Index >> Biodegradation of Pesticides and Pollutants >> Fate of Pesticides in Soil Biodegradation

In paddy fields under standing water, parathion undergoes biodegradation after repeated applications due to the enrichment of Flavobacterium sp. in soil. The standing water in fields turns yellow after the third addition of the insecticide due to the formation of p-nitrophenol. Earlier work, however, indicates that parathion breaks down in aerobic microbial cultures resulting in the formation of aminoparathion by the reduction of nitro group in the parent molecule of the insecticide. In Flavobacterium sp. (a. facultative anaerobe) inoculated culture, p¬nitrophenol is detected when the cultures are grown either aerobically or anaerobically but the secondary product is resistant to further degradation. Based on work in flooded soil and in pure culture. containing Flavobac¬terium sp., a scheme for parathion metabolism h s been proposed.

Data on the persistence of pesticides in soil are not only voluminous but also variable, probably due to differences in analytical techniques used in monitoring residues. The effective persistence of pesticides in soil varies from a few weeks to several years, depending on the structure and properties of the compound and to a certain extent on the availability of moisture in soil. The variability in persistence is best illustrated in the case of insecticides for instance, the highly toxic phosphates do not persist for more than three months in contrast to some of the chlorinated hydrocarbon insecticides which are known to persist for extended periods (4 to 5 years) at normal rates of application. From the agricultural point of view, accumulation of residues in soil may lead to increased absorption of such chemicals by plants to a level at which the consumption of plant products may prove deleterious to livestock and human beings.

Persistence is a relative term and, therefore, 75 to 100% loss of the pesticide in normal arable soil under recommended practices of application has been taken by several workers as the persistence value of a pesticide. Based on this parameter, the persistence of some chlorinated hydrocarbon insecticides is as follows: chlordane-5 years, DDT-21 years, BHC-3 years and heptachlor and aldrin which are metabolized in soil to their epoxides, heptachlor epoxide and dieldrin-16 years. On the other hand, the persistence of phosphate insecticides in soil is very low-diazinon (3 months), disulfoton (4 weeks), phorate (2 weeks) and malathion and parathion (up to 2 weeks).

The persistence of herbicides in soil may also vary from few weeks to 18 months, depending on the nature of the herbicide. Among the urea, triazine and picloram group of herbicides, propazineand picloran persist for 18 months followed by simazine (12 months), atrazine and monuron (10 months), fenron and diuron (8 months), linuron (4 months) and prometryne (3 months). In the group of benzoic acid and amide herbicides, 2,3,6-trichlorobenzoic acid persists for 12 months followed by bensulide (10 months) diphenamide (8 months), amibena (3 months) and others (less than 3 months). Among the phenoxy, toluidine and nitrile group of herbicides, the order of persistence is as follows: trifluralin (6 months) 2,4,5-T (5 months), dichlobenil (4 months) MCPA (3 months) and 2,4-0 (one month). The carbamate and aliphatic acid group of herbicides are comparatively less persistent among all types of herbicides with TCA having a persistence life of 12 weeks followed by dalapon (8 weeks) and barb an (2 weeks).

The disappearance of a pesticide depends on the initial concentration of the chemical in soil although factors such as volatilization, photodecomposition and erosion by water and wind contribute to the loss of pesticide from soil. When a biodegradable pesticide is applied to a newly cultivated soil, it disappears fairly rapidly after an initial lag phase but periodic application of the same chemical may lead to an accumulation of the substance depending on the period of persistence of the pesticide. Similarly, periodic application of heavy metal containing pesticides such as arsenic and mercurial pesticides results in a progressive accumulation of the heavy metals with every application even though a portion of the molecule of the pesticide gets degraded or lost. For instance, metallic mercury vapour and trace amount of phenyl mercuric acetate (PMA) have been detected in air surrounding PMA-treated soils. However, the problem of arsenic and mercury contamination in food chain of human beings is more im¬portant than their persistence in soil.

Mercury enters the food chain from seeds or crops treated with mer¬cury containing fungicides. A survey of mercury levels of plants in Britain, Canada, New Zealand and Scandinavia has revealed that translocation of mercury does occur into tissues of plants raised from seeds treated with mercury containing fungicides. One example of mercury containing seed protectants is methylmercury dicyanodiamide (panogen). It has been demonstrated through experiments that hens fed on grains obtained from crops raised from panogen-treated seeds concentrated mercury in their liver and eggs. The food chain of eggs and chicken from such contaminated hens may transfer mercury to man and the indications at preset are that mercury tends to accumulate in the tissues of brain. Fortunately, methyl mercury has a biological half-life of 60 to 74 days in human beings but it should be alarming to note that as little as 6 ppm in brain cells is sufficient to cause an irreversible brain damage.

Of all the pesticides investigated so far for residues in food chains, OOT has received universal attention and therefore, a reference to the persistence of DOT may prove useful in understanding the hazards faced by man through intensive use of chemicals to combat pests. ODT has been in use since World War II and has permeated our environment and con¬taminated human beings and wild life of the earth including such remote areas like the Antarctic. Apart from wind and water, migrating fish and birds can transport the chemical to long distances. DOT is sparingly soluble in water, has the property of entering into fatty substances of living beings, is extremely stable and is very slowly broken down in the environ¬ment. These attributes provide enough scope for the chemical to persist in living matter for extended periods and render DOT a potent source of contamination in food chains.

Several compounds by themselves are non-toxic but may be converted to toxic products by a process known as 'activation' which is also microbially mediated. This leads to the formation of carcinogens, teratogens, neurotoxins, phytotoxins, and insecticidal or fungicidal chemi¬cals. There are several examples of this kind, some of which are as follows: Many bacteria convert trichloroethylene (TCE) to vinyl chloride which is a potent carcinogen. TCE can also be converted to chloral hydrate (2,2, 2-trichloroacetaldehyde), a mutagen as well as a toxin, by methanotrophic bacteria. Nitrosamines are potential carcinogens, teratogens and mutagens, even at low concentrations. These compounds are products of microbial action of naturally formed NO2 ions from NO3 ions present in soil and water with synthetic chemicals such as secondary amines that are con¬stituents of many pesticides. These nitrosamines contaminate river waters and become part of sewage and other effluents. Some insecticides such as aldrin and parathion are liable to conversion in o more toxic compounds which are known to persist in soil microorganisms. Aldrin is converted to dieldrin and the latter compound is known to persist in soil for more than 15 years. Similarly parathion gets transformed to paraoxon. There have been instances, when some harmless precursors get converted by microor¬ganisms to potent toxic chemicals as exemplified hereunder: