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III. Effects of Pesticides on the Microorganisms and on the Total Activity of the Soil

III. Effects of Pesticides on the Microorganisms and on the Total Activity of the Soil

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EFFECTS OF PESTICIDES ON SOIL MICROFLORA



27



bufam (mentioned previously as having a positive effect), endothal, dalapon,

trichloracetate, chlorazine cycloate, and lenacil (Chulakov and Zharasov, 1975);

and DNOC, propham, pyrazon, TCA, DCPA, PCP,* and some substituted ureas

(Audus , 1970).

Finally, there is a herbicide category that causes depressive effects; dinoseb,

which is more toxic than bentazone, reduces the bacterial population

(Torstensson, 1975), but only at high rates. In some cases, the initial depressive

effects are followed by an increase in the bacterial number beyond the normal

level. This delayed stimulation is caused by the adaptation time of the bacteria

and by the increase in the environment of nutrients that come from weeds killed

by the treatment. According to some workers, it can also be explained by the

utilization of the herbicides as substrates, but only for herbicide quantities above

the normal rates of field applications. This group includes dinoseb, chlorazine,

and cycluron + chlorbufam (Nepomiluev et al., 1966). This depressive action

does not last long, with a few exceptions; dinoseb decreases the bacterial population for as long as three months, and dalapon and EPTC are effective on alfalfa

for two months, without, however, affecting the nitrifying bacteria.

The triazines often have no effect on the soil bacteria, as was shown in the

following studies: (a) Atrazine: in long-term trials on apple trees (Voets et al.,

1974) and in pots with wheat, bean, and corn (Houseworth and Tweedy, 1973);

the stimulating effect observed in this case is indirect and is not due to the

atrazine but to the fact that the degradation of the weeds killed by this product

leads to an increase in the bacterial and fungal populations. ( b ) Simazine: in pots

and in situ (Kulinska, 1967); in pots and in the laboratory (Freney, 1965; Kaiser

and Reber, 1970). (c) Simazine and atrazine: in the laboratory (Eno, 1962;

Amantaev et al., 1963), providing (with reference to the work of Eno) that the

normal rates are used.

These two triazines have no effect in field experiments (Zubets, 1973; Kruglov

et a l . , 1975a). Kruglov and co-workers, however, add that this conclusion is

valid only for a single application. The superposition of two- or three-year

treatments leads to a depressive action.

In some cases, triazines may also result (a) in stimulation-with methoprotryne in wheat rhizosphere (Micev, 1970); or with atrazine for two months and in

proportion to the rate used (Percich, 1975); or (b) in an evanescent depressive

effect, followed or not by stimulation-in sandy soil with atrazine and simazine

(Klyuchnikov et al., 1964); in apple orchards (Kwyndina, 1965); in forest

nurseries (Milkowska and Grozelak, 1966); in strawberry plants with simazine,

applied between the rows (Bakalivanov and Nikolova, 1969); in corn with

simazine (Simon-Sylvestre, 1974); in tomatoes for 100 days after an application

*PCP-product



used for potato tops.



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GINETTE SIMON-SYLVESTRE AND J.-C. FOURNIER



of metribuzin (Velev and Rankov, 1975); or in a perennial crop after three years

of treatment with atrazine (Kruglov et al., 1975b).

This negative action may also persist for one year, after eight consecutive

years of atrazine application, at normal doses, as counts carried out in an apple

orchard have shown (Simon-Sylvestre, 1974).

Some workers have mentioned a higher sensitivity of the spore-forming bacteria to the toxic actions of the triazines.

The insecticides have, in general, no effect on bacterial numbers when applied

at normal rates, according to counts carried out fifteen days after treatment (work

of Pathak et al., 1960-1961, on DDT, chlordane, and aldrin) and one month

after the application of chloro-insecticides (En0 and Everett, 1958).

