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Dynamics of Microbial Population in Soil as Influenced by Simazine and Ecological Factors
Zbl. Bakt. II. Abt., Bd. 133, S. 357-361 (1978)
[From the Division of Microbiology, Indian Agricultural Research Institute, New Delhi, India]
Dynamics of Microbial Population in Soil as Influenced by Simazine and Ecological Factors A. C.
GAUR
and K. C.
MISRA
With 4 Figures
Summary Simazine, even at normal rates of application, showed toxicity to bacteria and fungi. It was less toxic to actinomycetes, since toxicity up to 20 ppm of the herbicide was not observed. On the contrary, the normal rate of simazine stimulated both Azotobacter and actinomycetes population. The interaction of simazine with soil ecological factors, such as temperature, moisture, pH, and organic matter, affected soil microbial population differently. Simazine was relatively less toxic to bacteria under acidic and alkaline conditions of soil; they were not affected at 15 DC. Actinomycetes were comparatively not adversely affected even with 200 ppm of simazine under high soil moisture regime. The stimulatory effect of simazine on Azotobacter was also confirmed under different ecological conditions. The incorporation of 2 per cent of organic matter in soil mitigated the toxicity of simazine in respect to soil fungi. Simazine also appeared to be less toxic to soil fungi at lower temperatures, under acidic and alkaline conditions of soil, as well as under high moisture regime.
Zusammenfassung Simazin ist in normalen Gaben gegen Bakterien und Pilze toxisch, weniger gegen Aktinomyzeten. Die toxische Wirkung gegen Bakterien war am groBten in neutralen Boden, schwacher in saueren und alkalischen. Gegen Pilze wirkte Simazin immer ungilnstig, mit Ausnahme gleichzeitiger organischer Dungung. Die toxiwhe Wirkung des Herbizicles erhohte sich mit clem Tcmperaturanstieg.
Simazine (2-chloro-4,6-bis (ethyamino)-1,3,5-triazine) is a systemic soil herbicide that is used in controlling the growth of common weeds in crops like maize, sorghum, sugarcane, asparagus, strawberries, and around fruit trees. The final breakdown of simazine in soils may result from hydrolysis by catalysts in the soil, by soil microorganisms, and by uptake and decomposition by tolerant plants, such as corn. Its breakdown by microorganisms may be accelerated by high temperature, moisture, and organic matter. Thus, these factors and their interactions with simazine are of immense value in determining microbial equilibrium in soil. The information on the effect of simazine on soil microbial population is meagre (POCHON et al. 1960, NAYYAR et al. 1970, MISRA and GAUR 1971). Since there are no reports on the changes of soil microorganisms as affected by different soil ecological factors, the investigations were undertaken to examine the effect of simazine on soil bacteria, actinomycetes, fungi, and Azotobacter under the influence of varying soil organic matter, temperature, pH, and moisture regime.
358
A. C. GAUR and K. C. MISRA
Materials and Methods Alluvial soil (organic carbon 0.54 %, total nitrogen 0.059 %, pH 7.4) collected from 0-2:i em layer of the Indian Agricultural Research Institute farm, New Delhi, was air-dried and sieved through 2 mm meshes for these studies. Simazine was thoroughly mixed with 200 g of soil at the rate of 2.20 and 200 ppm and placed in plastic containers. These treatments were further subjected to soil ecological factors to be tested. Organic matter as farmyard manure was added at the rates of 0.5 and 2.0 per cent on soil weight basis. The effect of soil pH was examined by adjusting the pH of the soil (7.4) to 5.0 and 9.6. Moisture at two different levels (33 per cent and 60 per cent of water holding capacity of soil) were maintained. The effect of varying temperature (15,30, and 45°C) was also examined. The soils were incubated for 10 weeks at 30 2°C, except in the case of different temperature studies. Samples were drawn at weekly intervals for enumeration of microbial population. In the first week they were drawn twice. The procedure for making soil dilutions and plate counts were followed as outlined in "Experiments in Soil Bacteriology" by ALLEN (1957). The bacterial and actinomycetes counts were made on soil extract-agar and fungi on Martin's Rose-Bengal agar media. Jensen's medium was used for counting Azotobacter. The data were statistically analysed, using IBM computer facilities.
