Denitrification in soil: Effects of herbicides

Denitrification in soil: Effects of herbicides

Soil Bid. Biochem. Vol. 17, No. 4, pp. 447452, Printed in Great Britain. All rights reserved 0038.0717/M$3.00+ 0.00 Copyright 0 1985Pergamon Press Lt...

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Soil Bid. Biochem. Vol. 17, No. 4, pp. 447452, Printed in Great Britain. All rights reserved

0038.0717/M$3.00+ 0.00 Copyright 0 1985Pergamon Press Ltd

1985

DENITRIFICATION IN SOIL: EFFECTS OF HERBICIDES J. C. YEOMANS and J. M. BREMNER Department of Agronomy, Iowa State University, Ames, IA 50011, U.S.A.

Summary-The effects of 20 herbicides on denitrification of nitrate in three soils were studied by determining the effects of 10 and 50 pg g-l soil of each herbicide on the amounts of nitrate lost and the amounts of nitrite, N,O and N, produced when soil samples were incubated anaerobically after treatment with nitrate. The herbicides used were butylate, EPTC, chlorpropham, propham, diuron, linuron, monuron, siduron, alachlor, trifluralin, 2,4-D amine, 2,4-D ester, atrazine, cyanazine, metribuzin. simazine, dalapon, chloramben, dicamba and dinoseb. None of the herbicides studied significantly affected denitri~cation of nitrate when applied at the rate of IO yg g-’ soil, but dinoseb increased the ratio of N, to NtO in the gaseous products of denitri~cation when applied at this rate. Butylate, EPTC, diuron, simazine and dalapon had no significant effect on denitrification when applied at the rate of 50 p’gg-’ soil, whereas metribuzin and dinoseb enhanced denitrification when applied at this rate. The influence of the other herbicides on denitrification when applied at the rate of 5Opggg’ soil depended on the soil, but all enhanced or inhibited denitrification in at least one soil.

Herbicides are now used extensively in crop production. For example, in 1980, 92% of the corn acreage, 93% of the soybean acreage and 17% of the pasture acreage in Iowa received herbicides. The increasing use of herbicides with nitrogen fertilizers has created concern about the potential adverse effect of these pesticides on non-target organisms and the environment, and it has emphasized the need for information concerning the effects of herbicides on nitrogen transformations in soil. Numerous studies of the effects of herbicides on nitrification of ammonium in soil have been reported, but very little is known about the effects of herbicides on denitrification of nitrate in soil (Goring and Laskowski, 1982). Denitrifying bacterial populations in soil have been reported to be enhanced by PCP (pentachlorophenol) (Ishizawa and Matsu~uchi, 1966) and by 2,4-D (2,4dichlorophenoxyacetic acid) (Sethunathan, 1970), but to be decreased by MCPA (2-methyl-4-chlorophenoxyacetic acid) and propanil (De and Mukhopadhyay, 1971). Denitrification by cultures of denitrifying bacteria has been reported to be inhibited by diquat (Atkinson, 1973), MCPA, 2,4,5-T (Torstensson, 1974), 2,4-D, chlorbromuron, chloroxuron, fiuometuron, metoxuron, neburon, diuron, linuron, siduron (Hart and Larson, 1966; Bollag and Nash, 1974; Bollag and Henninger, 1976), dalapon (Grant and Payne, 1982), simazine and atrazine (McElhannon et al., 1984; Mills, 1984), to be enhanced by MCPA, 2,4,5-T, linuron and simazine (Torstensson, 1974), and to be unaffected by diquat (Atkinson, 1973), atrazine and simazine (Bollag and Henninger, 1976). Other herbicides found to have no effect on denitrification by cultures of denitrifying bacteria in&de fenuron, metobromuron, monuron, ametryne, endothal, hydroxysimazine and propham 447

