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The influence of some pesticides on soil microbial flora in relation to changes in nutrient level, rock phosphate solubilization and P release under laboratory conditions
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Agriculture Ecosystems Envlrorm'~nt ELSEVIER
Agriculture, Ecosystems and Environment 65 (1997) 59-68
The influence of some pesticides on soil microbial flora in relation to changes in nutrient level, rock phosphate solubilization and P release under laboratory conditions L.B. T a i w o a,*, B.A. Oso b a Soil Microbiology Unit, Institute of Agricultural Research and Training, P.M.B. 5029, Moor Plantation, Ibadan, Nigeria b Department of Botany and Microbiology, University oflbadan, lbadan, Nigeria Accepted 7 March 1997
Abstract Two soil types, loamy sand and sandy soils, were treated with atrazine, pyrethrin and a mixture of metobromuron and metolachor for eight weeks in the laboratory to determine the effect of the chemicals on soil microbial populations and their mineralization activities. The experiment was also aimed at evaluating the changes that occurred in soil nutrient levels as a result of the pesticide treatments. Significant reductions in microbial populations after an initial rise, as well as in percentage carbon, nitrogen, potassium and pH were recorded in the two treated soil types. Eight bacterial isolates, two actinomycetes and five fungal species were identified in the control loamy sand soil. Marked reductions in species numbers were recorded in the treated soils as compared to the control. Compared to similar species of organisms isolated from control soils, Pseudomonas, Achromobacter, Bacillus spp., Aspergillus niger and Streptomyces sp. isolated from the treated soils released less phosphorus when inoculated into a broth containing 'Sokoto' rock phosphate complex as the only source of P nutrition. © 1997 Elsevier Science B.V. Keywords: Microbes; Nutrients; Pesticides; Soil
1. Introduction Incessant and indiscriminate use of some agrochemicals in agricultural production has elicited fears of changes in microbial populations and the activities o f individual species of micro-organisms. Moorman (1989) found that total microbial populations in soils are unaffected or slightly affected by pesticide applications but populations and activities of individual groups of microorganisms (e.g. cellulose decom-
* Corresponding author.
posers) are largely affected. He also noted that pesticide concentrations greater than recommended doses do cause interruptions in microbial activities. For instance, Suneja and Dogra (1984) found a decrease in the number of rhizobial survivors on the seeds of Bengal gram (Cicer arietinum) treated with Aldrin and lindane and inoculated with chickpea rhizobium. The adverse effect resulted in a reduction in nodule number, yield and N content at all concentrations of insecticides in c o m p a r i s o n with the control. Heinonen-Tanski et al. (1989) studied the effects of a pesticide programme (thiram, hymexazol,
0167-8809/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 8 8 0 9 ( 9 7 ) 0 0 0 4 4 - 3
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L.B. Taiwo, B.A. Oso /Agriculture, Ecosystems and Environment 65 (1997) 59-68
dimethoate, phenmedipham and metamitron and alloxidin-Na) on soil microorganisms and sugar beet yields in Perinio and Laukaa and found that the most sensitive of the microorganisms were the ureolytic and the dehydrogenase types. Soil dehydrogenase and nitrification were earlier found to be affected only slightly by chlorsulfuron and glyphosphate (Heinonen-Tanski et al., 1985). However, Malkomes (1985) found that long-term soil microbial respiration was inhibited more strongly with increasing dosages of trichloroacetic acid (TCA) in the soil with higher humus content or with lucerne meal amendment. Early investigators, however, were of the opinion that not all pesticides have adverse effects on soil biota. For instance, Rosas and Carranza Destorani (1987) reported no effect of parathion on the growth and viability of Pseudomonas aeruginosa in soil. However, Schuster and Schroder (1990) found only slight and short-lived side effects on microbial activities which usually disappeared before further treatments. Studies particularly relating to the effects of continuous application of some of these pesticides (e.g. Atrazine, metobrumoron and metolachor and Pyrethrin commonly used in Nigeria) on soil microbial populations and their activities have not been fully reported in Nigeria. The experiments were, therefore, carried out to assess the effects of the pesticides on nutrient and microbial composition of loamy sand and sandy soils and the ability of some of the pesticide-affected microorganisms to acidulate and cause the release of phosphorus (P) from rock phosphate complex.
2. Materials and methods
2.1. Materials Two soil types, loamy sand and sandy soils, were used. Both soils were taken from plots previously grown to maize in the University of Ibadan and the institute of Agricultural Research and Training (IAR &T), Moor Plantation, both in Ibadan, Nigeria. These plots had no history of pesticide application. The samples of soil were taken from the upper layer of the arable soil (0-20 cm) and sieved with a 2.00 mm width mesh sieve.
2.2. Methods 2.2.1. Laboratory analysis The soil sub-sample pH before pesticide treatment was measured in water at 1:1 soil/water ratio. Soil nitrogen was determined using the micro-Kjeldahl method according to Bremer (1960) while organic carbon was determined by oxidation with sulphuric acid using the method of Walkley and Black (1934) as modified by Black (1965). Exchangeable cations were leached from soil with N ammonium acetate (pH 7) before K was determined on a (EEL) flame photometer (A.O.A.C., 1980). Available P was extracted with Bray I extractant (Bray and Kurtz, 1954) and measured on a spectro - 2 0 electro photometer following the procedure described by Jackson (1958). Hutchinson's agar, as modified by Bhat and Shetty (1949) and consisting of Glucose, 10.0 g; Peptone, 0.05 g; K2HPO 4, 0.50 g; MgSO 4. 7H20, 0.020 g; KNO 3, 0.05 g and agar, 20.0 g in 1000.0 ml distilled water (pH 6.8-7.0) was used for the enumeration of bacteria and actinomycetes. Potato dextrose agar with 10 ppm streptomycin was used to enumerate fungi. 2.2.2. Preliminary incubation experiment 1 kg of loamy sand soil per pesticide was treated with water solution of the pesticide in plastic cup at the following rates: 1. atrazine (100m151 -l of water) 2. metobromuron and metochor (110ml 51- ~ of water) 3. pyrethrin (20 ml 21-1 ). All the rates were twice the recommended dosages. The rates were chosen to simulate the quantities practically applied to the field with empty milk or tomato puree tins by the illiterate peasant farmers in Nigeria. The pesticides were mixed with the soil in each case and packed into a 21 plastic cup. The control soil was treated with distilled water. Each treatment was replicated four times and arranged on the laboratory bench at room temperature. At 5 and 10 weeks after treatment, the nutrient compositions of the pesticide-treated and control soils were determined. 2.2.2.1. Further investigation. In a further investigation, a similar experimental approach to the one described in the preliminary investigation above was
L.B. Taiwo, B.A. Oso / Agriculture, Ecosystemsand Environment 65 (1997) 59-68 adopted but glucose (energy source) was added to two of the four replicated soils. Populations of fungi, bacteria and actinomycetes were enumerated weekly by' means of the dilution plate counts using modified Hutchinson's agar described above as growth medium. Nutrient compositions of the soils were determined 8 weeks after incubation.
