Bull. Environ. Contam. Toxicol. (1990) 45:365-374

9 1990 Springer-VerlagNew York Inc.

~E~nvironmental ~C o n t a m i n a t i o n ~and Toxicology

Sorption of Bromacil, Diuron, Norflurazon, and Simazine at Various Horizons in Two Soils A. K. Alva and Megh Singh University of Florida, Institute of Food and Agricultural Sciences, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, Florida 33850 USA The i m p a c t of a g r i c u l t u r a l p r a c t i c e s on g r o u n d w a t e r q u a l i t y i s a matter of increasing public interest. Agricultural practices p r o v i d e n o n - p o i n t s o u r c e s f o r a v a r i e t y os p o t e n t i a l w a t e r c o n taminants. S o r p t i o n and m i c r o b i a l d e g r a d a t i o n of p e s t i c i d e s i n s o i l a r e i m p o r t a n t f a c t o r s in d e t e r m i n i n g l e a c h i n g b e h a v i o r o f p e s t i c i d e s and c o n t a m i n a t i o n o f g r o u n d w a t e r . Sorption of pestic i d e s v a r y c o n s i d e r a b l y d e p e n d i n g on t h e i r c h e m i c a l c h a r a c t e r i s tics. S o i l p r o p e r t i e s i n f l u e n c e the d e g r e e of s o r p t i o n o f p e s t i c i d e s , which in t u r n d e t e r m i n e t h e p o t e n t i a l f o r l e a c h i n g . S e v e r a l i n v e s t i g a t o r s h a v e shown t h a t t h e o r g a n i c f r a c t i o n o f the s o i l as t h e s i n g l e f a c t o r most h i g h l y r e l a t e d to h e r b i c i d e s o r p t i o n ( C a r r i n g e r e t a l . 1975, Hayes 1970, Kozak e t a l . 1983, S h e e t s e t a l . 1962, Upehureh and Mason 1962, Weber e t a l . 1969, Weed and Weber 1974). G r o v e r (1974) s t u d i e d the s o r p t i o n of trifluralin, triallate, and d i a l l a t e on s e v e r a l s o r p t i v e s u r faces. S o r p t i o n was e x t r e m e l y h i g h on a c t i v a t e d c h a r c o a l , and l e a s t on m o n t m o r i l l o n i t e . Weber e t a l . (1969) r e p o r t e d an i n c r e a s e in t h e s o r p t i o n of s - t r i a z i n e h e r b i c i d e s on s o i l o r g a n i c m a t t e r w i t h an i n c r e a s e in pH. The maximum s o r p t i o n o c c u r r e d a t pH l e v e l s in t h e v i c i n i t y o f the pKa v a l u e s os the r e s p e c t i v e herbicides. On t h e o t h e r hand, s o r p t i o n o f h e r b i c i d e s w i t h b a s i c p r o p e r t i e s ( b u t h i d a z o l e , VEL 3510, t e b u t h i u r o n , and f l u r i d o n e ; pKa v a l u e s 0 . 6 - 1 . 7 ) d e c r e a s e d w i t h i n c r e a s i n g pH (Weber 1980). S o i l pH, o r g a n i c m a t t e r , and m i c r o b i a l biomass a r e i m p o r t a n t f a c t o r s a f f e c t i n g h e r b i c i d e s o r p t i o n and d e g r a d a t i o n . The above p r o p e r t i e s v a r y c o n s i d e r a b l y in v a r i o u s h o r i z o n s f o r a g i v e n soil. A c c o r d i n g l y , s o r p t i o n o f h e r b i c i d e may v a r y a t d i f f e r e n t horizons. T h e r e f o r e , s o r p t i o n d a t a d e t e r m i n e d by u s i n g o n l y the s u r f a c e s o i l i s u s u a l l y not r e p r e s e n t a t i v e of the s u b s o i l h o r i z o n t h r o u g h which a h e r b i c i d e has to be t r a n s p o r t e d b e f o r e i t can contaminate the groundwater. Bromacil [5-bromo-6-methyl-3-(1-methylpropyl)-2,4(1H,3H) pyrimidlnedione], simazine [6-chloro-N-N'-diethyl-l,3,5-triazine-2,4diamine], norflurazon [4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-pyridazinone], and d i u r o n [ N ' - ( 3 , 4 - d i c h l o r o p h e n y l ) - N , N - d i m e t h y l u r e a ] a r e i m p o r t a n t h e r b i c i d e s r o u t i n e l y used in F l o r i d a c i t r u s g r o v e s . The o b j e c t i v e s of t h i s i n v e s t i g a t i o n Send r e p r i n t

requests

to Hegh Singh a t the a b o v e a d d r e s s .