(Bollag and Nash, 1974; Bollag and Henninger, 1976). PCP (Mitsui et al., 1964) and metolachlor (Bollag and Kurek, 1980) have been found to inhibit denitrification in soil. Eight herbicides reported to inhibit denitrification by cultures of denitrifying bacteria have also been reported to inhibit denitrification in soil. These are 2,4-D (Bollag and Henninger, 1976), fluometuron (Bollag and Kurek, 1980), dalapon (Weeraratna, 1980), diuron, Iinuron, neburon, simazine and atrazine (Rolston and Cervelli, 1980; McElhannon et at., 1984; Mills, 1984). Atrazine has also been reported to enhance denitrification in soil (Cervelli and Rolston, 1983) and to have no effect on denitrification is soil (Bollag and Henninger, 1976). Other herbicides that appear to have no effect on denitrification in s&l are PCP (Mitsui et at., 1964), diuron, linuron, simazine, ametryne, endothal, hydroxysima~ne, propham and siduron (Bolag and Henninger, 1976). Our purpose was to determine the effects of 20 extensively used herbici.des on denitrification of nitrate in soil. The herbicides studied are commonly used for weed control in corn and soybeans (butylate, EPTC, chlorpropham, linuron, alachlor, trifluralin, atrazine, cyanazine, metribuzin, simazine, ddapon, chloramben, dicamba and dinoseb) or for weed control in hay and pasture (propham, diuron, monuron, siduron, 2,4-D amine and 2,4-D ester).

MATERIALS

AND METHODS

The soils used (Table 1) were surface (O-15 cm) samples of soils representative of the Webster, Harps and Storden series used extensively for corn and soybean production in central Iowa. The analyses reported in Table 1 were performed as described by

and J. M. BREMNEK

J. C. YE~MANS

448

Table

I. Analvses

Series Webstel Harps Storden

Subgroup

of soils Organic carbon

Soil PH

Typic Haplaquoll Typic Calciaquoll Typic Udorthent

3.3 6.6 0.5

7.5 1.1 8.1

Zantua and Bremner (1975). Before use, each soil was air-dried and sieved (< 2 mm). The names, formulations and sources of the 20 herbicides studied are reported in Table 2. To study the effects of these compounds on denitrification of nitrate in soil, 30 g samples of air-dried soil were placed in 1.2 1 flasks fitted with standard taper (34145) ground-glass joints and were treated with 5 ml of Hz0 containing 9 mg of N as KNO, and with 10 ml of H,O containing 0, 0.3 or 1.5 mg of the herbicide under study. Each flask was then sealed with a glass stopper fitted with a standard taper (34/35) groundglass joint and a glass stopcock and was connected to a manifold system attached to a mercury (Hg) manometer. The flasks were evacuated via the stopcock and filled with helium (He) to slightly above atmospheric pressure, and this process was repeated three times. The flasks were then brought to atmospheric pressure with He and placed in an incubator at 30°C. The atmospheres in the flasks were sub-

name

Butylate EPTC Chlorpropham Propham Diuron Li”UtQ” Monuron Siduron Alachlor Trifluralin 2.4-D anune 2,4-D ester Atrarine Cykl”aZi”e Metribuzin Simazme Dalapon Chloramben DlC”mba Dinoseb

Chemical

0.24 0.50 0.05

Sand

Clay

31 IO I3

30 41 28

sequently sampled every 2 days to determine N,O, NO, N: and 0, by the CC-ultrasonic detector method described by Blackmer and Bremner (1977). Only the data obtained after 8 days are reported because the conclusions reached were not significantly affected by the time of sampling. The analyses for 0, were performed to check that anaerobic conditions had been maintained. The results of analyses for NO are not reported because only trace amounts of this gas could be detected. All experiments were performed in triplicate. Soil samples were analysed for nitrite by the colorimetric procedure described by Bremner (1965). Nitrate was determined by the steam distillation method described by Bremner and Keeney (1966). No loss of nitrate or production of nitrite, N,O, NO or N, could be detected when soil samples that had been sterilized by autoclaving at 121°C for 1 h were incubated (3O”C, 15 ml H,O) under He for 8 days after treatment with 9 mg of nitrate-N as KNO,.