2.2.3. Isolation and identification of microorganisms Plates were observed and isolation made of various groups of isolate colonies from growth plates of appropriate dilutions onto fresh agar medium. By repeated streaking, pure isolates were obtained and stored at 4°C. Biochemical characterization and identification of various bacterial isolates were carried out according to the methods of Holding and Colle (1971) and Buchanan and Gibbons (1974). Identification of actinomycetes was by the methods of the international Streptomyces project (Shirling and Gottlieb, 1966). Fungal isolates were identified with reference to Gilman (1975) and Barnett (1960). 2.2.4. Rock phosphate solubilization and estimation of P released.from 'Sokoto' rock phosphate powder Ammonium sulphate-yeast extract glucose (AYG) medium (Halder et al., 1990) was used in this experiment with 'Sokoto' powdered phosphate rock ( ~ 240 mesh size) obtained from Sokoto, Sokoto state of Nigeria as a source of insoluble phosphate. The medium contained: glucose, 20.0 g; ( N H 4 ) 2 8 0 4 , 1.0 g; Mg SO4 • 7H20, 0.5 g; yeast extract, 0.2 g; FeC13, 2.0 rag; MnSO4-H20, 4.0 mg in 1t of distilled water. After dissolving, the pH of the medium was adjusted to 6.8 before dispensing into a number of 200 ml flasks at 100 ml per flask. Following autoclaving at 121°C for 15min, the flasks were inoculated with the individual isolates in each case and incubated for 3 days on a rotary shaker at 28°C. Three flasks were used for each microbial isolate and three for the uninoculated control. The contents of the culture flasks at the end of incubation period were centrifuged at 15000g for 20min to remove biomass and unsolubilized matter. The soluble phosphates, expressed as equivalent phosphorus (P), were determined as a measure of the extent of solubilization of rock phosphate (Halder et al., 1990). Analysis of pesticide-treated soil samples for %C,
61
%N, available P and K were as previously described above.
3. Results and discussion
Results of soil analysis prior to pesticide treatment are shown in Table 1. Percentage organic carbon and nitrogen as well as the available phosphorus were low. The pH values indicated that the soils were of near neutral reaction. Figs. 1 and 2 show the effect of atrazine, metobromuron and metolachor and pyrethrin treatment on the loamy sand soil microflora. There was a general increase in bacterial and actinomycetes and fungal populations with bacterial and actinomycetes attaining a peak 3 weeks after treatment with the pesticides, while fungi reached a peak 2 weeks after treatment. In the pesticide-treated sandy soil, a similar trend of increase in microbial populations which attained a peak at about 3 weeks after treatment for bacteria and actinomycetes and 4 weeks for fungi is graphically presented in Figs. 3 and 4. This trend suggested an increase in nutrient availability and growth-promoting ability of the pesticides possibly owing to their structural modifications by the soil environment. For instance, Ali (1990) found in a study of the behaviour and interaction of pesticides with soil clays that the organic matter presence in soil is the most important factor that involved atrazine complexing in the soil clay. Calcium carbonate, montmorillonite and kaolinite also showed their ability of
Table 1 Some chemical and biological characteristics of the loamy sand and sandy soils prior to chemical treatment Loamysand Organic carbon (%) 0.93 Nitrogen (%) 0.07 Available P (ppm) 7.71 Exchangeable K (meq 100g- i ) 0.35 pH (soil: H20) 7.22 Sand (%) 89.2 Silt (%) 1.2 Clay (%) 9.6 Viable cell counts (bact. and actin.) 7.3 X 10 7 Fungi 2.3 x 104
Sandysoil 0.90 0.09 7.40 0.27 7.0 90.6 5.8 3.6 7.0X 107 2.0x 104
L.B. Taiwo, B.A. Oso /Agriculture, Ecosystems and Environment 65 (1997) 59-68
62
complexing with atrazine herbicide thus reducing its potency on target and non-target organisms. The complexing effect had probably allowed for the logarithmic growth (on soil nutrients) observed within 2 - 4 weeks after the commencement of pesticide treatments. Other factors that may be responsible for
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the logarithmic growth were accelerated decomposition of organic matter to release its nutrients and release of nutrients in the killed microbial biomass. This explanation is in harmony with the stimulations observed by Malkomes (1985) of nitrogen mineralization in humus soil after lucerne meal soil amend-
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Fig. 1. (a) Bacterial and actinomycetes population growth pattern in loamy sandy soil treated with: (a I) x x, atrazine plus glucose; (a 2) x---x, atrazine alone; (a 3) z x - - A, control. (b) Bacterial and actinomycetes population growth pattern in loamy sand soil treated with: (b I ) D - [3, metobromuron and metolachor plus glucose; (b 2) [3---[3, metobromuron and metolachor alone; (b 3) zx--zx, control. (c) Bacterial and actinomycetes population growth pattern in loamy sand soil treated with: (c I ) C) ©, pyrethrin plus glucose; (c 2) O - - - O , pyrethrin alone; (c 3) , x - - zx, control.
L.B. Taiwo, B.A. Oso // Agriculture, Ecosystems and Encironment 65 (1997) 59-68 40
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Fig. 2. (a) Fungal population growth pattern in loamy sand soil treated with: (a I ) x - - x , atrazine plus glucose; (a 2) x---x, atrazine alone; (a3) A - - - - - A , control. (b) Fungal population growth pattern in loamy sand soil treated with: (b l) [ ] I~, metobromuron and metolachor plus glucose; (b 2) [ ] - - - [ ] , metobromuron and metolachor alone; (b 3) -----A, control. (c) Fungal population growth pattern in loamy sand soil treated with: (c I ) (3 (3, pyrethrin plus glucose; (c_~) © - - - ( 3 , pyrethrin alone; (c 3) A - - - - - - A , control.
Fig. 3. (a) Bacterial and actinomycetes population growth pattern in sandy soil treated with: (a t ) x x, atrazine plus glucose; (a 2) x---x, atrazine alone; (a 3) a - - - - - z X , control. (b) Bacterial and actionomycetes population growth pattern in sandy soil treated with: (b~) [] E , metobromuron and metolachor plus glucose; (b 2) [ ] - - - [], metobromuron and metolachor alone; (b 3) z x - - zx, control. (c) Bacterial and actinomycetes population growth pattern in sandy soil treated with: (c I ) ( 3 - O, pyrethrin plus glucose; (c 2) (3---(3, pyrethrin alone; (c 3) a - - - A, control.