Focht and Josseph (1974) have not observed any modification in the microflora in general, even for tenfold rates of acephate and methamidophos. At very

high concentrations, however, Audus (1970) reports that the insecticides of the

chlordane group have a selective action in inhibiting completely the grampositive bacteria. Some other depressive effects are reported in the literature, but

they persist for only two weeks with the organophosphorus insecticides (Tu,

1970) and for sixteen weeks with lindane (Tu, 1975).

The number of bacteria increases in general after a fungicide treatment, probably as a result of the elimination of fungi and a decrease in competition. Several

papers mention this temporary increase as occurring on the seventh day after a

treatment with thiram (Agnihotri, 1974); during the second week after a treatment with benomyl (Yakolev and Stenina, 1974); and on the twenty-eighth day

after a treatment with benomyl, captan, thiram, dicloran, formaldehyde, or quintozene (Wainwright and Pugh, 1974).

Van Faassen (1974) also finds an increase in the number of bacteria after a

treatment with benomyl, and Wainwright and Pugh (1975), in assays in vitro

with captan, reveal the influence of the concentration of the product on the

amplitude of the bacterial growth and on the date of maximum growth. Hofer et

al. (1971), Ponchet and Tramier (1971), and Siege1 (1975), on the other hand, do

not observe any variation in the bacterial population after a treatment with benomyl. Chandra and Bollen (1961) report an opposite effect-a decrease in the

number of bacteria for 30 days, and a recovery after 60 days.

The soil fumigants (Audus, 1970) cause a decrease in the bacterial population

for varying periods of time: for several days with ethylene dibromide, for a few

weeks with DD, and for as long as six months with chloropicrin and formaldehyde. This decrease is followed immediately by a great increase in the number

of bacteria, and later by a return to a normal population. The same resultnamely, a temporary depressive effect-was obtained in the assays of Corden

and Young (1965) with metam-sodium and those of Tu (1972) with four

nematicides: DD, carbofuran, fensulfothion, and methylisothiocyanate.



EFFECTS OF PESTICIDES ON SOIL MICROFLORA



29



2 . Fungi

It is equally difficult to generalize about the results obtained in research on soil

fungi. This section will deal only generally with the action of pesticides on these

microorganisms. The special case of the pathogenic fungi is the subject of a

specific study discussed in another section (Section V).

The herbicides, in normal doses, often do not modify the soil fungal population. This is found for trifluralin (Tyunaeva ef nl., 1974),NaClO, (Karki et a/.,

1973), and atrazine and simazine (Eno, 1962;Amantaev et al.. 1963;Voets et

a/., 1974).

Some depressive effects have been reported, however, in laboratory experiments as well as in field assays with propanil (Oleinikov et al., 1975),propachlor (Husarova, 1972),2,4-dichlorophenoxyacetic acid (Mashtakov et d . ,

1962;Abueva and Bagaev, 1975),pyrazon (Zharasov et al., 1972),cycluron

chlorbufam (Zharasov et al., 1972;Kozaczenko, 1974),and even simazine and

atrazine (Mashtakov ef al., 1962;Bakalivanov and Nikolova, 1969;Malichenko,

1971;Simon-Sylvestre, 1974) and prometryn (Malichenko, 1971). These depressive effects decrease in time (Malichenko, 1971;Simon-Sylvestre, 1974).

Bakalivanov and Nikolova (1 969)show a quick recovery to a normal population

after ten days. Some negative actions are also observed after the use of higher

doses of certain herbicides such as paraquat (Camper et al., 1973)and substituted ureas (Grossbard and Marsh, 1974).

In contrast to these findings, a stimulation of the fungal population is reported

in the studies of Kaszubiak (1970)with prometryn, dinoseb, and 2,4-DB,of

Micev (1970)with methoprotryne, of Zubets (1973)with simazine and atrazine,

during the year following the treatment, and of Sharma and Saxena (1974)with

2,4-Dat a definite concentration.

The insecticides, in general, affect the soil fungi in the same way as they do

the soil bacteria. No action is seen, in soils of different types, with chloroinsecticides (Pathak ef al., 1960-1961)or with lindane alone, even with high

doses (Tu, 1975).However, Eno and Everett (1958)report a stimulation with

dieldrin one month after treatment, and Tu ( 1970)reports a brief decrease (after

one to two weeks) with organophosphorus insecticides.