±
Results and Discussion The results clearly show that the application of simazine to soil depressed bacterial population significantly right from the normal rate of field application, and the toxic effect was more pronounced at higher levels of simazine (Fig. 1). The toxicity of higher levels of simazine persisted throughout the incubation period. The results are in agreement with those of SOSNOVSKAYA and PASHCHENKO (1965) who also observed a sizable reduction in soil bacterial population, due to simazine application. Its persistence period in soil is relatively long and may stay as long as 17 months (TALBERT and FLETCHALL 1964). The application of simazine to soil amended with FYM, at both levels (0.5 and 2.0 per cent), resulted in a decreased bacterial population which was similar to unamended soil. The deleterious effect of simazine on bacterial population was noted at both 30 and 45 DC, however, at 15 DC its toxic effect was not observed. This suggests that the species of bacteria growing at 15 DC were not susceptible to simazine application. The bacterial population was affected differently, due to soil reaction. The normal rate (2.0 ppm) of simazine was harmful only in neutral soil, whereas in acidic and alkaline soil it was ineffective. The effect of higher levels of the herbicide was also less marked under acidic and alkaline conditions than in the neutral soil. This may be due to high tolerance of bacterial species that could grow under the deviated conditions. However, simazine proved toxic to bacteria at both soil moisture levels and the toxicity increased with increasing levels of the herbicide. Unlike bacteria, simazine up to 20 ppm had no adverse effect on the number of actinomycetes, but higher concentrations (200 ppm) inhibited the population significantly (Fig. 2). Normal rates of simazine stimulated the actinomycetes population on some days of the incubation which is substantiated by findings of NEPOMILUEV and KAZYAKINA (1967). Simazine reacted with actinomycetes in organic matter treated soil in a manner similar to unamended soil at 30 DC. However, at 45 DC, simazine was found to be toxic to actinomycetes even at 20 ppm concentration. Growth of actinomycetes at 15 DC was not obtained. At pH 5.0, simazine was found to be detrimental at all levels, whereas at pH 9.6 simazine had no significant effect on actinomycetes numbers. It is noteworthy that at a high soil moisture level even 200 ppm of simazine did not affect them adversely, whereas up to 20 ppm they were rather stimulated. This
Dynamics of Microbial Population in Soil F.YI'1
80
=-
2·0%[YI'I.
.~ ~L~ OS%Ern. i""--.. ......
............
o
80
2
20
CONTROL
200
FYI'1
2
20
""'"
JoOe
200
TEtfPERATURE
-'?>
60 r-+--+--~~z.o%Frtf
~....
.0 r:-+-=""';'--"'0-5%Frf1
~
CONTROL
'"~
~
""
i:: 20 '-'
"
0
~--...... :s ""-
-
I
20
2
------
,f·0
JO't
2 20 200 2 20 200 CONCENTRATION OF SltfAZINE (ppm)
-
---
N-.......,~- J3%W./iC -....;:::::
60%W.HC.
9S
200
~ i"--...
-
~ H
2
20
200
w'HC.
SOIL pH
-.J
~
WHC
SOIL pH
TEtlPERATURE
359
5·0
H
9'6
-......
33%W.HC 60%W./iC.