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L.B, Taiwo, B.A. Oso / Agriculture, Ecosystems and Enuironment 65 (1997) 59-68
ment. The population growth on these nutrients could be within a few days after soil application of pesticides. For example, Kunc et al. (1985) found that the amount of bacterial decomposers of 2,4-dichloropheno-oxyacetic acid in soil increased from initial 5% to 54% after 14 days and to 95% after 35 days of incubation. The authors' observation is consistent with Smith and Mayfield's (1977) results of proliferation of bacteria and fungi during a 14-day period of soil treatment with a commercial formulation of paraquat (1,1'-dimethyl-4,4'-bipyridinium). The soil microbial populations in this study could possibly have grown and proliferated on the applied chemicals. A similar observation was made by Venkatramesh and Agrihothrudu (1988) using captofol in soils. Following each peak, however, was a fall in the microbial population level. The decrease suggests that some of the microbial population that were tolerant of the pesticides were susceptible to the products of soil-pesticide interactions which could have possibly been bactericidal or fungicidal. This assumption could be true for the chloro-content of atrazine (2, chloro-4-ethylamino-6-isopropyl amino-s-triazine) and ionic radicals of metobromomuron ( N-(-4-bromophenyl)-N-methoxy-N-methyl urea) and metolachor (2-ethyl-6-methyl-N-) 2 methyloxyl-I-methyl) chloroaceto-nilide) which were capable of constituting organic inhibitors after ionization in the soil solution (Alexander, 1977). The impact may be more profound than the original pesticides depending, however, on the level of susceptibility of microbial activity especially respiration and nitrification (Alexander, 1977). For example, Corke and Thompson (1970) found that degradation products of some phenylamide herbicides were inhibitory to nitrification. The impacts of biodegradadon products of pesticides or products of soil-pesti-
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LB. Taiwo, B.A. Oso / Agriculture, Ecosystems and Environment 65 (1997) 59-68
65
Table 2 Microbial isolates from control soils and chemically treated loamy sand and sandy soils 8 weeks after treatment Treatments
Loamy sand soil
Sandy soil
Treated
Control soil
Treated
Control soil
Atrazine
Pseudomonas sp. Archromobacter sp. Bacillus sp. Streptomyces sp. Aspergillus niger
Pseudomonas sp. Achromobacter sp. Bacillus sp. Mycobacterium sp. Agrobacterium sp.
Pseudomonas sp. Bacillus sp.
Pseudomonas sp. Achromobacter sp. Bacillus sp. Thiobacillus sp.
Pyrethrin
Pseudomonas sp. Achromobacter sp. Bacillus sp.
Nitrosomonas sp. Nitrobacter sp. Thiobacillus sp. AspergiUus niger
Pseudomonas sp. Achromobacter sp. Bacillus sp.
Aspergillus niger Fusarium sp. Mucor sp. Streptomyces sp.
Metobromuron and metolachor
Bacillus sp. Pseudomonas sp.
Fusarium sp. Mucor sp. Cephalosporium sp. Streptomyces sp. Nocardia sp.
cide interactions are generally unknown and need in-depth investigations to unravel them. Nevertheless, the presence of combinations or sequences of several compounds in soil besides the application of maximum dosage of the chemicals under study may have resulted in the dramatic fall in population levels of bacterial and actinomycetes and gentle fall of fungi-fungi being able to sporulate heavily and fragment easily. Results of simultaneous addition of glucose and pesticides (pyrethrin or atrazine) to the test soils indicate that there were general increases in microbial population levels compared to pesticide treated soil without glucose. Specifically, glucose caused increased bacterial and actinomycetes population levels in atrazine and pyrethrin-treated loamy sand soil and fungal populations in atrazine treated loamy sand soil (Figs. 1 and 2). A similar trend of population increase in sandy soil was obtained (Figs. 3 and 4). The increases were presumably caused by the availability of metabolizable energy source in the soil solution. In a similar simultaneous addition of glucose and herbicide, Kunc et al. (1985) observed initial mineralization of glucose before the degradation of herbicide--the rate of degradation process accelerated in comparison with the variant without glucose suggesting increased microbial activities. Nevertheless a stimulated process of mineralization
Bacillus sp.
was not observed in loamy sand and sandy soil simultaneously treated with glucose and a mixture of metobromuron and metolachor especially after the 4th week. Instead a general decrease in bacterial and actinomycetes counts and in the fungal population levels when compared to similar treatments without glucose (Figs. 1 and 4) was obtained, suggesting a lethal effect of the chemical plus glucose. Table 2 shows the fungal, bacterial and actinomycetes isolates from the pesticide-treated loamy sand and sandy soils. Generally, there was a marked reduction in species number. For instance, Mycobac-
terium, Agrobacterium, Nitrosomonas, Nitrobacter, and Thiobacillus spp. were completely eliminated in Table 3 Rock phosphate solubilization and phosphorus released by test isolates from both pesticide-treated and untreated soils Organisms
No inoculation (reference) Streptomyces sp. Pseudomonas sp. Achromobacter sp. Bacillus sp. Aspergillus niger
Isolates from control soil
Isolates from treated soil
pH
P (ppm)
pH
P (ppm)
6.7 3.4 3.7 3.2 3.7 3.6
0.0 1.47 2.66 0.69 1.71 1.11
6.7 4.1 4.3 3.9 3.9 3.6
0.05 0.72 0.40 0.46 0.70 0.64
All values are means of three replicates.
L.B. Taiwo, B.A. Oso /Agriculture, Ecosystemsand Environment 65 (1997) 59-68
66
Table 4 Some chemical characteristics of the soil after 5 and 10 weeks of continuous treatment with each of atrazine, metobromunon and metolachor, and pyrethrin Treatment
Soil + water (control) Soil + atrazine Soil + metobromuron and metolachor Soil + pyrethrin CV %
5 weeks
10 weeks
%C
%N
Available P (ppm)
pH soil/ water 1: 1
%C
%N
Available P (ppm)
pH soil// water 1:1
1.15a 1.14a 1.13~
0.10 b 0.07 a 0.05 a
14.22 b 14.20 a 14.10 a
7.0 ~ 6.7 a 6.9 ~
1.37 b 1.10a 1.12 ~
0.12 b 0.07 b 0.04 a
13.2 a 15.2ab 14.7~
6.9 ~ 6.7 a 6.5 ~
1.15a 9
0.05 a 8
14.18 ~ 10
7.0 a 8
1.22 b 15
0.07 b 20
15.9b 24
6.& 9
Figures followed by the same letter in a column are not significantly different according to Duncan's Multiple Range Test (P < 0.05). the treated soils. Similarly, Fusarium, Mucor, Cephalosporium, Penicillium and Nocardia spp., found in the control soils w e r e e l i m i n a t e d in the c h e m i c a l l y treated soils. The selective and lethal effect on the non-target m i c r o o r g a n i s m s m a y possibly be due to group differences in sensitivity to a g r o c h e m i c a l s ; while s o m e pesticides are toxic to one group o f organisms, the pests they are d e s i g n e d to control, they are m e t a b o l i z e d to yield products inhibitory to entirely dissimilar organisms. G e n e r ally, pesticides used in agriculture often display toxic effects on various o r g a n i s m s including non-target m i c r o b e s because m a n y m e t a b o l i c processes are c o m m o n to all cellular organisms. T h e c o n s e q u e n c e o f these effects is the disruption or termination o f biological cycling processes associated with the affected organisms, thus stopping or altering the dyn a m i c e q u i l i b r i u m hitherto in place in the soil environment. Results o f rock phosphate solubilization and phosphorus released into the broth culture by s o m e o f the m i c r o b i a l isolates o b t a i n e d f r o m m e t o b r o m u r o n and m e t o l a c h o r - t r e a t e d and control soils are s h o w n in T a b l e 3. W h i l e the m i c r o b i a l isolates obtained f r o m control soils w e r e able to effect a release o f rela-
tively high a m o u n t o f phosphorus f r o m ' S o k o t o ' rock phosphate into the broth culture solution, the isolates obtained in m e t o b r o m u r o n and m e t o l a c h o r treated soil released less suggesting inhibition effect o f the pesticide on the isolate capacity to induce P mineralization through e n z y m e action. Shabana (1987) ascribed a similar observation to inhibition o f e n z y m e processes. Nutrient c o m p o s i t i o n o f l o a m y sand soils treated with atrazine, pyrethrin and m e t o b r o m u r o n and m e t o l a c h o r in the preliminary study is s h o w n in Table 4. C o m p a r e d to the control, atrazine, metobrom u r o n and m e t o l a c h o r elicited reductions in percentage carbon and soil nitrogen c o m p o s i t i o n s ; the reduction was significant ( P < 0.05) in nitrogen at both sampling periods for m e t o b r o m u r o n and metolachor indicating adverse e n v i r o n m e n t a l consequences o f repeated and relatively l o n g - t e r m application o f s o m e pesticides. H o w e v e r , phosphorus content m a r k e d l y increased with pesticide application. Results o f a further evaluation o f fertility status o f pesticide-treated l o a m y sand and sandy soil for 8 w e e k s are s h o w n in Tables 5 and 6 T h e r e was a decrease in p e r c e n t a g e carbon, nitrogen, available K and p H in the treated soil types c o m p a r e d to the
Table 5 Some chemical characteristics of the soil after 8 weeks of continuous treatment with various pesticides Pesticide treated soil and control
%C
%N
Available P (ppm)
K (meq 100 g- t )
pH soil/water 1:1
Soil + Soil + Soil + Soil +
0.93 a 0.72 b 0.51 a 0.81 c
0.07 c 0.05 abe 0.03 a 0.06 bc
7.7 a 12.5cd 12.7d 11.8 boa
0.35 b 0.27 a 0.33 ab 0.29 ab
7.2 b 6.7 c 5.3 a 6.1b
water (control) atrazine metobromuron and metolachor pyrethrin
Figures followed by the same letter in a column are not significantly different according to Duncan's Multiple Range Test (P < 0.05).