On the other hand, Cowley and Lichtenstein (1970)describe an inhibition of

the growth, on Czapek nutrient media, of several fungal species, isolated from

prairie soils, in the presence of aldrin, lindane, parathion, or carbaryl. This

inhibition decreases with the addition of yeast extract, asparagine, or some

ammonium salts to the culture media; it is completely suppressed with carbaryl

and is reduced to a large extent with aldrin.

As would be expected, the soil fungi are affected by afungicide treatment, as

Agnihotri (1974)demonstrated with thiram, and Foster (1975)and Siege1 (1975)



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30



GINETTE SIMON-SYLVESTRE AND J.-C. FOURNIER



observed with benomyl. This decrease may be selective; thus, Pugh et al. (1973)

found the cellulolytic fungi to be less tolerant than other fungi to the organomercuric products. Other workers believe, in particular for benomyl, that this is a

fungistatic action and not a fungicidal one (Hofer et af., 1971; Ponchet and

Tramier, 1971; Raynal and Ferrari, 1973).

Actually, this decrease in the number of fungi in the soil, which occurs even

with small doses of the fungicide, is only temporary. It is maximum after 2 days

with captan, in laboratory assays, and after 3-15 days with captan, dicloran,

drazoxolon, and triarimol, in the field (Wainwright and Pugh, 1975). After 2

weeks with 300 kg of benomyl per hectare (Yakolev and Stenina, 1974), 74% of

the fungi have disappeared, and only three species survive. The recovery to a

normal population takes place after 28 days (Wainwright and Pugh, 1974), with a

treatment using benomyl, captan, thiram, dicloran, and quintozene, and after 60

days (Chandra and Bollen, 1961) with nabam and dazomet. The inhibitory effect

of captan in all the fungi lasts longer than that of thiram (Houseworth and

Tweedy, 1973).

Paradoxically, fungicides may also have a positive effect on fungi. These

organisms are stimulated by high doses of benomyl (Avezdzhanova et al.,

1976). Wainwright and Pugh (1975) observed that four fungicides (captan, dicloran, triarimol, and drazoxolon), at normal field rates, lead to a reduction of fungi

3 and 15 days after the treatment; then, after 157 days, the fungal population

increases, sometimes beyond the normal level.

The nematicides decrease the number of soil fungi for 2 weeks in a sandy silt;

this occurs with ethoprop, dichloropropene, and methylisothiocyanate (Tu,

1973). This reduction is temporary, however, with metam-sodium (Corden and

Young, 1965), with carbofuran, DD, fensulfothion, and methylisothiocyanate

(Tu, 1972), and with metam-sodium and methylisothiocyanate (Welvaert, 1974);

it is incomplete with methylisothiocyanate and dazomet (Welvaert, 1974).

3 . Actinomycetes



The actinomycetes are, in general, very tolerant to most groups of herbicides

applied at normal rates. They are not affected by the phenoxy acids, DNOC,

NaC103, calcium cyanamide, or substituted ueas (Audus, 1970), phenylureas

(Doxtader, 1968), linuron ? lenacil (Balezina and Tretyakova, 1974), trifluralin

(Tyunyaeva et al., 1974), paraquat (Camper et al., 1973), pyrazon

(Mezharaupe, 1967), simazine (Kulinska, 1967), or simazine and atrazine, alone

or mixed (Zubets, 1973), during either the first year or the second year.

Depressive effects are mentioned, however, with dalapon and EPTC under

alfalfa (Rakhimov and Rybina, 1963), with methoprotryne under wheat (Micev,

1970), and with simazine used between the rows of strawberry plants

(Bakalivanov and Nikolova, 1969), but only temporarily. Owing to the fact that



EFFECTS OF PESTICIDES ON SOIL MICROFLORA



31



simazine may be used as a source of both carbon and nitrogen, the initial depressive effect is followed by a stimulation of the growth of actinomycetes with high

concentrations of 2,4-D (Sharma and Saxena, 1974) or with a series of treatments

for five years in an apple orchard with the mixture simazine + amitrole

(Teuteberg, 1968) or for seven years, under corn, with propachlor and atrazine

(Husarova, 1972).

