RELATIONSHIP OF MICROBIAL PROCESSES 301 



the plant stoichio metrically with the metal. After passing the root membrane, Pn(IV) is 

 translocated to the shoots in the xylem through formation of a number of organic 

 complexes with plant ligands. The form of plutonium differs in leaves and stems, but 

 greater than 90% of the soluble plutonium associated with these tissues was present as 

 complexed Pu(IV) after growth on soil to which ionic Pu(IV) had been added. 



A reevaluation of plant -to -animal transfer coefficients used presently in dose 

 assessments may be required since plutonium incorporated in plant tissues is markedly 

 more available to animals than Pu(IV) gavaged in the inorganic plutonium solutions that 

 were used for previous measurements. Differences in gastrointestinal transfer of 

 plutonium in stems and leaves of alfalfa are related to differences in plutonium solubility 

 in these tissues. Tims the form of plutonium in soils and plants may be closely related to 

 plutonium availability to 'animals. 



Although information leading to an understanding of the complex biochemical 

 interrelationships that exist between soils, plants, and animals is rapidly developing, these 

 phenomena are not sufficiently understood at present to be described by simple models. 



The major factor governing availability of the transuranic elements to plants will be their 

 solubility in soil since, for root uptake to occur, a soluble species must exist adjacent to 

 the root membrane for some finite period. The form of this soluble species will have a 

 strong influence on its stability in soil solution, on its mobility in soils, and on the rate 

 and extent of uptake and, perhaps, on its mobility and toxicity in the plant. 

 Furthermore, the results of preliminary studies discussed in this chapter suggest that the 

 concentration and chemical form of the element in the plant play a major role in 

 influencing its availabiUty to animals on ingestion. Thus any assessment of the long-term 

 behavior of the transuranic elements in the terrestrial environment must be based on the 

 determination of the factors influencing solubility and on the form of soluble species in 

 soil. These factors, illustrated in Fig. 1, include the concentration and chemical form of 

 the element entering soil; the influence of soil properties on the elemental distribution 

 between the solid and Hquid phase; and the effect of soil processes, such as microbial 

 activity, on the kinetics of sorption reactions, transuranic concentration, and the form of 

 soluble and insoluble chemical species. 



Portions of the soil chemical and microbiological sections of this chapter have been 

 published elsewhere by the U. S. Department of Energy (Wildung, Drucker, and Au, 

 1977) and the American Society of Agronomy (Keeney and Wildung, 1977). However, at 

 the request of the editor these have been reiterated to provide a complete treatment of 

 the subject. 



ANIMALS 

 PLANTS 



SOIL ^ / *'-'"- \ M SOIL 



PROCESSES^ /SOLUTION \ ^PROPERTIES 



SOURCE 

 Fig. 1 Factors influencing transuranic behavior in the terrestrial environment. 