L.B. Taiwo, B.A. Oso /Agriculture, Ecosystems and Environment 65 (1997) 59-68
67
Table 6 Some chemical characteristics of the soil after treatment with various pesticides for 8 weeks Pesticide treated soil and control
%C
%N
Available P (ppm)
K (meq 100 g - l )
pH soil/water 1:1
Soil + Soil + Soil + Soil + CV %
0.90 d 0.70 b 0.61 a 0.74 c 25
0.09 c 0.06 ab 0.4 ~ 0.07 bc 21
7.40 a 12.06 d 11.92 c 11.76 b 6
0.27 a 0.22 a 0.26 bed 0.26 Cd 6
7.0 c 6.2 a 6.1 ~ 6.6 b 8
water (control) atrazine metobromuron and metolachor pyrethrin
Figures followed by the same letter in a column are not significantly different according to Duncan's Multiple Range Test ( P < 0.05).
control indicating an impairment of nutrient cycling processes. Munch et al. (1989) attributed a similar observation to the modifications of the spectrum of certain (dormant) species of some soil flora. For instance, common genera of NO2-oxidizing bacteria were suppressed by some chemicals while new groups, e.g. NH4-oxidizing and denitrifying bacteria became very active. In contrast to reductions in carbon and nitrogen composition however, available P increased in all the treated soils compared to the control, indicating an enhanced solubilization of soil phosphate complex. Abdel-All et al. (1990) similarly increased the mineralization of organic phosphorus in sandy clay loam soil with gesaprim FW formulation, thus making more phosphorus available in soil solution. In contrast however, low amount of available P in the broth culture inoculated with the organisms isolated from metobromuron and metolachortreated soil (Table 3), when compared with organisms from control soil, indicated reduced capacity of the pesticide-treated isolate to optimally induce 'Sokoto' rock phosphate solubilization. The apparent inconsistency regarding the effect of pesticide on P released in soil and broth solution can be explained by the dissimilar conditions in soils and broth media. While there was a wide range of nutrients in soils for enhanced metabolic activities of micro organisms, limited nutrient composition in a defined medium such as yeast extract-mannitol (YEM) broth could be responsible for less activity. The nutrients, when present, as in soil, may have enhanced the phosphatase activities of the soil microflora, thus, releasing P from phosphate complex. Adsorption (through base exchange action) of the pesticide components by the clay fraction of the soil may have also reduced the potency of the pesticide. Generally, observations in this experiment indi-
cated a negative environmental consequence of doubling the dosages of the test pesticides on the soil microbial populations, their biological cycling activities and consequently, the mineralization of soil nutrients.
Acknowledgements We thank Mr. Nurudeen Olanrewaju for typing the manuscript.
References Abdel-All, A., El-Shahaat, M.S., Khattab, M.M., 1990. Soilpesticide interaction. 7. Effect of FW and WP formulations of Gesaprim and Primextra herbicides on nitrification and organic phosphorus mineralization processes in clay loam and sandy clay loam soils. Journal of Agricultural Research 14, 787-800. Alexander, M., 1977. Microbial transformation of phosphorus. In: Alexander, M., (Ed.), Introduction to Soil Microbiology. John Wiley and Sons, pp. 333-349. Ali, R.A., 1990. The behaviour and interaction of pesticides with soil clays in salt affected soils and its effect on the ions availability to monocotyledons and dicotyledons plants. Journal of Agricultural Research 14, 1991-2003. A.O.A.C. (Association of Official Analytical Chemists), 1980. Official Methods of Analysis, 13th edn. Washington, D.C. Barnett, H.L., 1960. Illustrated Genera of Imperfect Fungi, 2nd edn. Burgess Publishing Co., Minneapolis. Bhat, J.V., Shetty, M.V., 1949. A suitable medium for the enumeration of microorganisms in soil. Journal of the University of Bombay, Section B 13, 13-15. Black, C.A. (Ed.), 1965. Methods of soil analysis, part 2. Agronomy Monograph No. 9. American Society of Agronomy Inc. Publisher, Machison, Wisconsin, 1572 pp. Bray, R.H., Kurtz, L.T., 1954. Determination of total, organic and available forms of P in soil. Soil Science 59, 39--45. Bremer, J.M., 1960. Determination of nitrogen in soil by Kjeldahl
68
L.B. Taiwo, B.A. Oso / Agriculture, Ecosystems and Environment 65 (1997) 59-68
method. Journal of Agricultural Science (Cambridge) 55, I 123. Buchanan, R.E., Gibbons, W.E. (Eds.), 1974. Bergy's Manual of Determinative Bacteriology, 8th edn. Williams and Williams Company, Baltimore, pp. 1248. Corke, C.T., Thompson, F.R., 1970. Effects of some phenylamide herbicides and degradation products, on soil nitrification. Canadian Journal of Microbiology 16, 567-571. Gilman, J.C., 1975. A Manual of Soil Fungi, 2nd edn. Iowa State College Press, Ames. Halder, AIK., Mishra, A.K., Brattacharyya, P., Chaknabartlty, Y., 1990. Solubilization of rock phosphate by rhizobium and bradyrhizobium. Journal of General and Applied Microbiology 36, 81-92. Heinonen-Tanski, H., Montohen, L., Erivio, L.R., Junnila, S., Pessala, B., 1985. The effects of chlorsufuron, glyphosate, metrlbuzin and TCA on soil nitrification capacity and dehydrogenase activity. In: Comportement et effects secondaires des pesticides dans le sol, Versailles edn. Les Colloques de I'INRA, no. 31, INRA, Publication. Heinonen-Tanski, H., Simojoki, P., Raininko, K., Nuormala, N., Silvo, R., 1989. Effect of annual use of pesticides on soil microorganisms and sugar beet yields. Journal of Agricultural Science in Finland 61, 45-53. Holding, A.J., Colle, J.G., 1971. Isolation and characterization of bacteria in soils. In: Norris, J.R., Ribbons, D.W. (Eds.), Methods in Microbiology Academic Press, London 64, 1-32. Jackson, M.L. (Ed.), 1958. Soil Chemical Analysis. Prentice Hall Inc., Englewood Cliffs, New Jersey, pp. 498. Kunc, F., Tichy, P. and Vancura, V. (1985). 2,4 Dichlorophenoxyacetic acid in the soil: mineralization and changes in the counts of its bacterial decomposers. Comportement et effets secondaires des pesticides dans le sol, Versailles Ed. INRA Publ., (Les C011oques de I'INRA, No. 31). Malkomes, H.P. (1985). The influence of trichloroacetic acid (TCA) on microbial activities on soil under laboratory conditions. Comportment et effets secondaires des pesticides dans le