Table 2. Herbicides Common

Total nitrogen (“<,)

studied

name

Bis(2.methylpropyl)carbamothioic acid S-ethyl ester Dipropylcarbamothioic acid S-ethyl ester (3.Chlorophenyl)carbamx acid I-methylethyl ester Phenylcarbarmc aad I methylethyl ester 3.(3,4-DichlorophenyI)-l,I-dimethylurea N.-(3.4-Dichlorophenyl)-N-methoxy-Nmethylurea N.-(4.Chlorophenyl)-N,N-dimethylurea N-(2-MethylcyclohexyI)-N,-phenylurea 2.Chloro-N-(2.6.diethylphenyl)-N(methoxymethyl)acetamide 2.6.Dinitro-N.N-dipropyl-4.(trifluoromethyl)benxnamine (2,4-Dichlorophenoxy)acetic acid. dimethylamine salt (2,4-Dichlorophenoxy)acctic acid. butoxy ethanol ester 6.Chloro-N-ethvl-N;(I-methylethyl)-1.3.5. triazine-2.4.dkmine 2-([4-Chloro-6-(ethylamino).1,3,5-triarin2.yl]ami”o)-2.methylpropanenitrile 4-Amino-6.( 1, I -dimethylethyl)-3(methylthio)-l,2,4-triazin-5(4H)-one h-Chloro-N,N,-diethyl-l,3.5-triazine2.4.diamine 2.2.Dichloropropanolc acid 3-Amino-2.5.dichlorobenzoic acid 3.6.Dichloro-2-methoxybenzoic acid 2-( I-MethylpropyI)-4,6-dinitrophenol

Formulation”

SOtMC2

EC(800) EC(720)

S S

EC (480) EC (240) WP(80)

P P PN

WP(80) WP(80) WP(80)

PN PN PN

EC (480)

MO

EC(480)

E

L(480)

A

EC (480)

A

WP(80)

C

WP(80)

SC

WP(50)

M

WP(80) WP(85) L (240) L (480) L(360)

C D A V D

“EC, emulsifiable concentrate: WP, wettable powder; L, liquid. Values in parentheses after EC and L indicate grams of active ingredient per liter of formulation. Values in parentheses after WP indicate percent (w/w) of active ingredient in formulation. “S. Stauffer Chemical Co., Westport. CT; P, PPG Industries Inc., Pittsburgh. PA; PN, duPont de Nemours & Co. Inc., Wilmington. DE; MO, Monsanto Agricultural Products Co., St. Louis, MO; E, Elanco Products Co., Indianapolis, IN; A, Amchem Products Inc., Ambler, PA; C, Ciba-Geigy Corp., Greensboro, NC; SC. Shell Chemical Co., Houston. TX; M, Mobay Chemical Corp., St. Louis, MO; D. Dow Chemical U.S.A.. Midland. MI; V, Velsicol Chemical Corp., Chicago. IL.

449

Effects of herbicides on denitrification Table 3. Effects of 10 pg g-’ soil of various herbicides on denitrification soil

NOtI. Butylate EPTC Chlorpropham Propham DiUKXI Linuron MOllUVXl Siduron Alxhlor Trifluralin 2.4-D amine 2,4-D ester Atrazine Cyanazine Metribuzin Simazine Dalapon Chloramben Dicamba Dinoseb

RESULTS AND

0

120 122 120 118 I18

30 29 30 30 24 32 29 30 30 26 28 31 30 30 28 32 30 31 30 26 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

90 91 91 90 96 89 90 90 90 93 92 90 91 92 91 91 90 92 91 93 119

120 120 I21 120 120 121 II9 120 120 119 120 I21 121 122 119 123 120 123 I21 119 II9

rate of 100 pg gg’ soil. Bollag and Henninger (1976) also found that atrazine and simazine had no effect on denitrification when applied at the rate of lOOpgg_’ soil, and Grant and Payne (1982) found that atrazine at rates up to 1250 p g g-’ sediment had no effect on denitrification in a salt marsh sediment. However, atrazine has been reported to enhance denitrification in a soil column when applied at the rate of 3 pg gg’ soil (Cervelli and Rolston, 1983). Moreover, atrazine applied at the rate of 28.5 pg gg’ soil (Rolston and Cervelli, 1980) and atrazine and simazine applied at rates as low as 0.5 pgg-’ soil (McElhannon et al., 1984) have been reported to inhibit denitrification in soil. Dalapon has been reported to inhibit denitrification in soil when applied

