RELATIONSHIP OF MICROBIAL PROCESSES 327 



plutonium to be available to plants, it must pass through the solution phase. 

 Furthermore, in studies where appropriate measurements have been made, the quantities 

 taken up by plants exceeded the quantity present in the soil solution. Thus plutonium 

 was being resupplied to solution and plants from the solid phase. Since Pu(IV) ions would 

 hydrolyze in solution, precipitating on soil surfaces, it is likely that the plant available 

 fraction was stabilized in soil solution by complexation with inorganic or organic ligands 

 and/or was present in a different, more soluble, valence state. 



Circumstantial evidence suggested (Wildung and Garland, 1975; previous section) that 

 inorganic or organic ligands present in limited concentrations in soil stabilized plutonium 

 in soil solution. Furthermore, dissolution of plutonium from the solid phase has been 

 shown to be accelerated by complexing agents (Bondietti, Reynolds, and Shanks. 1976; 

 previous section). Organic ligands. which form the most stable plutonium complexes, are 

 generally derived from microbial processes in soil, and previous studies (Wildung, Garland, 

 and Drucker, 1973; previous section) have shown that plutonium solubility in soil and 

 availability and distribution in plants are influenced by niicrobial activity, although 

 mechanisms other than complexation may have been responsible. However, a synthetic 

 ligand (DTPA) was shown to maintain plutonium essentially soluble in soil for extended 

 periods. Thus it is likely that organic ligands of microbial origin, which differ markedly in 

 their form, concentration, and stability in soil (Keeney and Wildung, 1977), may play an 

 important role in stabilizing plutonium in solution for subsequent uptake by plants. A 

 key question is whether the relatively low uptake exhibited by plants from plutonium- 

 amended soils (reported concentration ratios of 10^^ to 10~^, Energy Research and 

 Development Administration, 1976) is due to limited solubility in soil as a result of 

 sorption on particulate surfaces or to discrimination by the plant. If discrimination is not 

 at the plant level, then the potential role of the soil microbiota in increasing plutonium 

 availability from the solid phase (oxide particles from the nuclear fuel cycle or soil 

 particles) becomes very important in influencing the long-term availability of plutonium 

 to plants. 



The role played by organic ligands in facilitating plant uptake of ions, particularly 

 hydrolyzable ions, has long been a subject of controversy. The question has been whether 

 the complex simply .serves as a means of delivering the ion to the root membrane, 

 supplying the ion to the root by dissociation, or is taken up intact by the plant. Perhaps 

 the most meticulous investigation of this phenomena has been the work of Tiffm and 

 co-workers (summarized by Tiffin, 1972; 1977). At least in the case of iron, the evidence, 

 derived in part from direct analyses of xylem exudates, strongly supports the role of 

 complexors in increasing uptake by plants but indicates that the complex serves mainly to 

 deliver the metal to the root membrane and that the ligand is not taken up by the plant 

 stoichiometrically with the metal. Recently, Malzer and Barber (1976) concluded that 

 less than 16% of several calcium and strontium chelating ligands was removed from 

 nutrient solution by corn (Zea mays), whereas over 90% of the calcium and strontium 

 was taken up by the plant. Both studies used several physicochemical as well as 

 radiochemical measures of chelate concentration in aqueous solution. It should be noted, 

 however, that detailed, exhaustive procedures are required to purify metal chelates, 

 particularly in the case of the transuranic elements (Swanson, Garland, and Wildung, 

 1975). Without this effort it is possible that studies using only ^^C analysis as a measure 

 of chelate concentrations in plants would overestimate uptake since low-molecular 

 impurities containing ^^C might account for the ^'^C present in the plant, particularly 

 where chelate uptake rates are low. 