sol, Versailles edn. Les Colloques de I'INRA, no. 31, INRA publication.. Moorman, T.B., 1989. A review of pesticide effects on microorganisms and microbial processes related to soil fertility. Journal of Crop Production 2, 14-23. Munch, J.C., Glott, B., Henneberger, C., 1989. The effect of terbuthylazine on soil microorganisms and on the nitrogen cycle. Mitteilungen der Deutschen Bodenk und Lichen Gesellschaft 59, 603-606. Rosas, S.B., Carranza Destorani, M.M., 1987. The action of pesticides On microorganisms. I. Parathion. Toxicity Assessment 2, 293-303. Schuster, E., Schroder, D., 1990. Side effect of sequentially applied pesticides on non-target soil microorganisms--field experiment. Soil Biology and Biochemistry 22, 367-373. Shabana, E.F., 1987. Use of batch assays to assess the toxicity of atrazine on some selected cyanobacteria II. Effect of atrazine on heterocyst frequency, nitrogen and phosphorus metabolism of four heterocystous cyanobacteria, Journal of Basic Microbiology 27, 215-223. Shirling, F.B., Gottlieb, D., 1966. Identification of streptomyces in soil. International Journal of System Bacteriology 16, 313317. Smith, E.A., Mayfield, C.I., 1977. Effects of paraquat on selected microbial activities in soil. Microbial Ecology 3, 333-343. Suneja, S., Dogra, R.C., 1984. Effect of aldrin and lindane seed treatment on symbiosis of chickpea rhizobium with Bengal gram (Cicer arietinum). Tropical Agriculture 62, 110-114. Venkatramesh, M., Agrihothrudu, V., 1988. Persistence of captofol in soils with and without amendments and its effects on soil microflora. Bulletin of Environmental Contamination and Toxicology 41,548-555. Walkley, A., Black, C.A., 1934. An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chronic acid titration method. Soil Science 37, 29-38.
Agriculture Ecosystems Envlrorm'~nt ELSEVIER
Agriculture, Ecosystems and Environment 65 (1997) 59-68
The influence of some pesticides on soil microbial flora in relation to changes in nutrient level, rock phosphate solubilization and P release under laboratory conditions L.B. T a i w o a,*, B.A. Oso b a Soil Microbiology Unit, Institute of Agricultural Research and Training, P.M.B. 5029, Moor Plantation, Ibadan, Nigeria b Department of Botany and Microbiology, University oflbadan, lbadan, Nigeria Accepted 7 March 1997
Abstract Two soil types, loamy sand and sandy soils, were treated with atrazine, pyrethrin and a mixture of metobromuron and metolachor for eight weeks in the laboratory to determine the effect of the chemicals on soil microbial populations and their mineralization activities. The experiment was also aimed at evaluating the changes that occurred in soil nutrient levels as a result of the pesticide treatments. Significant reductions in microbial populations after an initial rise, as well as in percentage carbon, nitrogen, potassium and pH were recorded in the two treated soil types. Eight bacterial isolates, two actinomycetes and five fungal species were identified in the control loamy sand soil. Marked reductions in species numbers were recorded in the treated soils as compared to the control. Compared to similar species of organisms isolated from control soils, Pseudomonas, Achromobacter, Bacillus spp., Aspergillus niger and Streptomyces sp. isolated from the treated soils released less phosphorus when inoculated into a broth containing 'Sokoto' rock phosphate complex as the only source of P nutrition. © 1997 Elsevier Science B.V. Keywords: Microbes; Nutrients; Pesticides; Soil
1. Introduction Incessant and indiscriminate use of some agrochemicals in agricultural production has elicited fears of changes in microbial populations and the activities o f individual species of micro-organisms. Moorman (1989) found that total microbial populations in soils are unaffected or slightly affected by pesticide applications but populations and activities of individual groups of microorganisms (e.g. cellulose decom-
* Corresponding author.
posers) are largely affected. He also noted that pesticide concentrations greater than recommended doses do cause interruptions in microbial activities. For instance, Suneja and Dogra (1984) found a decrease in the number of rhizobial survivors on the seeds of Bengal gram (Cicer arietinum) treated with Aldrin and lindane and inoculated with chickpea rhizobium. The adverse effect resulted in a reduction in nodule number, yield and N content at all concentrations of insecticides in c o m p a r i s o n with the control. Heinonen-Tanski et al. (1989) studied the effects of a pesticide programme (thiram, hymexazol,
0167-8809/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 8 8 0 9 ( 9 7 ) 0 0 0 4 4 - 3
60
L.B. Taiwo, B.A. Oso /Agriculture, Ecosystems and Environment 65 (1997) 59-68
dimethoate, phenmedipham and metamitron and alloxidin-Na) on soil microorganisms and sugar beet yields in Perinio and Laukaa and found that the most sensitive of the microorganisms were the ureolytic and the dehydrogenase types. Soil dehydrogenase and nitrification were earlier found to be affected only slightly by chlorsulfuron and glyphosphate (Heinonen-Tanski et al., 1985). However, Malkomes (1985) found that long-term soil microbial respiration was inhibited more strongly with increasing dosages of trichloroacetic acid (TCA) in the soil with higher humus content or with lucerne meal amendment. Early investigators, however, were of the opinion that not all pesticides have adverse effects on soil biota. For instance, Rosas and Carranza Destorani (1987) reported no effect of parathion on the growth and viability of Pseudomonas aeruginosa in soil. However, Schuster and Schroder (1990) found only slight and short-lived side effects on microbial activities which usually disappeared before further treatments. Studies particularly relating to the effects of continuous application of some of these pesticides (e.g. Atrazine, metobrumoron and metolachor and Pyrethrin commonly used in Nigeria) on soil microbial populations and their activities have not been fully reported in Nigeria. The experiments were, therefore, carried out to assess the effects of the pesticides on nutrient and microbial composition of loamy sand and sandy soils and the ability of some of the pesticide-affected microorganisms to acidulate and cause the release of phosphorus (P) from rock phosphate complex.