DISCUSSION

None of the herbicides had a significant effect on denitrification of nitrate when applied at the rate of 10 pg gg’ soil, but dinoseb increased the ratio of N, to N,O in the gaseous products of denitrification (i.e. promoted reduction of N,O to N, by soil microorganisms) (Tables 3-5). Butylate, EPTC, diuron, atrazine, simazine and dalapon had no significant effect on denitrification when applied at the rate of 5Opgg-’ soil (Tables 6-8). Rolston and Cervelli (1980) found that diuron inhibited denitrification in soil when applied at the rate of 30 pg g-’ soil, whereas Bollag and Henninger (1976) found that it had no effect when applied at the

Table 4. ElTects of IO pg g-’ soil of various

of nitrate in Webster

herbicides

EOi I

on denitrificatmn

of nitrate in

Harps

N produced NOT-N lost

~~~ ~~~~~~ NOT-N

Herbicide None Butylate EPTC Chlorpropham Propham Diuron Linuron Monuron Siduron Alachlor Trifluralin 2,4-D amine 2.4-D ester Atrazine Cyanazine Metribuzin Simazine Dalapon Chloramben Dicamba Dinoseb

142 143 141 143 143 141 142 142 140 142 I44 142 I41 142 142 142 140 140 140 143 I50

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

N,O-N (pgg-’ 35 34 36 32 29 34 36 33 35 36 32 36 35 35 33 34 37 33 35 36 0

N,-N soil) 107 I08 10s 110 1 I4 107 107 110 I05 106 112 107 I05 106 I08 108 104 107 106 107 I50

(NO,

+ N>O + N&N 142 142 141 142 143 I41 143 143 140 142 144 143 140 141 141 142 141 140 141 143 I50

450

J. C.

YEOMANS and

J. M.

Table 5. Effects of 1Opg 8-l soil of various herbicides soil

BREMNER

on denitrificatlon

of nitrate in Storden

N produced NO;-N IOSl

NOT-N

Herbicide NOtX

Butylate EPTC Chlcrpropham Propham Diuron Linuron Monuron Siduron Alachlor Trifluralin 2,4-D amine 2.4-D ester .Atrazine Cyanatine Metribuzin Simazine Dalapon Chloramben Dicamba Dinoseb

57 57 5h 57 57 55 60 58 59 57 60 58 61 55 57 59 60 57 51 59 60

14 13 12 I4 I3 14 16 13 I5 13 I4 16 15 15 13 14 17 13 15 16 0

4

5 6 4 4 6 4 4 4 5 5 4 6 6 4 5 5 4 5 5 0

(NO,

39 39 40 37 39 31 40 41 40 38 41 3R JO 36 40 40 38 40 31 31 61

+ N,O + N&N 57 57 sx 55 57 57 60 58 59 56 60 58 61 57 57 59 60 57 57 58 61

plied at the rate of 50 pg g-’ soil, but increased the ratio of N, to NzO in the gaseous products of denitrification (i.e. promoted N,O reduction by soil microorganisms) when applied at this rate. Chlorpropham and propham inhibited nitrite reduction in the Storden soil (Table 8). 2,4-D has been reported to inhibit denitrification in soil when applied at the rate of lOOpgg_’ soil, to inhibit denitrification by a culture of denitrifying bacteria when applied at the rate of lOO~grn1~’ culture medium (BoIlag and Henninger, 1976), and to inhibit denitrification in a salt marsh sediment when applied at the rate of 12.5 1.18g-’ sediment (Grant and Payne, 1982). Bollag and Henninger (1976) found that propham had no effect on denitrification in soil when applied at the

at the rate of JOpg gg’ soil ~eeraratna, 1980) and to inhibit denitrification in a salt marsh sediment when applied at the rate of 12.5 ,ugg-’ sediment (Grant and Payne, 1982). Tables 6-8 show that metribuzin and dinoseb enhanced denitrification in the three soils used when they were applied at the rate of 5Opgg-’ soil. They also show that dinoseb increased the ratio of N, to N,O in the gaseous products of denitri~cation in these soils and that met~buzin inhibited nitrite reduction in the Storden soil when applied at the rate of 50 ILg g-’ soil. Six of the herbicides (chloropropham, propham, alachlor, 2,4-D amine, 2,4-D ester and dicamba) either had no significant effect on denitrification, or enhanced denitrification, when ap-

