Enhanced Biodegradation of Pesticides in the Environment - American

Chapter 6

Enhanced Biodegradation of Insecticides in Midwestern Corn Soils 1

2

Kenneth D. Racke and Joel R. Coats 1

Environmental Chemistry Laboratory, DowElanco, 9001 Building, Midland, MI 48641-1706 Department of Entomology, Iowa State University, Ames, IA 50011

Downloaded by IMPERIAL COLL LONDON on June 28, 2014 | http://pubs.acs.org Publication Date: May 3, 1990 | doi: 10.1021/bk-1990-0426.ch006

2

An experimental strategy for the study of enhanced degradation is described based on its occurrence in Midwestern corn soils. The shift from recalcitrant chlorinated hydrocarbons to biodegradable organophosphorus and carbamate insecticides has resulted in the failure of some compounds, notably carbofuran and isofenphos, to provide adequate pest control following repeated use. Enhanced degradation of an insecticide involves its rapid degradation by a population of soil microorganisms that has adapted to beneficially catabolize it following exposure to it or a similar insecticide. For enhanced degradation to be thoroughly investigated studies must be carried out to demonstrate an increased rate of degradation in soils with prior insecticide exposure, to identify the rates and products of degradation in similar soils under controlled conditions, and to elucidate the microbiological mechanisms. Agriculture

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0097-6156/90/0426-0068$06.00/0 © 1990 A m e r i c a n Chemical Society

In Enhanced Biodegradation of Pesticides in the Environment; Racke, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


6. R A C K E & COATS

Degradation of Insecticides

69

c h l o r i n a t e d hydrocarbon i n s e c t i c i d e s provided e x c e l l e n t c o n t r o l o f t h e s e p e s t s ( 2 ) . The c y c l o d i e n e a l d r i n became t h e most w i d e l y used compound b y t h e mid 1950's a s use o f t h e s e s o i l - a p p l i e d i n s e c t i c i d e s blossomed. F o r example, s o i l i n s e c t i c i d e use i n Iowa went f r o m n o t h i n g i n 1950 t o 237,000 a c r e s i n 1954 ( 6 % o f c o r n a c r e s ) ( 3 ) . The c o n t r o l was s o e f f e c t i v e t h a t a b r o a d c a s t s o i l a p p l i c a t i o n i n many c a s e s p r o v i d e d c o n t r o l b o t h t h a t y e a r and the n e x t ! The p e r s p e c t i v e t h a t "The p e r s i s t e n t c h l o r i n a t e d h y d r o c a r b o n s appear t o be u n i q u e l y w e l l adapted f o r s o i l use" (3) r a t h e r q u i c k l y c o l l i d e d w i t h t h e r e a l i t y o f i n s e c t m i c r o e v o l u t i o n . The f i r s t e v i d e n c e o f rootworm c o n t r o l f a i l u r e s o c c u r r e d i n N e b r a s k a i n 1959, and b y 1962 r e s i s t a n c e o f rootworms t o the c y c l o d i e n e s had been c o n c l u s i v e l y documented ( 4 ) . The s p r e a d o f t h i s r e s i s t a n c e d u r i n g t h e 1960's, a l o n g w i t h e n v i r o n m e n t a l s c r u t i n y o f t h e e c o l o g i c a l impacts o f t h e s e p e r s i s t e n t compounds, c r e a t e d a demand f o r more e n v i r o n m e n t a l l y sound i n s e c t i c i d e s . B e g i n n i n g i n the mid t o l a t e 1960's t h e b i o d e g r a d a b l e organophosphorus and carbamate i n s e c t i c i d e s began t o r e p l a c e t h e c h l o r i n a t e d h y d r o c a r b o n s a s the rootworm c o n t r o l c h e m i c a l s o f c h o i c e . D i a z i n o n , o x y d i s u l f o t o n , and p h o r a t e were the f i r s t , and were q u i c k l y f o l l o w e d b y b u f e n c a r b , t r i m e t h a c a r b , f o n o f o s , and f e n s u l f o t h i o n . I n t h e mid t o l a t e 1970's the i n t r o d u c t i o n o f new c h e m i c a l s c o n t i n u e d a s e t h o p r o p , c a r b o f u r a n , c h l o r p y r i f o s and t e r b u f o s came i n t o use. Because rootworm i n s e c t i c i d e s a r e most commonly a p p l i e d a t p l a n t i n g , 4-8 weeks p r i o r t o rootworm egg h a t c h , i t was e a r l y r e c o g n i z e d t h a t t h e i n h e r e n t l y r a p i d d e g r a d a t i o n r a t e o f some o f t h e s e i n s e c t i c i d e s p r e c l u d e d t h e i r e f f e c t i v e use ( 5 ) . However, most p r o v i d e d e f f e c t i v e c o n t r o l w i t h o u t the u n d e s i r a b l e e n v i r o n m e n t a l impact o f t h e c h l o r i n a t e d hydrocarbons. By 1973 r o u g h l y 20-30 m i l l i o n a c r e s o f midwest c o r n were t r e a t e d a n n u a l l y w i t h s o i l i n s e c t i c i d e s ( 6 ) , a n d c o r n had become the l e a d i n g c r o p f o r use o f s o i l i n s e c t i c i d e s . Enhanced I n s e c t i c i d e D e g r a d a t i o n