2. Materials and methods
2.1. Materials Two soil types, loamy sand and sandy soils, were used. Both soils were taken from plots previously grown to maize in the University of Ibadan and the institute of Agricultural Research and Training (IAR &T), Moor Plantation, both in Ibadan, Nigeria. These plots had no history of pesticide application. The samples of soil were taken from the upper layer of the arable soil (0-20 cm) and sieved with a 2.00 mm width mesh sieve.
2.2. Methods 2.2.1. Laboratory analysis The soil sub-sample pH before pesticide treatment was measured in water at 1:1 soil/water ratio. Soil nitrogen was determined using the micro-Kjeldahl method according to Bremer (1960) while organic carbon was determined by oxidation with sulphuric acid using the method of Walkley and Black (1934) as modified by Black (1965). Exchangeable cations were leached from soil with N ammonium acetate (pH 7) before K was determined on a (EEL) flame photometer (A.O.A.C., 1980). Available P was extracted with Bray I extractant (Bray and Kurtz, 1954) and measured on a spectro - 2 0 electro photometer following the procedure described by Jackson (1958). Hutchinson's agar, as modified by Bhat and Shetty (1949) and consisting of Glucose, 10.0 g; Peptone, 0.05 g; K2HPO 4, 0.50 g; MgSO 4. 7H20, 0.020 g; KNO 3, 0.05 g and agar, 20.0 g in 1000.0 ml distilled water (pH 6.8-7.0) was used for the enumeration of bacteria and actinomycetes. Potato dextrose agar with 10 ppm streptomycin was used to enumerate fungi. 2.2.2. Preliminary incubation experiment 1 kg of loamy sand soil per pesticide was treated with water solution of the pesticide in plastic cup at the following rates: 1. atrazine (100m151 -l of water) 2. metobromuron and metochor (110ml 51- ~ of water) 3. pyrethrin (20 ml 21-1 ). All the rates were twice the recommended dosages. The rates were chosen to simulate the quantities practically applied to the field with empty milk or tomato puree tins by the illiterate peasant farmers in Nigeria. The pesticides were mixed with the soil in each case and packed into a 21 plastic cup. The control soil was treated with distilled water. Each treatment was replicated four times and arranged on the laboratory bench at room temperature. At 5 and 10 weeks after treatment, the nutrient compositions of the pesticide-treated and control soils were determined. 2.2.2.1. Further investigation. In a further investigation, a similar experimental approach to the one described in the preliminary investigation above was
L.B. Taiwo, B.A. Oso / Agriculture, Ecosystemsand Environment 65 (1997) 59-68 adopted but glucose (energy source) was added to two of the four replicated soils. Populations of fungi, bacteria and actinomycetes were enumerated weekly by' means of the dilution plate counts using modified Hutchinson's agar described above as growth medium. Nutrient compositions of the soils were determined 8 weeks after incubation.
2.2.3. Isolation and identification of microorganisms Plates were observed and isolation made of various groups of isolate colonies from growth plates of appropriate dilutions onto fresh agar medium. By repeated streaking, pure isolates were obtained and stored at 4°C. Biochemical characterization and identification of various bacterial isolates were carried out according to the methods of Holding and Colle (1971) and Buchanan and Gibbons (1974). Identification of actinomycetes was by the methods of the international Streptomyces project (Shirling and Gottlieb, 1966). Fungal isolates were identified with reference to Gilman (1975) and Barnett (1960). 2.2.4. Rock phosphate solubilization and estimation of P released.from 'Sokoto' rock phosphate powder Ammonium sulphate-yeast extract glucose (AYG) medium (Halder et al., 1990) was used in this experiment with 'Sokoto' powdered phosphate rock ( ~ 240 mesh size) obtained from Sokoto, Sokoto state of Nigeria as a source of insoluble phosphate. The medium contained: glucose, 20.0 g; ( N H 4 ) 2 8 0 4 , 1.0 g; Mg SO4 • 7H20, 0.5 g; yeast extract, 0.2 g; FeC13, 2.0 rag; MnSO4-H20, 4.0 mg in 1t of distilled water. After dissolving, the pH of the medium was adjusted to 6.8 before dispensing into a number of 200 ml flasks at 100 ml per flask. Following autoclaving at 121°C for 15min, the flasks were inoculated with the individual isolates in each case and incubated for 3 days on a rotary shaker at 28°C. Three flasks were used for each microbial isolate and three for the uninoculated control. The contents of the culture flasks at the end of incubation period were centrifuged at 15000g for 20min to remove biomass and unsolubilized matter. The soluble phosphates, expressed as equivalent phosphorus (P), were determined as a measure of the extent of solubilization of rock phosphate (Halder et al., 1990). Analysis of pesticide-treated soil samples for %C,
61
%N, available P and K were as previously described above.
3. Results and discussion
Results of soil analysis prior to pesticide treatment are shown in Table 1. Percentage organic carbon and nitrogen as well as the available phosphorus were low. The pH values indicated that the soils were of near neutral reaction. Figs. 1 and 2 show the effect of atrazine, metobromuron and metolachor and pyrethrin treatment on the loamy sand soil microflora. There was a general increase in bacterial and actinomycetes and fungal populations with bacterial and actinomycetes attaining a peak 3 weeks after treatment with the pesticides, while fungi reached a peak 2 weeks after treatment. In the pesticide-treated sandy soil, a similar trend of increase in microbial populations which attained a peak at about 3 weeks after treatment for bacteria and actinomycetes and 4 weeks for fungi is graphically presented in Figs. 3 and 4. This trend suggested an increase in nutrient availability and growth-promoting ability of the pesticides possibly owing to their structural modifications by the soil environment. For instance, Ali (1990) found in a study of the behaviour and interaction of pesticides with soil clays that the organic matter presence in soil is the most important factor that involved atrazine complexing in the soil clay. Calcium carbonate, montmorillonite and kaolinite also showed their ability of
Table 1 Some chemical and biological characteristics of the loamy sand and sandy soils prior to chemical treatment Loamysand Organic carbon (%) 0.93 Nitrogen (%) 0.07 Available P (ppm) 7.71 Exchangeable K (meq 100g- i ) 0.35 pH (soil: H20) 7.22 Sand (%) 89.2 Silt (%) 1.2 Clay (%) 9.6 Viable cell counts (bact. and actin.) 7.3 X 10 7 Fungi 2.3 x 104
Sandysoil 0.90 0.09 7.40 0.27 7.0 90.6 5.8 3.6 7.0X 107 2.0x 104
L.B. Taiwo, B.A. Oso /Agriculture, Ecosystems and Environment 65 (1997) 59-68
62
complexing with atrazine herbicide thus reducing its potency on target and non-target organisms. The complexing effect had probably allowed for the logarithmic growth (on soil nutrients) observed within 2 - 4 weeks after the commencement of pesticide treatments. Other factors that may be responsible for
17.0
the logarithmic growth were accelerated decomposition of organic matter to release its nutrients and release of nutrients in the killed microbial biomass. This explanation is in harmony with the stimulations observed by Malkomes (1985) of nitrogen mineralization in humus soil after lucerne meal soil amend-
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Fig. 1. (a) Bacterial and actinomycetes population growth pattern in loamy sandy soil treated with: (a I) x x, atrazine plus glucose; (a 2) x---x, atrazine alone; (a 3) z x - - A, control. (b) Bacterial and actinomycetes population growth pattern in loamy sand soil treated with: (b I ) D - [3, metobromuron and metolachor plus glucose; (b 2) [3---[3, metobromuron and metolachor alone; (b 3) zx--zx, control. (c) Bacterial and actinomycetes population growth pattern in loamy sand soil treated with: (c I ) C) ©, pyrethrin plus glucose; (c 2) O - - - O , pyrethrin alone; (c 3) , x - - zx, control.
