The results of this study suggest that either maize root
exudates or oxalate/citrate do not influence phenanthrene
desorption from the soils in the presence of NaN3. This
is different from the previous work in which maize root
exudates (42.3 mg C/kg) were observed to enhance the
desorption of DDT and DDE from soils (Luo et al., 2006).
Such difference is perhaps ascribed to the differences of
contaminant properties and further researches are necessary
to elucidate the mechanism. Without the addition of
NaN3, maize root exudates promoted phenanthrene desorption
while oxalate and citrate inhibited phenanthrene
desorption, which could contribute to the degradation of
phenanthrene when root exudates were used for desorption.
Moreover, a faster loss of phenanthrene occurred
for maize root exudates than for deionized water, oxalate
and citrate, further suggesting phenanthrene degradation
happened in the presence of exudates. Phenanthrene degradation
can contribute to the effect of phenolic substrates
and enzymes released by root exudates. Preliminary results
have indicated that enzyme activity is generally higher
when root exudates exist (Badalucco and Kuikman, 2001)
and phenolic substrates released by plant roots can act
as an inducer of PAH metabolism by stimulating PAH
dioxygenase activity