Geoscience Reference
In-Depth Information
Table 8.12
Reduction in compound availability for soil microbial degradation as a result of
aging
Compound Soil Aging period (d)
Naphthalene Colwood loam 365
Naphthalene Mt. pleasant silt loam 68
Phenanthrene Mt. pleasant silt loam 110
Phenanthrene 16 soils 200
Anthracene Lima loam 203
Fluoranthene Lima loam 140
Pyrene Lima loam 133
Atrazine Ravenna silt loam 90
Atrazine 16 soils 200
4-Nitrophenol Lima loam 103
4-Nitrophenol Edwards muck 103
Reprinted with permission from Alexander (
2000
). Copyright 2005 American Chemical Society
residues. Use of solvents milder than those used in this experiment lead to different
bound residue values.
The effect of native soil microorganisms on formation of bound residues is
illustrated by the study of Gevao et al. (
2005
) on formation and release of
''nonextractable''
14
C-dicamba. The impact of microorganisms was determined by
following the behavior of this compound under sterile and nonsterile regimes
during 90 days of incubation, using a mild extraction solvent (0.01 M CaCl
2
aqueous solution). The reported results indicate that, one day following the
treatment, about 5 % of the added contaminant was not extractable. The fraction of
nonextractable dicamba increased exponentially to a maximum between 14 and
21 days after application, followed by a decrease that led, at the end of the
experiment (90 days), to about 65 % nonextractable contaminant. This behavior
characterizes the nonsterile soil. Different patterns appeared in a sterile incubation:
A gradual increase in the amount of nonextractable residue was formed, reaching
about 20 % of the initial activity two weeks following treatment. After this period,
there were no significant changes in the amount of nonextractable residues formed
for the entire incubation period. Additional treatment (e.g., fresh soil added to aged
soil) led to the hypothesis that microorganisms play a dual role in the formation
and eventual release of nonextractable residues. When a pesticide is added to a
catabolically active soil, the degrading microorganisms convert the pesticide to
one or more metabolites, which are capable of interacting with subsurface OM. In
the case of dicamba, the major metabolite (3, 6 dichlorosalicylic acid) is adsorbed
to a much greater extent than the parent compound; this strong adsorption of the
metabolite prevents its extraction in CaCl
2
solution. As the concentration of the
available portion of added pesticide drops, the microbes revert to utilizing sub-
surface OM for their energy needs. The microbes consume subsurface OM as their
primary substrate, so the bound pesticide-metabolite molecules are freed from
their entrapment with humic macromolecules.