L.B. Taiwo, B.A. Oso // Agriculture, Ecosystems and Encironment 65 (1997) 59-68 40
63
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Fig. 2. (a) Fungal population growth pattern in loamy sand soil treated with: (a I ) x - - x , atrazine plus glucose; (a 2) x---x, atrazine alone; (a3) A - - - - - A , control. (b) Fungal population growth pattern in loamy sand soil treated with: (b l) [ ] I~, metobromuron and metolachor plus glucose; (b 2) [ ] - - - [ ] , metobromuron and metolachor alone; (b 3) -----A, control. (c) Fungal population growth pattern in loamy sand soil treated with: (c I ) (3 (3, pyrethrin plus glucose; (c_~) © - - - ( 3 , pyrethrin alone; (c 3) A - - - - - - A , control.
Fig. 3. (a) Bacterial and actinomycetes population growth pattern in sandy soil treated with: (a t ) x x, atrazine plus glucose; (a 2) x---x, atrazine alone; (a 3) a - - - - - z X , control. (b) Bacterial and actionomycetes population growth pattern in sandy soil treated with: (b~) [] E , metobromuron and metolachor plus glucose; (b 2) [ ] - - - [], metobromuron and metolachor alone; (b 3) z x - - zx, control. (c) Bacterial and actinomycetes population growth pattern in sandy soil treated with: (c I ) ( 3 - O, pyrethrin plus glucose; (c 2) (3---(3, pyrethrin alone; (c 3) a - - - A, control.
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L.B, Taiwo, B.A. Oso / Agriculture, Ecosystems and Enuironment 65 (1997) 59-68
ment. The population growth on these nutrients could be within a few days after soil application of pesticides. For example, Kunc et al. (1985) found that the amount of bacterial decomposers of 2,4-dichloropheno-oxyacetic acid in soil increased from initial 5% to 54% after 14 days and to 95% after 35 days of incubation. The authors' observation is consistent with Smith and Mayfield's (1977) results of proliferation of bacteria and fungi during a 14-day period of soil treatment with a commercial formulation of paraquat (1,1'-dimethyl-4,4'-bipyridinium). The soil microbial populations in this study could possibly have grown and proliferated on the applied chemicals. A similar observation was made by Venkatramesh and Agrihothrudu (1988) using captofol in soils. Following each peak, however, was a fall in the microbial population level. The decrease suggests that some of the microbial population that were tolerant of the pesticides were susceptible to the products of soil-pesticide interactions which could have possibly been bactericidal or fungicidal. This assumption could be true for the chloro-content of atrazine (2, chloro-4-ethylamino-6-isopropyl amino-s-triazine) and ionic radicals of metobromomuron ( N-(-4-bromophenyl)-N-methoxy-N-methyl urea) and metolachor (2-ethyl-6-methyl-N-) 2 methyloxyl-I-methyl) chloroaceto-nilide) which were capable of constituting organic inhibitors after ionization in the soil solution (Alexander, 1977). The impact may be more profound than the original pesticides depending, however, on the level of susceptibility of microbial activity especially respiration and nitrification (Alexander, 1977). For example, Corke and Thompson (1970) found that degradation products of some phenylamide herbicides were inhibitory to nitrification. The impacts of biodegradadon products of pesticides or products of soil-pesti-
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LB. Taiwo, B.A. Oso / Agriculture, Ecosystems and Environment 65 (1997) 59-68
65
Table 2 Microbial isolates from control soils and chemically treated loamy sand and sandy soils 8 weeks after treatment Treatments
Loamy sand soil
Sandy soil
Treated
Control soil
Treated
Control soil
Atrazine
Pseudomonas sp. Archromobacter sp. Bacillus sp. Streptomyces sp. Aspergillus niger
Pseudomonas sp. Achromobacter sp. Bacillus sp. Mycobacterium sp. Agrobacterium sp.
Pseudomonas sp. Bacillus sp.
Pseudomonas sp. Achromobacter sp. Bacillus sp. Thiobacillus sp.
Pyrethrin
Pseudomonas sp. Achromobacter sp. Bacillus sp.
Nitrosomonas sp. Nitrobacter sp. Thiobacillus sp. AspergiUus niger
Pseudomonas sp. Achromobacter sp. Bacillus sp.
Aspergillus niger Fusarium sp. Mucor sp. Streptomyces sp.
Metobromuron and metolachor
Bacillus sp. Pseudomonas sp.
Fusarium sp. Mucor sp. Cephalosporium sp. Streptomyces sp. Nocardia sp.
cide interactions are generally unknown and need in-depth investigations to unravel them. Nevertheless, the presence of combinations or sequences of several compounds in soil besides the application of maximum dosage of the chemicals under study may have resulted in the dramatic fall in population levels of bacterial and actinomycetes and gentle fall of fungi-fungi being able to sporulate heavily and fragment easily. Results of simultaneous addition of glucose and pesticides (pyrethrin or atrazine) to the test soils indicate that there were general increases in microbial population levels compared to pesticide treated soil without glucose. Specifically, glucose caused increased bacterial and actinomycetes population levels in atrazine and pyrethrin-treated loamy sand soil and fungal populations in atrazine treated loamy sand soil (Figs. 1 and 2). A similar trend of population increase in sandy soil was obtained (Figs. 3 and 4). The increases were presumably caused by the availability of metabolizable energy source in the soil solution. In a similar simultaneous addition of glucose and herbicide, Kunc et al. (1985) observed initial mineralization of glucose before the degradation of herbicide--the rate of degradation process accelerated in comparison with the variant without glucose suggesting increased microbial activities. Nevertheless a stimulated process of mineralization
Bacillus sp.
was not observed in loamy sand and sandy soil simultaneously treated with glucose and a mixture of metobromuron and metolachor especially after the 4th week. Instead a general decrease in bacterial and actinomycetes counts and in the fungal population levels when compared to similar treatments without glucose (Figs. 1 and 4) was obtained, suggesting a lethal effect of the chemical plus glucose. Table 2 shows the fungal, bacterial and actinomycetes isolates from the pesticide-treated loamy sand and sandy soils. Generally, there was a marked reduction in species number. For instance, Mycobac-
terium, Agrobacterium, Nitrosomonas, Nitrobacter, and Thiobacillus spp. were completely eliminated in Table 3 Rock phosphate solubilization and phosphorus released by test isolates from both pesticide-treated and untreated soils Organisms
No inoculation (reference) Streptomyces sp. Pseudomonas sp. Achromobacter sp. Bacillus sp. Aspergillus niger
Isolates from control soil
Isolates from treated soil
pH
P (ppm)
pH
P (ppm)
6.7 3.4 3.7 3.2 3.7 3.6
0.0 1.47 2.66 0.69 1.71 1.11
6.7 4.1 4.3 3.9 3.9 3.6
0.05 0.72 0.40 0.46 0.70 0.64
All values are means of three replicates.