2 20 200 2 20 200 CONCENTRATION OF Sft1AZINE (ppm)
Fig. 1. Changes in bacterial population in soil treated with simazine under the influence of different ecological factors during incubation period of 10 weeks. Fig. 2. Changes in actinomycetes population in soil treated with simazine under the influence of different ecological factors during incubation period of 10 weeks.
indicates that some of the soil actinomycetes may be involved in degrading simazine into various non-toxic intermediate compounds. By improving the moisture status, raising the temperature, and liming the soil, the period of effectiveness of 2,4-D and some other herbicides (NEWMAN and NORMAN 1947, HERNANDEZ and WARREN 1950) was diminished. Azotobacter population was significantly increased, due to application of simazine at the recommended field rate (2 ppm), where as higher levels up to 200 ppm had no adverse effect (Fig. 3). Stimulatory effect of triazine herbicides on Azotobacter population was recorded by other workers as well (TIMOFEEV and MONSEEV 1965, GOGUADZE 1967, and KULINSKA 1967). Simazine had almost similar effect on the number of Azotobacter in the FYM-amended soil as well. Azotobacter population was unaffected by different dosages of simazine at 45 DC, whereas at 15 DC Azotobacter cells were not enhanced as a result of simazine application. The influence of simazine on Azotobacter was more or less the same in all the three conditions of soil reaction. The stimulatory effect of simazine on Azotobacter was noticed irrespective of soil moisture regimes. It is presumed that simazine might have been utilized by Azotobacter as carbon and nitrogen source.
360
~
A. C.
GAUR
and K. C.
FY.I1. 60,.-,--,--------,
MISRA'
TEI'fPERATURE
WHC.
SOIL pH
~ 40H--t------i ~ 1--":"--~--J33% WIiC.
~==~=~'O% WIiC.
flO
FY.!'1.
.....
~
o
I
--
TEI'fPERATURE
I
2
20
CONTROL
I - - 30°C
~
200
~-
l s"c
~SOC
2 20 200 2 20 200 CONCENTRATION OF S//'fAZINE (ppm)
---
2
20
200
WHC.
SOIL pH
2·0 % F.y.11. OS%F.Y.tf..~
9·, S·O
I
H
s·o
9·6
-
~-I--- 33% WHC
60% WHC.
2 20 200 2 20 200 CONCENTRATION OF SII'fAZINE (ppm)
Fig. 3. Changes in Azotobacter population in soil treated with simazine under the influence of different ecological factors during incubation period of 10 weeks. Fig. 4. Changes in fungal population in soil treated with simazine under the influence of different ecological factors during incubation period of 10 weeks.
The addition of simazine to soil at different concentrations reduced the soil fungal population during the entire test period (Fig. 4). At a normal rate and at a 1O-fold rate of simazine, a slight reduction in fungal population was observed, but with 100 times application the inhibitory effect was further accentuated. But NEPOMILUEV and KAZYAKINA (1967) reported that simazine and atrazine at 4.5 and 10 kg per hectare stimulated the growth of both Penicillium and Mucor spp. which actively participated in cellulose decomposition. Simazine application proved inhibitory to soil fungi in the presence of 0.5 per cent of FYM, but 2 per cent of FYM mitigated the toxicity of simazine and caused a significant stimulation of fungal population in soils treated with higher doses of the herbicide (20 and 200 ppm). Simazine was found to be more toxic to soil fungi at higher temperature, even at a normal dosage than at mesophilic and psychrophilic temperatures. The toxicity of the herbicide to the fungal flora was less under acidic and alkaline conditions of soil, as compared to neutral soil. This suggests that the fungal flora of neutral soil are more susceptible to simazine than the fungal flora, growing in soil adjusted to acidic and alkaline reactions. Similarly, simazine appeared to be less toxic to fungi under high soil moisture conditions than under low soil moisture regime. Acknowledgement The authors are grateful to Dr. N. viding the facilities.