and C o n t r o l F a i l u r e

A l t h o u g h i t was r e a l i z e d t h a t v a r i o u s e n v i r o n m e n t a l f a c t o r s c o u l d i n f l u e n c e rootworm c o n t r o l w i t h s o i l - a p p l i e d i n s e c t i c i d e s ( 7 ) , t h e f a i l u r e s o f s e v e r a l r e l i a b l e i n s e c t i c i d e s wrought some d e g r e e o f c o n s t e r n a t i o n w i t h i n the a g r i c u l t u r a l community. As v a r i o u s compounds f e l l from common use d u r i n g t h e 1960's and 1970's due t o poor performance ( d i a z i n o n , o x y d i s u l f o t o n , b u f e n c a r b , t r i m e t h a c a r b , f e n s u l f o t h i o n ) r e s i s t a n c e was examined a s a p o s s i b i l i t y and f o u n d not t o p r o v e s u f f i c e n t f o r e x p l a i n i n g the o b s e r v e d f a i l u r e s ( 6 , 8 ) . The c o n t i n u i n g i n t r o d u c t i o n o f new i n s e c t i c i d e s f o r t h e c o r n rootworm market tended t o b a l a n c e out the o v e r a l l e f f e c t o f t h e l o s s e s o f f a i l u r e - p r o n e products. R e s e a r c h o n c o n t r o l f a i l u r e s d u r i n g the l a t e 1970's and e a r l y 1980's f o c u s e d o n c a r b o f u r a n , a compound t h a t had e a r l i e r p r o v i d e d e x c e l l e n t c o n t r o l and had c a p t u r e d a c o n s i d e r a b l e s h a r e o f t h e rootworm i n s e c t i c i d e market (e.g. 20% o f t r e a t e d a c r e s i n Iowa i n 1977)(9). A l t h o u g h some s t u d i e s f o u n d no c o r r e l a t i o n between p r i o r use o f c a r b o f u r a n and a n enhanced r a t e o f c a r b o f u r a n d e g r a d a t i o n (10.11) b o t h m o n i t o r i n g o f f i e l d r e s i d u e s (12-14) and l a b o r a t o r y d e g r a d a t i o n t e s t s (15-17) c o n c l u s i v e l y d e m o n s t r a t e d enhanced m i c r o b i a l d e g r a d a t i o n as the cause o f d e c r e a s e d c a r b o f u r a n p e r s i s t e n c e i n f i e l d s w i t h h i s t o r i e s o f c a r b o f u r a n use. Thus, a



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ENHANCED BIODEGRADATION OF PESTICIDES IN THE ENVIRONMENT

phenomenon t h a t had f i r s t been n o t e d w i t h t h e p h e n o x y a l k a n o i c h e r b i c i d e s d u r i n g t h e 1940*s and had l a n g u i s h e d a s an academic c u r i o s i t y became a p r i m a r y c o n c e r n i n the M i d w e s t e r n c o r n b e l t . See F e l s o t (18) f o r a more complete account o f t h e r e c o g n i t i o n o f c a r b o f u r a n enhanced d e g r a d a t i o n . As c o n t r o l f a i l u r e s o f o t h e r i n s e c t i c i d e s f o r t h e rootworm market ( i s o f e n p h o s , t e r b u f o s , f o n o f o s , c l o e t h o c a r b , b e n d i o c a r b ) o c c u r r e d , enhanced d e g r a d a t i o n began t o be b l i t h e l y i n v o k e d as t h e u n i v e r s a l cause f o r many o f t h e s e f a i l u r e s (19). With t h i s a s s u m p t i o n many e x t e n s i o n a g e n t s began recommending r o t a t i o n s between rootworm i n s e c t i c i d e s as a p o s s i b l e means o f a v o i d i n g t h i s p e r c e i v e d problem (e.g.,20). The i n f e r e n c e o f enhanced d e g r a d a t i o n became a m a r k e t i n g weapon o f some a g r o c h e m i c a l companies t o r a i s e q u e s t i o n s r e g a r d i n g the performance o f c o m p e t i t o r ' s p r o d u c t s . I t was o b v i o u s t h a t a r a t i o n a l , e x p e r i m e n t a l a p p r o a c h was needed t o a d d r e s s t h e q u e s t i o n o f which i n s e c t i c i d e s had a c t u a l l y undergone enhanced degradation. An E x p e r i m e n t a l