L.B. Taiwo, B.A. Oso /Agriculture, Ecosystemsand Environment 65 (1997) 59-68
66
Table 4 Some chemical characteristics of the soil after 5 and 10 weeks of continuous treatment with each of atrazine, metobromunon and metolachor, and pyrethrin Treatment
Soil + water (control) Soil + atrazine Soil + metobromuron and metolachor Soil + pyrethrin CV %
5 weeks
10 weeks
%C
%N
Available P (ppm)
pH soil/ water 1: 1
%C
%N
Available P (ppm)
pH soil// water 1:1
1.15a 1.14a 1.13~
0.10 b 0.07 a 0.05 a
14.22 b 14.20 a 14.10 a
7.0 ~ 6.7 a 6.9 ~
1.37 b 1.10a 1.12 ~
0.12 b 0.07 b 0.04 a
13.2 a 15.2ab 14.7~
6.9 ~ 6.7 a 6.5 ~
1.15a 9
0.05 a 8
14.18 ~ 10
7.0 a 8
1.22 b 15
0.07 b 20
15.9b 24
6.& 9
Figures followed by the same letter in a column are not significantly different according to Duncan's Multiple Range Test (P < 0.05). the treated soils. Similarly, Fusarium, Mucor, Cephalosporium, Penicillium and Nocardia spp., found in the control soils w e r e e l i m i n a t e d in the c h e m i c a l l y treated soils. The selective and lethal effect on the non-target m i c r o o r g a n i s m s m a y possibly be due to group differences in sensitivity to a g r o c h e m i c a l s ; while s o m e pesticides are toxic to one group o f organisms, the pests they are d e s i g n e d to control, they are m e t a b o l i z e d to yield products inhibitory to entirely dissimilar organisms. G e n e r ally, pesticides used in agriculture often display toxic effects on various o r g a n i s m s including non-target m i c r o b e s because m a n y m e t a b o l i c processes are c o m m o n to all cellular organisms. T h e c o n s e q u e n c e o f these effects is the disruption or termination o f biological cycling processes associated with the affected organisms, thus stopping or altering the dyn a m i c e q u i l i b r i u m hitherto in place in the soil environment. Results o f rock phosphate solubilization and phosphorus released into the broth culture by s o m e o f the m i c r o b i a l isolates o b t a i n e d f r o m m e t o b r o m u r o n and m e t o l a c h o r - t r e a t e d and control soils are s h o w n in T a b l e 3. W h i l e the m i c r o b i a l isolates obtained f r o m control soils w e r e able to effect a release o f rela-
tively high a m o u n t o f phosphorus f r o m ' S o k o t o ' rock phosphate into the broth culture solution, the isolates obtained in m e t o b r o m u r o n and m e t o l a c h o r treated soil released less suggesting inhibition effect o f the pesticide on the isolate capacity to induce P mineralization through e n z y m e action. Shabana (1987) ascribed a similar observation to inhibition o f e n z y m e processes. Nutrient c o m p o s i t i o n o f l o a m y sand soils treated with atrazine, pyrethrin and m e t o b r o m u r o n and m e t o l a c h o r in the preliminary study is s h o w n in Table 4. C o m p a r e d to the control, atrazine, metobrom u r o n and m e t o l a c h o r elicited reductions in percentage carbon and soil nitrogen c o m p o s i t i o n s ; the reduction was significant ( P < 0.05) in nitrogen at both sampling periods for m e t o b r o m u r o n and metolachor indicating adverse e n v i r o n m e n t a l consequences o f repeated and relatively l o n g - t e r m application o f s o m e pesticides. H o w e v e r , phosphorus content m a r k e d l y increased with pesticide application. Results o f a further evaluation o f fertility status o f pesticide-treated l o a m y sand and sandy soil for 8 w e e k s are s h o w n in Tables 5 and 6 T h e r e was a decrease in p e r c e n t a g e carbon, nitrogen, available K and p H in the treated soil types c o m p a r e d to the
Table 5 Some chemical characteristics of the soil after 8 weeks of continuous treatment with various pesticides Pesticide treated soil and control
%C
%N
Available P (ppm)
K (meq 100 g- t )
pH soil/water 1:1
Soil + Soil + Soil + Soil +
0.93 a 0.72 b 0.51 a 0.81 c
0.07 c 0.05 abe 0.03 a 0.06 bc
7.7 a 12.5cd 12.7d 11.8 boa
0.35 b 0.27 a 0.33 ab 0.29 ab
7.2 b 6.7 c 5.3 a 6.1b
water (control) atrazine metobromuron and metolachor pyrethrin
Figures followed by the same letter in a column are not significantly different according to Duncan's Multiple Range Test (P < 0.05).
L.B. Taiwo, B.A. Oso /Agriculture, Ecosystems and Environment 65 (1997) 59-68
67
Table 6 Some chemical characteristics of the soil after treatment with various pesticides for 8 weeks Pesticide treated soil and control
%C
%N
Available P (ppm)
K (meq 100 g - l )
pH soil/water 1:1
Soil + Soil + Soil + Soil + CV %
0.90 d 0.70 b 0.61 a 0.74 c 25
0.09 c 0.06 ab 0.4 ~ 0.07 bc 21
7.40 a 12.06 d 11.92 c 11.76 b 6
0.27 a 0.22 a 0.26 bed 0.26 Cd 6
7.0 c 6.2 a 6.1 ~ 6.6 b 8
water (control) atrazine metobromuron and metolachor pyrethrin
Figures followed by the same letter in a column are not significantly different according to Duncan's Multiple Range Test ( P < 0.05).
control indicating an impairment of nutrient cycling processes. Munch et al. (1989) attributed a similar observation to the modifications of the spectrum of certain (dormant) species of some soil flora. For instance, common genera of NO2-oxidizing bacteria were suppressed by some chemicals while new groups, e.g. NH4-oxidizing and denitrifying bacteria became very active. In contrast to reductions in carbon and nitrogen composition however, available P increased in all the treated soils compared to the control, indicating an enhanced solubilization of soil phosphate complex. Abdel-All et al. (1990) similarly increased the mineralization of organic phosphorus in sandy clay loam soil with gesaprim FW formulation, thus making more phosphorus available in soil solution. In contrast however, low amount of available P in the broth culture inoculated with the organisms isolated from metobromuron and metolachortreated soil (Table 3), when compared with organisms from control soil, indicated reduced capacity of the pesticide-treated isolate to optimally induce 'Sokoto' rock phosphate solubilization. The apparent inconsistency regarding the effect of pesticide on P released in soil and broth solution can be explained by the dissimilar conditions in soils and broth media. While there was a wide range of nutrients in soils for enhanced metabolic activities of micro organisms, limited nutrient composition in a defined medium such as yeast extract-mannitol (YEM) broth could be responsible for less activity. The nutrients, when present, as in soil, may have enhanced the phosphatase activities of the soil microflora, thus, releasing P from phosphate complex. Adsorption (through base exchange action) of the pesticide components by the clay fraction of the soil may have also reduced the potency of the pesticide. Generally, observations in this experiment indi-
cated a negative environmental consequence of doubling the dosages of the test pesticides on the soil microbial populations, their biological cycling activities and consequently, the mineralization of soil nutrients.
Acknowledgements We thank Mr. Nurudeen Olanrewaju for typing the manuscript.
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