Table 6. Effects of 50 fig 8-l soil

N,O-N NT-N (ux e-’ soil)

ofvarious herbicides

on denit~ific~tion

of nitrate in Webster

soil N produced NOT-N lost

NOT-N

None Butylate EPTC Chlorpropham Propham Diuron Linuron MOtlUKXl Siduron Alachlor Trifluralin 2,4-D amine 2.4-D ester Atrazine Cyanazine Metribuzin Simazine Dalapon Chloramben Dicamba Dinoseb

N,O-N

(j.tgg’

Herbicide 120 119 120 128 159 118 110 II8 I18 129 116 120 117 122 I14 13s 118 I15 119 II8 I46

0 I 1 0 I 0 0 0 0 0 I 0 0 0 0 0 0 0 I 0 0

30 15 18 6 22 36 32 40 28 0 35 19 22 32 36 39 29 30 IO 0 0

N,-N soil) 90 103 IO1 121 136 83 78 78 90 129 80 IO1 95 90 71 97 90 87 108 11% I46

(NO;

+ N,O + N&N I20 II9 120 127 IS9 119

I10 I18 118 129 116 120 I17 122 113 136 I19 117 119 118 146

Effects of herbicides Table 7. Effects of 50 pg g-’ soil of various

451

on denitrification

herbicides

on denitrification

of nitrate in Harps


N produced NOT-N lost

NOT-N

Herbicide NOW Butylate EPTC Chlorpropham Propham Diuron Linuron M0lWr0o Siduron Alachlor Trifluralin 2,4-D amine 2,4-D ester Atrarine Cyanazine Metribuzin Simarine Dalapon Chloramben Dicamba Dinoseb

142 149 142 150 I59 145 144 150 I44 I52 166 I52 160 I48 143 I52 140 142 I57 I52 172

0 0 0 0 0 0’ 0 0 0 0 0 0 0 0 0 0 0 0 0 I 0

rate of 1OO~gg~’ soil or on denitrification by a culture of denitrifying bacteria when applied at the rate of 100 pgmlF’ culture medium. When applied at the rate of 5O~gg~’ soil, the effects of six of the herbicides studied (linuron, monuron, siduron, trifluralin, cyanazine and chloramben) depended on the soil used. Linuron inhibited denitrification in the Webster soil (Table 6) had no effect on denitrification in the Harps soil (Table 7) and inhibited nitrite reduction and N,O production in the Storden soil (Table 8). Linuron has been reported to inhibit denitrification by a culture of denitrifying bacteria when applied at rates of 50, 100 200 pg ml-’ culture medium (Bollag and Nash, 1974) and to inhibit denitrification in soil when applied at the rate of 26.5 ;Lg g-’ soil (Rolston and Cervelli,

N,O-N N,-N (pg g- ’ soil) 35 38 48 40 IO 32 31 30 20 I4 39 20 I6 31 28 34 28 32 38 6 0

(NO;

107 112 94 I IO I50 I12 I13 120 125 I38 127 132 146 II7 II6 120 112 III 120 145 172

+ NzO + N&N 142 150 142 150 160 144 144 150 I45 I52 166 I52 I62 148 I44 I54 140 143 158 151 172

1980). In contrast, linuron has been reported to have no effect on denitrification in soil when applied at the rate of 100 pg g- ’ soil (Bollag and Henninger, 1976). Monuron and siduron enhanced denitrification in the Storden soil (Table 8) and increased the ratio of N, to N,O in the gaseous products of denitrification in the Harps and Storden soils when applied at the rate of 50 pg g-’ soil (Tables 7 and 8). Monuron also inhibited NzO reduction in the Webster soil (Table 6) and inhibited nitrite reduction in the Storden soil when applied at this rate (Table 8). Monuron has been reported to have no effect on denitrification by a culture of denitrifying bacteria when applied at rates up to 200 pg ml-’ culture medium (Bollag and Nash, 1974). Siduron has been reported to inhibit denitrification in soil when applied at the rate of