Stimulating effects are also observed with prometryn, dinoseb, and 2,4-DB

(Kaszubiak, 1970) and with chlorpropham (CIPC) under cotton (Taha et a l . ,

1972).

Very few papers deal with the effects of the insecticides on the actinomycetes.

According to Audus (1970), these germs are resistant to DDT, BHC, toxaphene,

aldrin, dieldrin, and parathion applied at normal rates, and Tu (1975) confirms

this statement for lindane. Only in pure cultures do the actinomycetes show some

sensitivity.

The fungicides and soil fumigants are in general toxic toward the actinomycetes, even at normal rates, as indicated by the work of Corden and Young

(1965) with metam-sodium, nabam, and dazomet, and that of Agnihotri (1971)

with captan. A temporary effect was observed by Agnihotri (1974) with thiram

and by Siege1 (1975) with benomyl.

Only Wainwright and Pugh (1975), in laboratory experiments, report a

stimulating effect with a low dose of captan, and Van Faassen (1974) a lack of

action with benomyl.

4 . Algae



A last group of organisms, somewhat apart in the soil micropopulation, deserves to be mentioned. These are algae, of little importance in cultivated soils

and studied only by some ecologists. Their research has usually been limited to

the effects of herbicides the inhibitory power of which is in general proportional

to their concentration (Wright, 1972). In fact, these data have made it possible to

perfect a technique, built on the sensitivity of the green algae, that detects the

presence of certain herbicides in the soil according to the quantity of chlorophyllian pigment extracted. A current publication (Lefebvre-Drouet and Calvet,

1978) will contribute to the precision of this technique, already used by several

research workers.

All the triazines exert a negative action on algae, more significant after one

application on a light soil than after five treatments on a heavy soil (Pantera,

1970). Kruglov and Kvyatrovskaya (1975) report that the sensitivity of Chlorelfu

to phenylureas depends on the chemical structure of the herbicide and on the soil

content in humus. At very small concentrations (0.001-0.01 ppm), however,

substituted ureas and atrazine have a stimulating action on algae. At higher

concentrations (0.1-1 ppm) the growth of the algae is inhibited (Pillay and



32



GINETTE SIMON-SYLVESTRE AND J.-C. FOURNIER



Tchan, 1972). Lenacil, alone or mixed with linuron, would be toxic, whereas

monolinuron, associated with linuron or alone, would not (Balezina and Tretyakova, 1974). According to Kirkwood and Fletcher (1970), MCPB inhibits

growth, respiration, and phosphorus absorption more strongly than does MCPA,

in the case of three unicellular algae. Phenazone, at high rates, also reduces the

growth of algae (Pociene et al., 1974), whereas with 4 kg/ha there is no effect.

Chlorella are not affected by MSMA in liquid medium, but photosynthesis is

stopped by a 2-hour exposure to 5 x

M prometryn or diuron and to 5 x

lo-’ M fluometuron (Davis et al., 1976).

The sensitivity of algae varies, depending on the herbicide, but generally

Chlamydomonas and Chlorococcum are more sensitive than Chlorella and Nostoc, according to the findings of Cullimore and MacCann (1977) on the effects of

four herbicides (2,4-D, trifluralin, MCPA, and TCA) on algae isolated from a

grassland loam soil. A herbicide treatment, in the top layer of the soil, is followed by a reduction in the cell numbers of sensitive algae and an increase in the

population of Chlorella. The authors also note an overall reduction in cell numbers for the algae, growing preferentially on a nitrogen-free medium. With

metribuzin, Arvik et al. ( 1973) find the same classification-Chlamydomonas

the most sensitive, and Chlorella the most resistant.