365

were: (i) To s t u d y the s o r p t i o n of the above four h e r b i c i d e s a t v a r i o u s h o r i z o n s in two s o i l s r e p r e s e n t a t i v e o f Flatwood and Ridge s o i l s ; and (2) To examine the r e l a t i o n s h i p s between h e r b i c i d e s o r p t i o n and v a r i o u s p r o p e r t i e s o f the above s o i l s . MATERIALS AND METHODS Soil samples were obtained from 0-25-, 25-45-, 45-90-, 90-105-, and i05-150-cm depths from an uncultivated Myakka sand, siliceous, hyperthermic Aeric Haplaquods series (representative of Flatwood soils) adjacent to an abandoned citrus grove, and from 0-30-, 30-60-, 60-90-, and 90-120-cm depths from an uncultivated Candler fine sand (Hyperthermic, uncoated, Typic Quartizipsamments) series (representative of Central Florida Ridge soils). Sampling depths in the Myakka sand were chosen on the basis of distinct horizonation. However, sampling depths in the Candler fine sand were arbitrary since no distinct horizonation was noticeable. Soil samples were taken using a 10-cm diameter, core sampler. Fifteen soil cores at each depth were taken randomly from an area of 50 x 50 m. The core samples from each depth were mixed together, air-dried, ground, and sieved through a 2-mm sieve. Subsamples were taken from the bulk soil for chemical analyses and sorption study. Bromacil, simazine, norflurazon, and diuron, were used in this study. These herbicides represent a wide range of physical and chemical properties (Table I) and are important in Florida citrus production. Sorption of these herbicides was studied by Table i.

Physical and chemical characteristics of herbicides a HalfSolubility Partition life Leaching in water coefficient in soil potential at 25~ Koc Ionizability t l / 2 Koc/tl/2 Herbicide (ppm)

(mg/L)

(pKa)

(day)

Bromacil 815 72 Acidic (9.1) 90 0.8 Simazine 5 138 Basic (1.65) 75 1.9 Norflurazon 28 248 Nonionic 45 5.5 Diuron 42 400 Nonionic I00 4.0 ~Herbicide Handbook. 1989. Weed Science Society of America, Champaign, IL (Sixth Ed.) employing the b a t c h e q u i l i b r a t i o n t e c h n i q u e . Air-dried soil ( 2 . 5 g) was weighed i n t o 10-mL p o l y e t h y l e n e v i a l s and 2.5-mL o f 0 . 0 0 1 M CaC12 was added. To each v i a l , a 100-~L s o l u t i o n o f e i t h e r 14C l a b e l l e d or u n l a b e l l e d h e r b i c i d e was added. The q u a n t i t y o f h e r b i c i d e added was 12.5 ~g (5 ~ g / g s o i l ) , and the t o t a l r a d i o a c t i v i t y a p p l i e d ( i n the c a s e o f l a b e l l e d t r e a t m e n t ) was a p p r o x i m a t e l y 10,000 DPM. A t r e a t m e n t w i t h o u t the s o i l was i n c l u d e d to d e t e r m i n e the t o t a l amount o f 14C a p p l i e d . No l o s s o f r a d i o a c t i v i t y in the tube w i t h o u t s o i l was e v i d e n t d u r i n g the equilibration period. Each t r e a t m e n t was r e p l i c a t e d t h r e e t i m e s . The s u s p e n s i o n was shaken f o r 24 hr on a v i b r a t i n g s h a k e r and c e n t r i f u g e d f o r 20 min. One mL of s u p e r n a t a n t was t r a n s f e r r e d 366

to

scintillation vials containing lO-mL of scintillation cocktail (ScintiVer~? II, Fisher Scientific Co., Fairlawn, NJ). The amount of ~aC was quantitated in a liquid scintillation spectrometer (LS 1800, Beckman Inst. Co., Brea, CA). The supernatant used for counting 14C activity was very clear, therefore, quench correction was not required. The counts from unlabelled treatments (background counts for the soil) were subtracted from the counts for the respective labelled treatments. I~C in supernatant at equilibrium Herbicide in solution (%) -