Approach f o r the Study o f Enhanced

Degradation

In an e f f o r t t o d e t e r m i n e the c r i t e r i a t h a t s h o u l d be used t o i n v o k e c a s e s o f enhanced d e g r a d a t i o n , an e x p e r i m e n t a l a p p r o a c h f o r i t s s t u d y was d e v e l o p e d t h a t f o c u s e d on l a b o r a t o r y i n v e s t i g a t i o n s w i t h field-collected soils. I t was o b v i o u s t h a t i n s e c t i c i d e c o n t r o l f a i l u r e s were common o c c u r r e n c e s and c e r t a i n l y not a l l due t o enhanced d e g r a d a t i o n , as i n v e s t i g a t i o n s o f f a u l t y a p p l i c a t i o n methods and u n u s u a l e n v i r o n m e n t a l c o n d i t i o n s have shown (18). The i d e a l a p p r o a c h t o t h e s t u d y o f enhanced d e g r a d a t i o n would i n v o l v e c o n t r o l l e d f i e l d r e s e a r c h i n which p e s t i c i d e p e r s i s t e n c e and c o n t r o l e f f i c a c y were b o t h measured a t many l o c a t i o n s o v e r a number o f y e a r s . However, t h e tremendous c o s t i n time and e f f o r t and c o n f o u n d i n g o f r e s u l t s by e n v i r o n m e n t a l v a r i a b l e s make a c o n t r o l l e d l a b o r a t o r y approach d e s i r a b l e . The l i m i t a t i o n o f l a b o r a t o r y e f f o r t s f o c u s e d e x c l u s i v e l y on t h e s o i l - i n s e c t i c i d e i n t e r a c t i o n i s t h a t t h e y cannot f u l l y a d d r e s s t h e a d d i t i o n a l i n s e c t - i n s e c t i c i d e and i n s e c t - c r o p i n t e r a c t i o n s p r e s e n t i n the f i e l d . T h i s means t h a t c a u t i o n must be e x c e r c i s e d when p r o o f o f enhanced d e g r a d a t i o n i s d i s c o v e r e d i n t h e l a b o r a t o r y , f o r t h i s does not n e c e s s a r i l y mean t h a t i n s e c t c o n t r o l and c r o p y i e l d w i l l be a d v e r s e l y a f f e c t e d under f i e l d c o n d i t i o n s . W i t h t h i s c a u t i o n i n mind, r e s u l t s o f l a b o r a t o r y i n v e s t i g a t i o n s c a n be kept i n p r o p e r p e r s p e c t i v e f o r what t h e y c a n b e s t p r o v i d e : a mechanistic understanding of s o i l - m i c r o b e - i n s e c t i c i d e i n t e r a c t i o n s . Rapid

I n s e c t i c i d e Degradation

Assay

The f i r s t p a r t o f t h e l a b o r a t o r y methodology f o r s t u d y o f enhanced d e g r a d a t i o n i n v o l v e d use o f a r a p i d d e g r a d a t i o n a s s a y by which an i d e a o f t h e r a t e o f i n s e c t i c i d e d e g r a d a t i o n c o u l d be o b t a i n e d f o r a l a r g e number o f s o i l s . The s o i l s f o r t h i s a s s a y were c o l l e c t e d d u r i n g t h e f a l l f r o m Iowa c o r n f i e l d s w i t h known h i s t o r i e s o f i n s e c t i c i d e use. In some c a s e s t h e s o i l s c o l l e c t e d were s p e c i f i c a l l y from f i e l d s i n which an i n s e c t i c i d e had been used f o r s e v e r a l y e a r s and no l o n g e r p r o v i d e d s u i t a b l e c o n t r o l o f s o i l i n s e c t p e s t s . In t h e s e c a s e s a s e p a r a t e s o i l was a l s o c o l l e c t e d f r o m an a d j a c e n t f i e l d or f r o m t h e f i e l d fencerow.