It is difficult to draw conclusions from such varied results on the counts of the

microorganisms of the soil. The statement of Kaszubiak (1970), “Bacteria were

stimulated or depressed by all herbicides, depending upon the preparation, its

dose, and sampling time, although it concerns only bacteria and herbicides,

very well sums up the complexity of the facts, which characterizes all the studies

carried out on soil microorganisms and pesticides.

However, other causes of variation must be added:





Nature of the soil and its composition: According to Karki et al. (1973),

CaClO, has a more marked effect on acid soils. On the other hand, its harmful

actions are less important in soils that are rich in organic matter, for greater

amounts of the chemical products are adsorbed (Fusi and Franci, 1971;

Beckmann and Pestemer, 1975; Rankov and Velev, 1975; Vlassak and Livens,

1975). The addition of silica and clay to a light soil decreases the inhibitory effect

observed in soil microorganisms (except in fungi) after a treatment with atrazine

or prometryn (Panterowa et al., 1975).

Climatic conditions: In a wet year, the sugar-beet herbicides, such as cycluron

chlorbufam, have a stronger effect on the soil fungi (Zharasov et al., 1972).

Duration of the treatment: Sobieszczanski et al. (1975) mention an adaptation

of bacteria to certain herbicides-atrazine, simazine, prometryn-used continuously.



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EFFECTS OF PESTICIDES ON SOIL MICROFLORA



33



B. TOTAL ACTIVITY OF THE SOIL



I . Respiratory Activity

As with the counts of the microorganisms, the results for total respiratory

activity vary considerably, depending on the pesticides studied and the experimental conditions.

Some herbicides do not affect this activity when they are applied at normal

field rates; these include simazine and atrazine (Eno, 1962), simazine (Kulinska,

1967), and triallate and MCPA (Grossbard, 1971). Even after a series of seven to

eight treatments, picloram (Grover, 1972), metoxuron (Grossbard and Marsh,

1974), NaCIO, (Karki and Kaiser, 1974), and 2,4-D and 2,4,5-T (Ruffin, 1974)

have no effect; this is found also with isoproturon and triazophos at hundredfold

concentrations (Neven et al., 1975).

However, some of these herbicides partially inhibit the respiration of the

microflora of the soil if they are used at high rates-for example, simazine

(Kulinska, 1967), metoxuron and linuron (Grossbard and Marsh, 1974), and

dinoseb acetate (Neven et al., 1975)-or if the treatments are repeated-for

example, simazine and linuron (Grossbard, 197 1).

Some herbicides inhibit the respiratory activity of the soil. One of these is

monuron (Tolkachev, 1974). This inhibition disappears in time, however, and

stimulation takes place. Other herbicides, inhibitory at normal doses, become

favorable at high concentrations, such as dalapon and 2,4-D (Bliev, 1973) in

podzols and lawn soils, and pyrazon (Lauss and Danneberg, 1975).

Other herbicides stimulate the gaseous exchanges in the soil. Examples of

these are simazine and ioxynil (Smith and Weeraratna, 1975). This stimulation is

sometimes followed by inhibition-for example, after treatment with chloroxuron, diuron, fluometuron, metobromuron, or monuron (Grossbard and Marsh,

1974); dalapon, 2,4-D, TCA, or simazine (Kozlova et al., 1974) (assays in

vitro; or simazine or atrazine (Kruglov et a / . , 1975a). The negative action

appears only after the third or fourth treatment.

Whereas DDT and HCH have no effect on the respiratory activity of the soil

when applied in normal doses (Drouineau, et al., 1947), nor do DDT, dieldrin,

and malathion at tenfold concentrations, (Ruffin, 1974), other insecticides lead

to an increase in the oxygen uptake for nine weeks, as shown in Tu’s assays

(1975) with lindane, or an increase in CO, evolution, as found in the experiments

of Eno and Everett (1958) with toxaphene and dieldrin and those of Tu (1970)

with organophosphorus insecticides. In contrast, an inhibition of the respiration

is found with mephosfolan (Purushothaman et al., 1974).