Table 8. Effects of 50 pg g-’ soil of various herbicides soil

on denitrification

of nitrate in Storden

N uroduced

Butylate EPTC Chlorpropham Propham Diuron LlllUrOll MOIlWOll Siduron Alachlor Triflurdh

2.4-D amine 2.4-D ester Atrdzine

Cyanazine Metriburin Simazine Dalapon Chloramben Dicamba Dinoseb

54 50 69 60 64 62 80 75 51 82 85 92 60 65 70 61 61 59 64 85

4 I 30 I3 I4 I9 19 2 6 I4 9 IO 5 4 I2 6 6 7 5 0

6 2 3 4 8 5 7 9 3 II 18 I7 5 I4 I4 I3 I3 IO 8 0

44 47 35 43 42 39 54 66 42 58 58 65 50 47 45 42 42 42 50 85

54 50 68 60 64 63 80 77 51 83 85 92 60 65 71 61 61 59 63 85

452

J. C. YEOMANS and J. M. BREMNEK

IOOpgg-’ soil and to inhibit denitrification by a culture of denitrifying bacteria when applied at the rate of 100,ugml~’ culture medium (Bollag and Nash, 1974; Bollag and Henninger, 1976). Trifluralin enhanced denitrification in the Harps and Storden soils when applied at the rate of 5Opg g- ’ soil (Table 7 and S), but inhibited nitrite reduction in the Storden soil when applied at this rate. Cyanazine inhibited denitrification and caused a decrease in the ratio of N, to N,O in the gaseous products of denitrification when applied at the rate of 50 pg gg’ soil to the Webster soil (Table 6), but it had no effect on denitrification in the Harps soil (Table 7), and it enhanced denitrification in the Storden soil when applied at this rate (Table 8). Chloramben had no effect on denitrification in the Webster (Table 6) and Storden (Table 8) soils when applied at the rate of 50 pg g-’ soil, but it enhanced denitrification in the Harps soil when applied at this rate (Table 7). It also increased the ratio of N, to N,O in the gaseous products of denitrification in the Webster soil (i.e. promoted reduction of N,O by soil microorganisms) (Table 6). To summarize, none of the herbicides studied had a significant effect on denitrification when applied at the rate of 10 pg g-’ soil. Butylate, EPTC, diuron, simazine and dalapon had no significant effect on denitrification when applied at the rate of 50 pg g- ’ soil, whereas metribuzin and dinoseb enhanced denitrification when applied at this rate. The influence of the other herbicides on denitrification when applied at the rate of SOpgg-’ soil depended on the soil, but all enhanced or inhibited denitrification in at least one soil. Acknowledgemenrs-Journal Paper No. J-l 1690 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project 2655. This work was supported in part by the Tennessee Valley Authority. REFERENCES Atkinson G. (1973) Effects of diquat on the microbiological organisms in the soil of lakes. Proceedings of the Pollution Research Conference, Wairakei. New Zealand. DSIR Information Series No. 97, pp. 529-538. Blackmer A. M. and Bremner J. M. (1977) Gas chromatographic analysis of soil atmospheres. Soil Science Society of America Journal 41, 908-912. Bollag J.-M. and Nash C. L. (1974) Effect of chemical structure of phenylureas and anilines on the denitrification process. Bulletin of Environmental Contaminafion and Toxicology 12, 241-248. Bollag J.-M. and Henninger N. M. (1976) Influence of pesticides on denitrification in soil and with an isolated bacterium. Journal of Environmental Quality 5, 15-18. Bollag J.-M. and Kurek E. J. (1980) Nitrite and nitrous oxide accumulation during denitrification in the presence

