Geoscience Reference
In-Depth Information
2,4-D-decomposing bacteria more than a year later after 2,4-D treatment was dis-
continued. Under such conditions, repeated herbicide treatment with 2,4-D becomes
much less effective, and it is advisable to use a different herbicide for continued treat-
ment.
Enzyme adaptations by bacteria or other microorganisms are usually specific to
one chemical compound. Occasionally, the specificity is not absolute and cross ad-
aptations may occur. Thus the two herbicides MCPA and 2,4-D have something in
common in their chemical structure, and microorganisms that attack MCPA are able
to degrade 2,4-D also. However, the converse does not apply, since treatment of soil
with 2,4-D does not accelerate subsequent breakdown of MCPA.
Adapted microbes can utilize these herbicides as a source of food and energy and
achieve complete breakdown of the substance to carbon dioxide and water and ionic
chlorine. Many other synthetic chemicals are not completely metabolized in this way
but may be partially degraded by the process known as co-oxidation or co-metabol-
ism. Then a succession of different microorganisms may be necessary to effect com-
plete decomposition of the compound.
Microorganisms are largely responsible for the breakdown or detoxification of
insecticides and fungicides, and their interactions are of great interest. For example,
the organophosphorus insecticides owe their activity to interference with the acetyl
cholinesterase enzyme of insects. It is fortunate, therefore, that some soil bacteria
possess
constitutive
'phosphatase' enzymes which, in a few cases, can hydrolyse the
phosphate ester grouping in the insecticide molecule, and so inactivate the insecticide
completely. The very toxic parathion can be inactivated by several soil bacteria in this
way.
As with herbicides, repeated additions of an insecticide to soil can induce the
growth of a population of bacteria able to decompose it. An example is the use of
diazinon to control the lettuce root aphid. The failure of control after repeated use of
diazinon was shown to be due to a population of a
Flavobacterium
species that could
decompose diazinon. A similar phenomenon was seen in the case of carbofuran that
was used repeatedly in soils to control a rice pest. The effectiveness of the insecticide
declined after repeated use.
A vast amount of information is now available on the behaviour and breakdown
of pesticides in soil, and repeatedly the activities of microorganisms have been shown
to be vital to maintaining soils in a healthy, fertile condition. There still remain certain
very stable chemicals which resist biological attack. These are referred to as recal-
citrant substances and include the persistent insecticides and herbicides already men-
tioned, for example DDT and 2,4,5-T.
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EACHING OF NITRATES