X i00 14C applied

Herbicide sorbed = (I00 - Percent of herbicide in solution). Some chemical properties of the soil samples representative for various horizons of sampling are shown in Table 2. Soil pH was measured in water and 0.001 M CaCl= at i:I ratio of soil:solution. Organic matter content was measured by the Walkley-Black method (Walkley and Black 1934). For measuring cations, 20 mL of Mehlich I extractant (Mehlich 1953) was added to 5 g of soil. The suspension was shaken for 5 min and filtered through Whatman No. 42 filter paper. The concentrations of cations were determined by inductively coupled plasma emission spectroscopy. The cation concentrations were expressed as cmol kg -i . RESULTS AND DISCUSSION Myakka sand is very acidic with the pH ranging from 4.3-5.0 for various horizons (Table 2). The organic matter content was 3.4% in the top 25-cm of soil immediately followed by a horizon (25-45-cm) with only 0.23% organic matter. The organic matter content increased to 1.87% in the spodic horizon (45-90-cm) and decreased to 0.70% at i05-150-cm depth. Although soil pH of the spodic horizon was slightly greater (0.3 pH unit) than that of surface soil, the concentration of aluminum was 7.5-fold greater in the former than the latter horizon sample. The aluminum content was extremely low in the 25-45-cm horizon sample. The pH of the Candler fine sand decreased from 6.9 in 0-30-cm sample to 5.7 in 90-120-cm depth sample. The organic matter content also decreased from 1.13 to 0.30%. The CEC of the surface 30-cm of soil was 5.79 cmol kg -I which decreased to 2.89, 2.05, and 2.03 cmol kg -I in samples at 30-60-, 60-90-, and 90-120-cm, respectively. Among the four herbicides, the fraction of herbicide sorbed decreased in the order: diuron > norflurazon > simazine > bromacil in both the soils, regardless of horizons of sampling (Fig. I). Jain and Singh (1989) using Astatula fine sand (0-30-cm) also reported similar trends with respect to sorption of the above herbicides. In the Myakka sand, the sorption of bromacil was 62% in the top 25-cm sample, and decreased to 5% in the 25-45-cm depth. However, the sorption increased to 30% in the spodic horizon and slightly decreased to 19 and 15% at 90-105- and I05-150-cm samples, respectively. In field situations, bromacil if leached through the top 25-cm of soil is subjected to rapid leaching in the 25-45-cm horizon. However, as it reaches the spodic horizon, 367

4.3 4.4 4.6 5.0 4.9

6.9 6.6 5.8 5.7

0-30 30-60 60-90 90-120

pH (water)

6.4 5.8 4.9 4.7

3.1 3.5 3.7 4.0 3.9

pH (I mM CaCI2)

1.13 0.43 0.40 0.30

3.40 0.23 1.87 1.20 0.70

0.39 0.06 0.09 0.21 0.14

Myakka sand

Ca

0.21 0.03 0.03 0.07 0.08

Mg

4.17 1.16 0.52 0.43

0.60 0.33 0.17 0.15

0.02 0.02 0.02 0.02

0.16 0.01 0.01 0.02 0.02

K

0.03 0.01 0.01 0.02

0.05 0.03 0.06 0.04 0.06

Na

0.97 1.37 1.33 1.41

0.72 0.17 5.38 5.65 4.63

A1

5.79 2.89 2.05 2.03

1.53 0.30 5.57 5.99 4.93

E cations

Mehlich i extractable cations [cmol kg -I ]

Candler fine sand

Organic matter (%)

Important chemical properties of soils used for sorption of four herbicides

0-25 25-45 45-90 90-105 105-150

Horizons (cm)

Table 2.

aa

Myakka sand

100

b 80 -O

(D

b e

60

c d

b

d

N

t_

O

40

6O

(D 2O -I3 O 0

o

e

~

~

N

0-25

25-45

45-90

9 0 - 1 0 5 105-150

~100 s

,, 8o

a

60

fq

Candler fine sand

a

c

40 20 o

i

c

0-30

30-60

60-90

c d

i

90-120

Depth of soil horizons (cm) m

Bromacil

gr~ Norflurazon

~

Simazine

Diuron

Figure I. Sorption of bromacil, simazine, norflurazon, and diuron in two soils at various horizons. The letters "a" through "e" indicate relative sorption at different depths for a given soil, with "a" indicating the depth at which sorption was highest, etc. Means followed by similar letters s each herbicide for a given soil are not significantly different at P = O.05. 369

