316 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



microbial types and numbers and soil respiration rate (Wildung, Garland, and Drucker, 

 1973; Wildung et al., 1974). A noncalcareous Ritzville silt loam (pH 6.7) was amended 

 with ^^^Pu (N03)4 at levels of 0.05, 0.5, and lO^uCi/g and with starch, nitrogen, and 

 water to provide optimal microbial activity. Subsamples of soil were periodically removed 

 to determine the changes in types and numbers of soil microflora with time. During this 

 period soil respiration rate was monitored by continuous collection of soil-evolved COj. 

 The growth curve of fungi (Fig. 5) was generally typical of the growth response for 

 other classes of microorganisms. Total microbial numbers were compared at the end of 



Fig. 5 Influence of plutonium concentration on the growth of fungi in soil (Wildung 

 et al., 1974). Arrows denote time intervals at which growth rates and total numbers were 

 compared with other microbial types (Table 4). 



logarithmic growth. The organisms generally reached this stage after 8 to 14 days of 

 incubation. Growth rates were compared over the intervals of maximum microbial growth 

 for each organism at each plutonium concentration. The results are summarized in 

 Table 4. 



The plutonium did not generally affect the rate of growth, but it decreased the total 

 numbers of all classes of microorganisms at levels as low as 0.05 juCi/g or 7 /Jg/g. The 

 fungi were the exception, differing from the controls only at a plutonium concentration 

 of 1 juCi/g or 144 /ig/g. Thus the plutonium did not affect the maximum generation rate 

 but rather it affected the kg period or onset of the stationary phase, which limited 

 microbial numbers. 



The accumulative COi curve generally corresponded to the growth curve of the fungi. 

 For the other classes of organisms, maximum logarithmic growth occurred before the rate 

 of CO2 evolution reached minimum levels. The rate of CO2 evolution and cumulative 

 CO2 over the incubation period were significantly reduced only at the 144-jUg/g level of 

 plutonium amendment, although numbers of all classes of organisms except the fungi 

 were depressed below this level (Table 4). This is in marked contrast to the results of 

 studies with a number of other heavy metals (Drucker et al., 1973), such as silver and 

 mercury, in which respiration rate was a sensitive measure of metal effect at levels as low 

 as 1 jug/g in soil. Differences in the effects of the metals may be related to differences in 