Varied actions of the fungicides are also mentioned, from the absence of

effect, observed in the work of Hofer et al. (1971) and of Van Faassen (1974)



34



GINETTE SIMON-SYLVESTRE AND J.-C. FOURNIER



with benomyl, to the inhibition of respiration, also found with benomyl (Weeks

and Hedrick, 1975) and with captan (Agnihotri, 1971). In this last example, the

inhibition is followed by a stimulation, probably resulting from the use of the

decomposition remains of the captan by the microorganisms.

For the tzematicides-carbofuran, fensulfothion , and ethoprop-the oxygen

uptake increases with the concentration of the product used in the soil (Tu, 1972,

1973).

2 . Enzymatic Activity

Experiments on enzymatic activity are still very limited, but already the results

show considerable diversity. Most studies concern dehydrogenase activity, associated with the total respiratory activity, according to some workers. For

Ulasevich and Drach (1971), one normal application of atrazine to a cultivated

field, and two applications to a bare soil, are enough to increase the dehydrogenase activity, whereas the use of the same product for fifteen years, in apple

orchards, leads to a decrease in all enzymatic activity (Voets et a f . , 1974). In the

assays of Hofer et al. (197 1) and Van Faassen (1974), benomyl remains without

effect on the activity of the dehydrogenases. On the other hand, sodium chlorate

(Karki and Kaiser, 1974) and mephosfolan (Purushothaman et al., 1974) inhibit

this enzyme group.

In regard to enzymatic activity in general, a whole range of results is possible,

from no effect with simazine at normal doses (Kulinska, 1967) or with benomyl

(Hofer et al., 1971), to inhibition with ethofumesate and pyrazon (Verstraete and

Voets, 1974), to stimulation with aldicarb (Verstraete and Voets, 1974).

Urease is sometimes the only enzyme affected (Zubets, 1973) by simazine or

atrazine, alone or mixed, and sometimes it is the only one remaining untouched

after treatment with paraquat (Giardina et al., 1970). Lethbridge and Burns

(1976) also find an inhibition of the soil urease by organophosphorus insecticides; malathion is the least potent inhibitor of the three insecticides studied

(fenitrothion and phorate are the other two). Many of the microorganisms, inhibited at the beginning of the experiment (the first 48 hours), develop a tolerance

after 21 days, if the application rate is not too high.

Catalase activity increases with dalapon and 2,4-DA in the laboratory as well

as in soils of varied nature, while the activity of amylase remains constant (Bliev,

1973).

This range of pesticide action on soil enzymes is clearly indicated in the

research of Karanth et a f . (1975) with fenaminosulf; the inhibition caused by this

fungicide depends on its concentration and on the incubation period. Thus,

invertase, urease, and tryptophanase are stimulated by 20 ppm of fenaminosulf,

but are inhibited by 100 and 200 ppm.



EFFECTS OF PESTICIDES ON SOIL MICROFLORA



35



C. CONCLUSIONS



The great diversity of experimental protocols, both in siru and in the laboratory, leads to highly varied conclusions regarding the effects of pesticides on the

soil microflora-ranging from one extreme to the other, from inhibition to stimulation. It is therefore difficult to classify these results or to express them in

generalities, or even to try to elucidate the part played by each factor responsible

for the effects observed. In the laboratory studies, carried out under controlled

conditions, the observation and evaluation of the microorganisms are more precise and the facts are more reproducible, but the concentrations of the products

used are often higher than those applied in agricultural practice, and the methods

of application are so different that it is difficult to compare the results with those

obtained in field experiments.

This diversity in the effects of pesticides on the soil microflora is increased

even more by the great differences in the chemical formulas of the productssometimes quite simple and mineral, sometimes complex and organic.

IV. Effects of Pesticides on the Biological Cycles of the Soil



The preceding section presented data on the major groups of soil microorganisms and total soil activity. This section deals with more specific questions. It

is concerned with the consequences of pesticide treatments on the biological

cycles of the soil, the importance of which in numerous processes of synthesis

and mineralization has already been shown.