S. SUBBA RAO,
Head of the Division of Microbiology, for pro·
Dynamics of Microbial Population in Soil
361
References ALLEN, O. N.: Experiments in soil bacteriology. Burges Publishing Co., Minneapolis, Minnesota 1957. GOGUADZE, V. D.: Effect of simazine on microflora development under various soil conditions in Western Georgia (USSR): Subtrop. Kul'tury (1967),148. HERNANDEZ, T. P., and WARREN, G. F.: Some factors affecting the rate of inactivation and leaching of 2,4-D in different soils. Proc. Amer. Soc. Hort. Sci. 56 (1950), 287. KULINSKA, D.: The effect of simazine on soil microorganisms. Roczn. Nauk roln. 93A (1967), 229. MISRA, K. C., and GAUR, A. C.: Tolerance of Azotobacter to some herbicides. Ind. J. Weed. Sci. 3 (1971), 99. NAYYAR, V. K., RANDHAWA, N. S., and CHOPRA, S. L.: Effect ofsimazine on nitrification and microbial population in sandy loam soil. Ind. J. Agric. Sci. 40 (1970), 445. NEPOMILUEV, V. F., and KAzYAKINA, T. I.: Effect of herbicides on microflora of peat bog soils. Izv. Timiryazev. Selkhoz. Akad. 4 (1967), 84. NEMAN, A. S., and NORMAN, A. G.: Effect of soil microorganisms on the persistence of plant growth regulators in soil. J. Bact. 54 (1947), 37. POCHON, J., TARDIEUX, P., and CHARPENTIER, M.: Recherches sur les interactions entre les amino triazines herbicides et la microflore bacterienne tellurique. Compt. Rend. 250 (1960), 1555. SOSNOVSKAYA, E. A., and PASHCHENKO, P. D.: The effect of herbicides on microflora of the soil under maize. Mater. Tezisy VI. Konf. Khim. Sel' Khoz Oyenburg. 1 (1965), 179. TALBERT, R. E., and FLETCHALL, O. H.: Inactivation of simazine and atrazine in the field. Weeds 12 (1964),33. TIMOFEEV, V. A., and MONSEEV, A. N.: Micropopulation of serozems and the rhizosphere of maizein relation to the use of herbicides. 'trudy Kirgiz. nauchnoissled. Inst. Zeml. 5 (1965), 123. Authors' address: Dr. A. C. GAUR, and Dr. K. C. MISRA, Division of Microbiology, Indian Agricultural Research Institute, New Delhi-llOOI2, India.
24 Zbl. Bakt. II. Abt., Bd, 133
[From the Division of Microbiology, Indian Agricultural Research Institute, New Delhi, India]
Dynamics of Microbial Population in Soil as Influenced by Simazine and Ecological Factors A. C.
GAUR
and K. C.
MISRA
With 4 Figures
Summary Simazine, even at normal rates of application, showed toxicity to bacteria and fungi. It was less toxic to actinomycetes, since toxicity up to 20 ppm of the herbicide was not observed. On the contrary, the normal rate of simazine stimulated both Azotobacter and actinomycetes population. The interaction of simazine with soil ecological factors, such as temperature, moisture, pH, and organic matter, affected soil microbial population differently. Simazine was relatively less toxic to bacteria under acidic and alkaline conditions of soil; they were not affected at 15 DC. Actinomycetes were comparatively not adversely affected even with 200 ppm of simazine under high soil moisture regime. The stimulatory effect of simazine on Azotobacter was also confirmed under different ecological conditions. The incorporation of 2 per cent of organic matter in soil mitigated the toxicity of simazine in respect to soil fungi. Simazine also appeared to be less toxic to soil fungi at lower temperatures, under acidic and alkaline conditions of soil, as well as under high moisture regime.
Zusammenfassung Simazin ist in normalen Gaben gegen Bakterien und Pilze toxisch, weniger gegen Aktinomyzeten. Die toxische Wirkung gegen Bakterien war am groBten in neutralen Boden, schwacher in saueren und alkalischen. Gegen Pilze wirkte Simazin immer ungilnstig, mit Ausnahme gleichzeitiger organischer Dungung. Die toxiwhe Wirkung des Herbizicles erhohte sich mit clem Tcmperaturanstieg.