of pesticide derivatives. Applird urld Enrironmentul Microbiology 39, 845--849. Bremner J. M. (1965) Inorganic forms of nitrogen. In Method7 qf Soil Analysis, Vol. 2 (c. A. Black et crl., Eds), pp. Il79-~1237. American Society of .Agronomy. Madison, WI. Bremner J. M. and Keeney D. R. (1966) Determination and isotope-ratio analysis of different forms of nitrogen in soils. 3. Exchangeable ammonium. nitrate. and nitrite by extraction-distillation methods. SolI Scic~~, Socielr of Ameriru Proceedings 30, 577-582. Cervelli S. and Ralston D. E. (1983) Influence of atrazine on denitrification in soil columns. Journcd of Em,ironmental Quality 12, 482-486. De B. K. and Mukhopadhyay S. (1971) Effect of MCPA and Stam F-34 on the occurrence of some nutritional groups of bacteria in the rice fields of West Bengal. International Rice Commission Newsletter 20(3). 35-40. Goring C. A. I. and Laskowski D. A. (1982) The effects of pesticides on nitrogen transformations in soils. In Nitrogm in Agriculturcd Soils (F. J. Stevenson. Ed.), pp. 689-720. American Society 01‘ Agronomy. Madison, WI. Grant M. A. and Payne W. J. ( 1982) Effects of pesticides on denitrifying activity in salt marsh sediments. Journal of Envionmcntal Q&it>. I I, 369-372. Hart L. T. and Larson A. D. (1966) Elfect of 2,4-dichlorophenoxyacetic acid on different metabolic types of bacteria. Bullctin o/‘~tll.ironn?enl(ll Contamination and Toxicology 1. 108-- 120. Ishizawa S. and Matsuguchi T. (1966) Effects of pesticides and herbicides upon microorganisms in soil and water under waterlogged conditions. Bulletin of the Nutional Institute o/’ Ag~kdtural Scicwe. Japan, Series B 16. I-90. McElhannon W. S.. Mills H. A. and Bush P. B. (19X4) Simazine and atrazine-- -suppression of denitrification. HortSciencr 19, 21%219. Mills H. A. (1984) / A .nrocedure for ranidlv evaluating the . Communications 1d irr effect of chemicals on denitrificatlon. Soil Science and Plant iina1ysi.c 15, 1007-1016. Mitsui S.. Watanabe I., Honma M. and Honda S. (1964) The effect of pesticides on the denitritication in paddy soil. Soil Science rrnd Plant Nutrition 10, 10771 15. Rolston D. E. and Cervelli S. (1980) Denitrification as affected by irrigation frequency and applied herbicides. In Agrochemical Residue-hiota Inteructions in Soil und Aquatic Ecos~~stems, pp. 1X9-199. International Atomic Energy Agency. Vienna. Sethunathan N. (1970) Foliar sprays of growth regulators and rhizosphere effects in Cajunus capn Millsp. II. Qualitative changes in the rhizosphere and certain metabolic changes in the plant. Plum and Soil 33, 71L80. Torstensson L. (1974) Effects of MCPA, 2.4.5-T. linuron. and simazine on some functional groups of soil microorganisms. Swedish Journal of’ .4griculturul Rr.cearc,h 4, 151Ll60. Weeraratna C. S. (198Oj Ellect of dalapon-sodium on nitrification and denitrification in a tropical loam soil. Weed Research 20, 29 l--293. Zantua M. I. and Bremner J. M. (1975) Comparison of methods of assaying urease activity in soils. Soil Riolog). & Biochemistry 7, 291L295.