an increased sorption would minimize the downward movement of bromacil. The results clearly reveal that the leaching potential of bromacil is strongly dependent on properties of the soil at various horizons. The sorption trend with soil horizons was very similar for simazine, norflurazon, and diuron as discussed above for bromacil. The greatest difference in sorption of various herbicides was evident in the 25-45-cm depth sample. Accordingly, in this horizon sample, sorption of diuron was 54% while that of bromacil was only 5%. The sorption of all four herbicides was lower in the Candler fine sand than in the Myakka sand. This could be partly due to the difference in organic matter content (Table 2). The organic matter content of top 30-cm of Candler fine sand (1.13%) was very similar to that at 90-i05-cm depth sample of Myakka sand (1.20%). The sorption of bromacil, simazine, norflurazon, and diuron were 21, 33, 51, and 73%, respectively, in the former soil, while 19, 40, 52, and 71%, respectively, in the latter soil. Thus, except for simazine, sorption of other herbicides was very similar in the above two samples. Therefore, organic matter content plays a major role in determining sorption of herbicides regardless of differences in soil series, pH, and horizon of sampling. In the Candler fine sand, the sorption of bromacil was only 21% in the top 30-cm of soil. The sorption decreased to 9, 4, and 3%, respectively, in the 30-60-, 60-90-, and 90-120-cm depth samples. The sorption of simazine, norflurazon, and diuron at the 90-120-cm depth decreased by 23, 44, and 53% of the respective sorption in the top 30-cm of soil. Therefore, in the Candler fine sand, sorption of all four herbicides consistently decreased along with a decrease in organic matter content as the depth of sampling increased. Walker et al. (1989) also reported a decrease in sorption of chlorsulfuron and metsulfuron-methyl at 40-60- than at O-20-cm depth samples of eight different soils. Since a decrease in sorption of herbicide partly contributes to a greater potential for its leaching, the determination of sorption using only the top soil is likely to underestimate the leaching potential of a given herbicide. In the case of Myakka soil, although the horizon (25-45-cm) immediately below the top 25-cm soil is extremely permeable, the spodic horizon (45-90-cm)and the underlying soil down to 150-cm are likely to restrict the leaching of herbicides because of an increased sorption. The sorption of bromacil increased by 7-23% and 3-7% in the Myakka sand and Candler fine sand, respectively, when pH of the suspension was increased to 6.5 prior to addition of bromacil (Fig. 2). Increase in pH may increase negative charges in the soil, thus resulting in increased sorption of bromacil. Organic matter content of soil was highly correlated with sorption of all four herbicides (Fig. 3). The relationship was linear for bromacil and simazine (R ~ =40.94 to 0.99), while curvilinear for norflurazon and diuron (R ~ = 0.84 to 0.94).

370

6o. - o 60 (D

A

Myakka sand

a

B

2 40. 0 9

C b

c

D

20

"-~- 0 L . 80 0

0-25

25-45

45-90

90-105 105-150

N

c- 60 9 L) (D 4-0

n

Candler fine send A

20 0

0-30

30-60

60-90

90-120

Depth of soil horizons (cm) m

Without pH adjustment pH odjusted to 6.5 on all horizon samples

Figure 2. Sorption of bromacil without the pH adjustment and with pH adjusted to 6.5 in two soils at various horizons. Means s by similar letters are not signis different at P = 0.05. (Mean comparisons between horizon samples without the pH adjustment and with pH adjusted to 6.5 are shown by lower case and upper case letters, respectively.)

371

1 O0 80

Bromacil Y = - 0 . 7 2 + 17.95x

1 I

I

0

Simazine

R2 --- 0.987**

-~

4o

0

20

9~

0

5x

~"~L100

..~ eo

13.. 40

2o

~'0

y = 5._88 + 48.41x - 8.03x2J

o 0 0

yZ=o.~3s,,

,

/

.

1

3

4.

0

2

R2 = 0.841"*

~ y = 22.37 § 5 0 . 1 2 , - 8.18,~ 2

Soil o r g a n i c

.

.

matter

.

1

2

3

4.

(%)

Figure 3. Sorption of bromacil, simazine, norflurazon, and diuron in two soils as a function of soil organic matter.

The ultimate objective of a herbicide sorption study is to evaluate the potential of a given herbicide for leaching through the soil profile and finally contamination of groundwater. In the absence of microbial degradation and uptake by weeds, sorption by soil components is the main factor determining the leaching of a herbicide in question. In the present study, the amounts of bromacil, simazine, norflurazon and diuron leached below the depth of sampling in the two soils were estimated. For the sake of convenience, the fractions of herbicide subjected to degradation and uptake by weeds were considered to be negligible. The estimation of leaching was based on the application of herbicide on the surface of soil at 5 pg/g soil rate, and the amount which was not sorbed in the first sampling depth was considered as leached below this depth of sampling. Considering 5 ~g/g soil each of bromacil, simazine, norflurazon and diruon applied on the surface of Myakka sand, approximately 1.92, 0.47, 0.30 and 0.08 pg/g soil respective herbicides were not sorbed in the top 25-cm horizon of sell (Fig. 4), therefore, considered as potentially leachable. Drawing on the data presented in Figure 2, the remaining points

372

4

Myakka sand 3 !

