 

 tions even when total transuranic-element soil concentration is low. Soil organisms may 

 be expected to be present at highest levels in the immediate vicinity of soil colloids 

 (Alexander, 1961) where, from the aqueous chemistry of the transuranic elements and on 

 the basis of recent information on transuranic-element chemistry in soil (previous 

 section), the transuranic elements are likely to be concentrated. It is therefore necessary 

 to determine the toxicity of the transuranic elements to soil microorganisms since 

 microorganisms exhibiting resistance to the chemical effects of the transuranic elements 

 may have the highest potential for participating in alteration of transuranic-element form. 

 However, the transuranic-element series does not contain stable isotopes, and organisms 

 chemically resistant to these elements must exhibit a degree of radiation resistance, which 

 is dependent, in large part, on the radiochemistry of the isotope. Resistance to the 

 chemical effects of transuranic elements can occur by three general mechanisms, 

 including (1) inability of the transuranic elements to produce a toxic effect on cell 

 metabolism at the cytoplasmic or exocytoplasmic levels; (2) inability of organisms to 

 transport the transuranic elements; or (3) ability of the organisms to convert transuranic 

 elements, by the direct and indirect mechanisms discussed in a previous section, to a form 

 that is either less capable of entering the cell or is not toxic to the cell. The last 

 mechanism is likely the most important in the alteration of transuranic-element form in 

 soil. 



Effect on Microbial Types, Numbers, and Activity. The etTect of soil plutonium 

 concentration on the soil microflora has been measured as a function of changes in 



