Pesticides are the chemical substances
that kill pests and herbicides are the chemicals that kill weeds. In the
context of soil, pests are fungi, bacteria insects, worms, and nematodes etc.
that cause damage to field crops. Thus, in broad sense pesticides are
insecticides, fungicides, bactericides, herbicides and nematicides that are
used to control or inhibit plant diseases and insect pests.
1.
Effects of pesticides: Pesticides reaching the soil in
significant quantities have direct effect on soil microbiological aspects,
which in turn influence plant growth.
Some
of the most important effects caused by pesticides are:
(1)
alterations in ecological balance of the soil microflora, (2) continued
application of large quantities of pesticides may cause everlasting changes in
the soil microflora, (3) adverse effect on soil fertility and crop
productivity, (4) inhibition of N2 fixing soil microorganisms such as Rhizobium, Azotobacter,
Azospirillum etc. and cellulolytic and phosphate solubilizing
microorganisms, (5) suppression of nitrifying bacteria, Nitrosomonas and Nitrobacter by soil
fumigants ethylene bromide, Telone, and vapam have also been reported, (6)
alterations in nitrogen balance of the soil, (7) interference with
ammonification in soil, (8) adverse effect on mycorrhizal symbioses in plants
and nodulation in legumes, and (9) alterations in the rhizosphere microflora,
both quantitatively and qualitatively.
2.
Persistence of pesticides in soil: How long an insecticide, fungicide, or herbicide
persists in soil is of great importance in relation to pest management and
environmental pollution. Persistence of pesticides in soil for longer period is
undesirable because of the reasons: a) accumulation of the chemicals in soil to
highly toxic levels, b) may be assimilated by the plants and get accumulated in
edible plant products, c) accumulation in the edible portions of the root
crops, d) to be get eroded with soil particles and may enter into the water
streams, and finally leading to the soil, water and air pollutions. The
effective persistence of pesticides in soil varies from a week to several years
depending upon structure and properties of the constituents in the pesticide and
availability of moisture in soil. For instance, the highly toxic phosphates do
not persist for more than three months while chlorinated hydrocarbon
insecticides (eg. DOT, aldrin, chlordane etc) are known to persist at least for
4-5 years and some times more than 15 years.
From
the agricultural point of view, longer persistence of pesticides leading to
accumulation of residues in soil may result into the increased absorption of
such toxic chemicals by plants to the level at which the consumption of plant
products may prove deleterious / hazardous to human beings as well as
livestock's. There is a chronic problem of agricultural chemicals, having
entered in food chain at highly inadmissible levels in India, Pakistan,
Bangladesh and several other developing countries in the world. For example,
intensive use of DDT to control insect pests and mercurial fungicides to
control diseases in agriculture had been known to persist for longer period and
thereby got accumulated in the food chain leading to food contamination and
health hazards. Therefore, DDT and mercurial fungicides has been, banned to use
in agriculture as well as in public health department.
3.
Biodegradation of Pesticides in Soil: Pesticides reaching to the soil are acted upon by
several physical, chemical, and biological forces. However, physical and
chemical forces are acting upon/degrading the pesticides to some extent,
microorganism’s plays major role in the degradation of pesticides. Many soil
microorganisms have the ability to act upon pesticides and convert them into
simpler non-toxic compounds. This process of degradation of pesticides and
conversion into non-toxic compounds by microorganisms is known as
“biodegradation”. Not all pesticides reaching to the soil are biodegradable and
such chemicals that show complete resistance to biodegradation are called
“recalcitrant”.
The
chemical reactions leading to biodegradation of pesticides fall into several
broad categories which are discussed in brief in the following paragraphs.
a)
Detoxification:
Conversion of the pesticide molecule to a non-toxic compound. Since a single
chance in the side chain of a complex molecule may render the chemical
non-toxic.
b)
Degradation:
The breaking down / transformation of a complex substrate into simpler products
leading finally to mineralization. e.g. Thirum (fungicide) is degraded by a
strain of Pseudomonas
and the degradation products are dimethlamine, proteins,
sulpholipaids, etc.
C.
Conjugation (complex formation or addition reaction): In which an organism make the
substrate more complex or combines the pesticide with cell metabolites.
Conjugation or the formation of addition product is accomplished by those
organisms catalyzing the reaction of addition of an amino acid, organic acid or
methyl crown to the substrate, for e.g., in the microbial metabolism of sodium
dimethly dithiocarbamate, the organism combines the fungicide with an amino
acid molecule normally present in the cell and thereby inactivate the
pesticides/chemical.
d) Activation: It is the conversion of non-toxic substrate into a toxic molecule, for eg. Herbicide, 4-butyric acid (2, 4-D B) and the insecticide Phorate are transformed and activated microbiologically in soil to give metabolites that are toxic to weeds and insects.
d) Activation: It is the conversion of non-toxic substrate into a toxic molecule, for eg. Herbicide, 4-butyric acid (2, 4-D B) and the insecticide Phorate are transformed and activated microbiologically in soil to give metabolites that are toxic to weeds and insects.
e)
Changing the spectrum of toxicity: Some fungicides/pesticides are designed to
control one particular group of organisms / pests, but they are metabolized to
yield products inhibitory to entirely dissimilar groups of organisms, for e.g.
the fungicide PCNB fungicide is converted in soil to chlorinated benzoic acids
that kill plants.
Biodegradation
of pesticides / herbicides is greatly influenced by the soil factors like
moisture, temperature, PH and organic matter content, in addition to microbial
population and pesticide solubility. Optimum temperature, moisture and organic
matter in soil provide congenial environment for the break down or retention of
any pesticide added in the soil. Most of the organic pesticides degrade within
a short period (3-6 months) under tropical conditions. Metabolic activities of
bacteria, fungi and actinomycetes have the significant role in the degradation
of pesticides.
4.
Criteria for Bioremediation / Biodegradation: For successful biodegradation
of pesticide in soil, following aspects must be taken into consideration. i)
Organisms must have necessary catabolic activity required for degradation of
contaminant at fast rate to bring down the concentration of contaminant, ii)
the target contaminant must be bioavailability, iii) soil conditions must be congenial
for microbial /plant growth and enzymatic activity and iv) cost of
bioremediation must be less than other technologies of removal of contaminants.
According
to Gales (1952) principal of microbial infallibility, for every naturally
occurring organic compound there is a microbe / enzyme system capable its
degradation.
Article
compiled by Mr. Amol Vijay Shitole (Ph.D. Scholar)
Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola (M.S.)