Simazine (2-chloro-4,6-bis (ethyamino)-1,3,5-triazine) is a systemic soil herbicide that is used in controlling the growth of common weeds in crops like maize, sorghum, sugarcane, asparagus, strawberries, and around fruit trees. The final breakdown of simazine in soils may result from hydrolysis by catalysts in the soil, by soil microorganisms, and by uptake and decomposition by tolerant plants, such as corn. Its breakdown by microorganisms may be accelerated by high temperature, moisture, and organic matter. Thus, these factors and their interactions with simazine are of immense value in determining microbial equilibrium in soil. The information on the effect of simazine on soil microbial population is meagre (POCHON et al. 1960, NAYYAR et al. 1970, MISRA and GAUR 1971). Since there are no reports on the changes of soil microorganisms as affected by different soil ecological factors, the investigations were undertaken to examine the effect of simazine on soil bacteria, actinomycetes, fungi, and Azotobacter under the influence of varying soil organic matter, temperature, pH, and moisture regime.
358
A. C. GAUR and K. C. MISRA
Materials and Methods Alluvial soil (organic carbon 0.54 %, total nitrogen 0.059 %, pH 7.4) collected from 0-2:i em layer of the Indian Agricultural Research Institute farm, New Delhi, was air-dried and sieved through 2 mm meshes for these studies. Simazine was thoroughly mixed with 200 g of soil at the rate of 2.20 and 200 ppm and placed in plastic containers. These treatments were further subjected to soil ecological factors to be tested. Organic matter as farmyard manure was added at the rates of 0.5 and 2.0 per cent on soil weight basis. The effect of soil pH was examined by adjusting the pH of the soil (7.4) to 5.0 and 9.6. Moisture at two different levels (33 per cent and 60 per cent of water holding capacity of soil) were maintained. The effect of varying temperature (15,30, and 45°C) was also examined. The soils were incubated for 10 weeks at 30 2°C, except in the case of different temperature studies. Samples were drawn at weekly intervals for enumeration of microbial population. In the first week they were drawn twice. The procedure for making soil dilutions and plate counts were followed as outlined in "Experiments in Soil Bacteriology" by ALLEN (1957). The bacterial and actinomycetes counts were made on soil extract-agar and fungi on Martin's Rose-Bengal agar media. Jensen's medium was used for counting Azotobacter. The data were statistically analysed, using IBM computer facilities.
±
Results and Discussion The results clearly show that the application of simazine to soil depressed bacterial population significantly right from the normal rate of field application, and the toxic effect was more pronounced at higher levels of simazine (Fig. 1). The toxicity of higher levels of simazine persisted throughout the incubation period. The results are in agreement with those of SOSNOVSKAYA and PASHCHENKO (1965) who also observed a sizable reduction in soil bacterial population, due to simazine application. Its persistence period in soil is relatively long and may stay as long as 17 months (TALBERT and FLETCHALL 1964). The application of simazine to soil amended with FYM, at both levels (0.5 and 2.0 per cent), resulted in a decreased bacterial population which was similar to unamended soil. The deleterious effect of simazine on bacterial population was noted at both 30 and 45 DC, however, at 15 DC its toxic effect was not observed. This suggests that the species of bacteria growing at 15 DC were not susceptible to simazine application. The bacterial population was affected differently, due to soil reaction. The normal rate (2.0 ppm) of simazine was harmful only in neutral soil, whereas in acidic and alkaline soil it was ineffective. The effect of higher levels of the herbicide was also less marked under acidic and alkaline conditions than in the neutral soil. This may be due to high tolerance of bacterial species that could grow under the deviated conditions. However, simazine proved toxic to bacteria at both soil moisture levels and the toxicity increased with increasing levels of the herbicide. Unlike bacteria, simazine up to 20 ppm had no adverse effect on the number of actinomycetes, but higher concentrations (200 ppm) inhibited the population significantly (Fig. 2). Normal rates of simazine stimulated the actinomycetes population on some days of the incubation which is substantiated by findings of NEPOMILUEV and KAZYAKINA (1967). Simazine reacted with actinomycetes in organic matter treated soil in a manner similar to unamended soil at 30 DC. However, at 45 DC, simazine was found to be toxic to actinomycetes even at 20 ppm concentration. Growth of actinomycetes at 15 DC was not obtained. At pH 5.0, simazine was found to be detrimental at all levels, whereas at pH 9.6 simazine had no significant effect on actinomycetes numbers. It is noteworthy that at a high soil moisture level even 200 ppm of simazine did not affect them adversely, whereas up to 20 ppm they were rather stimulated. This
Dynamics of Microbial Population in Soil F.YI'1
80
=-
2·0%[YI'I.