!

o m

2

O O3 UD [D I

1

0

0

~,:'z.:{-.-_-....,._-.-.-.-....-....-~-..,.=~...=..,=...=...=...~

4

Candler fine sand

I

tO

-O

3

e ~

O

_Q k_

"A_

El. oo

o ~

--.

""

~176

"A

o.

2

"El,,.

-.~

I

.... I~ ............ 0.

[]

~ .~, ~ ,o

....

0

lk,,,

30

0

O- . . . . . . . .

._0

I

,,,I

I

60

90

120

Depth of soil horizons (cm) o--o

Bromacil

A--a

Simazine

[] ..... []

Norflurazon

o .... o

Diuron

Figure 4. Estimated amounts of bromacil, simazine, norflurazon, and diuron leached through 150- and 120-cm depth of Myakka sand and Candler fine sand, respectively. The rate of each herbicide applied on the surface of soil is 5 ~g/g soil.

373

,I

150

in Figure 4 were estimated in a similar fashion. However, the potentially leachable quantities of the respective herbicides below 150-cm horizon are 0.89, 0.06, 0.02 and 0.0007 ~g/g soil. The fractions of potentially leachable herbicides in the Candler fine sand were much greater than those in the Myakka sand. It appears that 3.34, 2.27, 1.43 and 0.43 ~g/g soil bromacil, simazine, norflurazon and diuron, respectively, is leachable below 120-cm depth of soil in the Candler fine sand. Acknowledgments. We thank Dr. C. A. Anderson and Mr. J. DeVore for their valuable contribution to this study. Florida Agricultural Experiment Station Journal Series No. R-00418. REFERENCES

Carringer RD, Weber JB, Monaco TJ (1975) Adsorption-desorption of selected pesticides by organic matter and montmorillonite. J Agric Food Chem 23:568-572 Grover R (1974) Adsorption and desorption of trifluralin, triallate, and diallate by various adsorbents. Weed Sci 22:405-408 Hayes MBH (1970) Adsorption of triazine herbicides on soil organic matter, including a short review on soil organic matter chemistry. Res Rev 32:131-174 Jain R, Singh M (1989) Effect of a synthetic polymer on adsorption and leaching of herbicides in soil. Proc 2nd Int Symp Adj Kozak J, Weber JB, Sheets TJ (1983) Adsorption of prometryn and metalachlor by selected soil organic matter fractions. Soil Sci 136:94-101 Mehlich A (1953) Determination of P, Ca, Mg, K, Na, and NH~. North Carolina Soil Test Mimeo. North Carolina State Univ., Raleigh, NC Sheets TJ, Crafts AC, Drever HJ (1962) Influence of soil properties on the phytotoxicities of the s-Triazine herbicides. J Agric Food Chem 10:458-462 Upchurch RP, Mason DD (1962) The influence of soil organic matter on the phytotoxicity of herbicides. Weeds 10:9-14 Walker A, Cotterill EG, Welch SJ (1989) Adsorption and degradation of chlorsulfuron and metsulfuron-methyl in soils from different depths. Weed Res 29:281-287 Walkley A, Black IA (1934) An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Sci 37:29-38 Weber JB (1980) Adsorption of Buthidazole, VEL 3510, Tebuthiuron and Fluridone by organic matter, montmorillonite clay, exchange resins, and a sandy loam soil. Weed Sci 28:478-483 Weber JB, Weed SB, Ward TM (1969) Adsorption of s-triazines by soil organic matter. Weed Sci 17:417-421 Weed SB, Weber JB (1974) Pesticide-organic matter interactions. In: Guenzi WD (ed) Pesticides in Soil and Water, Soil Sci Soc Am, Madison, WI Received January 15, 1990; accepted March 4, 1990

374

Sorption of bromacil, diuron, norflurazon, and simazine at various horizons in two soils.

Bull. Environ. Contam. Toxicol. (1990) 45:365-374 9 1990 Springer-VerlagNew York Inc. ~E~nvironmental ~C o n t a m i n a t i o n ~and Toxicology So...
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