A. THE CARBON CYCLE



Microbiological studies on the microorganisms active in the carbon cycle,

especially the cellulolytic organisms, relatively are limited. Nevertheless, the

facts revealed are fairly complex.

Among the herbicides that have no effect on the cellulolytic organisms when

applied at normal doses, Audus ( 1970) mentions the synthesis phytohormones,

monuron, the substituted phenols, cycluron chlorbufam, DCU, and TCA. In

an earlier report, Shklyar (1961) mentioned 2,4-D, simazine, monuron, dinoseb,

and chlorpropham (CIPC), used in pre-emergent applications. Later research by

Micev (1970) and by Wolf et al. (1975) added methoprotryne and bromacil to

this list.

A reduction in the decomposition of cellulose appears, however, in experiments in siru with simazine and atrazine (Klyuchnikov, et al., 1964), carbaryl

(Atlavinyte et al., 19741, linuron (Grossbard and Marsh, 1974), certain



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36



GINETTE SIMON-SYLVESTRE AND J.-C. FOURNIER



+



triazines, used for seven years, linuron, monolinuron, and cycluron

chlorbufam (Kozaczenko and Sobieraj, 1973) (pot experiments), and atrazine in a

long-term experiment (fifteen years) in apple orchards (Voets et al., 1974). A

similar reduction is found in experiments in pots with linuron, monolinuron, and

cycluron + chlorbufam (Kozaczenko, 1974). Mainly the Cytophaga are affected.

Klyuchnikov et al. (1964) found the inhibitory action to be more severe with

atrazine than with simazine, which is less water-soluble and penetrates less

deeply into the soil. These workers, however, note an action of these two herbicides down to 25-35 cm in light soils. This decrease in activity never results

from a reduction in the number of cellulolytic organisms. A modification of the

species may occur after eight sprayings of atrazine (Simon-Sylvestre, 1974). The

initial depressive effect disappears in certain cases, followed by a cellulose

decomposition greater than that found in untreated soils.

An immediate stimulation of cellulolytic activity is also mentioned with 2,4-D

(Abueva and Bagaev, 1975), dalapon, or TCA at high rates and mixed with

2,4-D (Kozlova et al., 1974), atrazine (Percich, 1975), linuron (Miklaszenski,

1975), and propachlor (Rankov and Velev, 1975) at low concentrations. The

opposite effect is observed after high doses.

In pure cultures, Audus (1970) found that certain herbicides have no effect

even at very high concentrations; these include atrazine, simazine, dalapon,

diuron, and chlorthal. Others become harmful only at concentrations of 0.1% and

completely inhibit the cellulose decomposition; dicamba, tricamba, and 2,3,6TBA are examples. Sometimes toxicity appears at lower rates-for example,

with propanil, DMPA, 2,4-D, and paraquat (Szegi, 1970). For 2,4-D, a slight

inhibition is followed by a stimulating effect. Grossbard (1974) also mentions

glyphosate, metoxuron, paraquat, and amitrole as products that cause cellulose

breakdown, in pure cultures as well as under field conditions.

Only at very high concentrations do the insecticides affect the cellulolytic

organisms in the soil (Audus, 1970). To detect an effect on their number, the

normal dose must be multiplied by five with DDT, and by seventy with parathion; one must use rates twenty times as high with chlordane, forty times as high

with demeton, and two hundred times as high with dieldrin, to affect cellulose

decomposition. Audus also mentions a positive action on the cellulolytic population with heavy applications of BHC and parathion.

Audus (1970) demonstrates a negative action, even at normal rates, of the

fungicides and the soilfumigants DD and methyl bromide on cellulose decomposition, and a tolerance to metam-sodium in the cellulolytic fungi. On the other

hand, the studies of Simon-Sylvestre (1974), carried out in situ on two different

soils, show that DD does not affect the number of aerobic cellulolytic organisms,

whereas metam-sodium decreases it considerably; by 27 and 33 days after the

treatment, only 4170and 68%, respectively, of the cellulolytic population remain



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