.~ ~L~ OS%Ern. i""--.. ......
............
o
80
2
20
CONTROL
200
FYI'1
2
20
""'"
JoOe
200
TEtfPERATURE
-'?>
60 r-+--+--~~z.o%Frtf
~....
.0 r:-+-=""';'--"'0-5%Frf1
~
CONTROL
'"~
~
""
i:: 20 '-'
"
0
~--...... :s ""-
-
I
20
2
------
,f·0
JO't
2 20 200 2 20 200 CONCENTRATION OF SltfAZINE (ppm)
-
---
N-.......,~- J3%W./iC -....;:::::
60%W.HC.
9S
200
~ i"--...
-
~ H
2
20
200
w'HC.
SOIL pH
-.J
~
WHC
SOIL pH
TEtlPERATURE
359
5·0
H
9'6
-......
33%W.HC 60%W./iC.
2 20 200 2 20 200 CONCENTRATION OF Sft1AZINE (ppm)
Fig. 1. Changes in bacterial population in soil treated with simazine under the influence of different ecological factors during incubation period of 10 weeks. Fig. 2. Changes in actinomycetes population in soil treated with simazine under the influence of different ecological factors during incubation period of 10 weeks.
indicates that some of the soil actinomycetes may be involved in degrading simazine into various non-toxic intermediate compounds. By improving the moisture status, raising the temperature, and liming the soil, the period of effectiveness of 2,4-D and some other herbicides (NEWMAN and NORMAN 1947, HERNANDEZ and WARREN 1950) was diminished. Azotobacter population was significantly increased, due to application of simazine at the recommended field rate (2 ppm), where as higher levels up to 200 ppm had no adverse effect (Fig. 3). Stimulatory effect of triazine herbicides on Azotobacter population was recorded by other workers as well (TIMOFEEV and MONSEEV 1965, GOGUADZE 1967, and KULINSKA 1967). Simazine had almost similar effect on the number of Azotobacter in the FYM-amended soil as well. Azotobacter population was unaffected by different dosages of simazine at 45 DC, whereas at 15 DC Azotobacter cells were not enhanced as a result of simazine application. The influence of simazine on Azotobacter was more or less the same in all the three conditions of soil reaction. The stimulatory effect of simazine on Azotobacter was noticed irrespective of soil moisture regimes. It is presumed that simazine might have been utilized by Azotobacter as carbon and nitrogen source.
360
~
A. C.
GAUR
and K. C.
FY.I1. 60,.-,--,--------,
MISRA'
TEI'fPERATURE
WHC.
SOIL pH
~ 40H--t------i ~ 1--":"--~--J33% WIiC.
~==~=~'O% WIiC.
flO
FY.!'1.
.....
~
o
I
--
TEI'fPERATURE
I
2
20
CONTROL
I - - 30°C
~
200
~-
l s"c
~SOC
2 20 200 2 20 200 CONCENTRATION OF S//'fAZINE (ppm)
---
2
20
200
WHC.
SOIL pH
2·0 % F.y.11. OS%F.Y.tf..~
9·, S·O
I
H
s·o
9·6
-
~-I--- 33% WHC
60% WHC.
2 20 200 2 20 200 CONCENTRATION OF SII'fAZINE (ppm)
Fig. 3. Changes in Azotobacter population in soil treated with simazine under the influence of different ecological factors during incubation period of 10 weeks. Fig. 4. Changes in fungal population in soil treated with simazine under the influence of different ecological factors during incubation period of 10 weeks.
The addition of simazine to soil at different concentrations reduced the soil fungal population during the entire test period (Fig. 4). At a normal rate and at a 1O-fold rate of simazine, a slight reduction in fungal population was observed, but with 100 times application the inhibitory effect was further accentuated. But NEPOMILUEV and KAZYAKINA (1967) reported that simazine and atrazine at 4.5 and 10 kg per hectare stimulated the growth of both Penicillium and Mucor spp. which actively participated in cellulose decomposition. Simazine application proved inhibitory to soil fungi in the presence of 0.5 per cent of FYM, but 2 per cent of FYM mitigated the toxicity of simazine and caused a significant stimulation of fungal population in soils treated with higher doses of the herbicide (20 and 200 ppm). Simazine was found to be more toxic to soil fungi at higher temperature, even at a normal dosage than at mesophilic and psychrophilic temperatures. The toxicity of the herbicide to the fungal flora was less under acidic and alkaline conditions of soil, as compared to neutral soil. This suggests that the fungal flora of neutral soil are more susceptible to simazine than the fungal flora, growing in soil adjusted to acidic and alkaline reactions. Similarly, simazine appeared to be less toxic to fungi under high soil moisture conditions than under low soil moisture regime. Acknowledgement The authors are grateful to Dr. N. viding the facilities.
S. SUBBA RAO,
Head of the Division of Microbiology, for pro·
Dynamics of Microbial Population in Soil
361
References ALLEN, O. N.: Experiments in soil bacteriology. Burges Publishing Co., Minneapolis, Minnesota 1957. GOGUADZE, V. D.: Effect of simazine on microflora development under various soil conditions in Western Georgia (USSR): Subtrop. Kul'tury (1967),148. HERNANDEZ, T. P., and WARREN, G. F.: Some factors affecting the rate of inactivation and leaching of 2,4-D in different soils. Proc. Amer. Soc. Hort. Sci. 56 (1950), 287. KULINSKA, D.: The effect of simazine on soil microorganisms. Roczn. Nauk roln. 93A (1967), 229. MISRA, K. C., and GAUR, A. C.: Tolerance of Azotobacter to some herbicides. Ind. J. Weed. Sci. 3 (1971), 99. NAYYAR, V. K., RANDHAWA, N. S., and CHOPRA, S. L.: Effect ofsimazine on nitrification and microbial population in sandy loam soil. Ind. J. Agric. Sci. 40 (1970), 445. NEPOMILUEV, V. F., and KAzYAKINA, T. I.: Effect of herbicides on microflora of peat bog soils. Izv. Timiryazev. Selkhoz. Akad. 4 (1967), 84. NEMAN, A. S., and NORMAN, A. G.: Effect of soil microorganisms on the persistence of plant growth regulators in soil. J. Bact. 54 (1947), 37. POCHON, J., TARDIEUX, P., and CHARPENTIER, M.: Recherches sur les interactions entre les amino triazines herbicides et la microflore bacterienne tellurique. Compt. Rend. 250 (1960), 1555. SOSNOVSKAYA, E. A., and PASHCHENKO, P. D.: The effect of herbicides on microflora of the soil under maize. Mater. Tezisy VI. Konf. Khim. Sel' Khoz Oyenburg. 1 (1965), 179. TALBERT, R. E., and FLETCHALL, O. H.: Inactivation of simazine and atrazine in the field. Weeds 12 (1964),33. TIMOFEEV, V. A., and MONSEEV, A. N.: Micropopulation of serozems and the rhizosphere of maizein relation to the use of herbicides. 'trudy Kirgiz. nauchnoissled. Inst. Zeml. 5 (1965), 123. Authors' address: Dr. A. C. GAUR, and Dr. K. C. MISRA, Division of Microbiology, Indian Agricultural Research Institute, New Delhi-llOOI2, India.
24 Zbl. Bakt. II. Abt., Bd, 133