6.

RACKE & COATS

71


A 25-g p o r t i o n o f e a c h s o i l was p l a c e d i n a g l a s s j a r and treated with C - i n s e c t i c i d e a t 5 μg/g. I n s e c t i c i d e s used f o r t h i s a s s a y i n c l u d e d t h o s e f o r which f i e l d s w i t h some t r e a t m e n t h i s t o r i e s c o u l d be i d e n t i f i e ^ d : C-carbonyl-carbofuran, C-ring-isofenphos, C-ring-fonofos, C-ethoxy-terbufos, C-ethoxy-phorate, and C-ethyl-ethoprop. A f t e r e a c h s o i l was moistened t o 0.3 b a r s o i l - m o i s t u r e t e n s i o n , g l a s s v i a l s c o n t a i n i n g 0. 1 Ν NaOH were p l a c e d i n s i d e e a c h j a r t o s e r v e a s C 0 2 t r a p s . The j a r s were i n c u b a t e d i n t h e d a r k a t 25°C f o r 1 week, d u r i n g which t h e t r a p s were a n a l y z e d d a i l y f o r trapped CO2. R e s u l t s o f the r a p i d degradation assay (Table I) a r e expressed f o r each i n s e c t i c i d e as cumulative m i n e r a l i z a t i o n i n h i s t o r y s o i l s ( t r e a t e d w i t h t h a t p e s t i c i d e a t l e a s t one p r e v i o u s y e a r ) and non-history s o i l s (untreated o r treated with other i n s e c t i c i d e s ) . A l l s o i l s from f i e l d s p r e v i o u s l y t r e a t e d w i t h c a r b o f u r a n e x h i b i t e d a much h i g h e r m i n e r a l i z a t i o n r a t e t h a n s o i l s from n o n - h i s t o r y f i e l d s . P r e v i o u s r e p o r t s have c o n f i r m e d t h e enhanced d e g r a d a t i o n o f c a r b o f u r a n , and i t s r a p i d m i n e r a l i z a t i o n i n e a c h c a r b o f u r a n - h i s t o r y s o i l i s i n d i c a t i v e o f i t s g r e a t s u s c e p t i b i l i t y t o enhanced degradation. Even t h e f e n c e r o w s o i l s s u r r o u n d i n g c a r b o f u r a n - h i s t o r y f i e l d s e x h i b i t e d i n c r e a s e d c a r b o f u r a n m i n e r a l i z a t i o n r a t e s , and had a p p a r e n t l y been c o n t a m i n a t e d by c a r b o f u r a n o r c a r b o f u r a n - t r e a t e d soil. Up t o 5 y e a r s had e l a p s e d s i n c e t h e l a s t c a r b o f u r a n s o i l application to history soils.

Table

I. E f f e c t o f S o i l I n s e c t i c i d e Use H i s t o r y o n t h e M i n e r a l i z a t i o n o f C - I n s e c t i c i d e s t o CCte i n S o i l s D u r i n g a 1-Week L a b o r a t o r y Assay 1 4

14

% M i n e r a l i z a t i o n + SE Carbofuran History Nonhistory H i s t o r y Fencerow Isofenphos History Nonhistory Fonofos History Nonhistory Terbufos History Nonhistory Phorate History Nonhistory Ethoprop History Nonhistory

84. 0 + 18. 1 + 39..7 +

No. o f S o i l s

8. 0 7. 6 8. 2

8 11 2

35. 9 + 22. 4 2. 4 + 1. 0

6 19

20. 8 + 13. 6 9. 1 + 3. 5

13 16

12. 5 + 9. 7 +

4. 0 3. 2

11 5

13. 1 + 8. 0 +

4. 1 1. 3

4 12

32. 7 + 18. 9 19. 9 + 11. 6

2 8

F o r i s o f e n p h o s , t h e r e a l s o was a n i n c r e a s e d m i n e r a l i z a t i o n r a t e in isofenphos-history soils. Very l i t t l e m i n e r a l i z a t i o n o c c u r r e d i n



72

ENHANCED BIODEGRADATION OF PESTICIDES IN T H E ENVIRONMENT

n o n h i s t o r y s o i l s , and i n a l l c a s e s t h e r a t e o f m i n e r a l i z a t i o n was g r e a t e r i n h i s t o r y s o i l s , but v a r i e d i n magnitude from 5.1 t o 64.4%. Some d i f f e r e n c e i n t h e m i n e r a l i z a t i o n o f f o n o f o s i n t h e h i s t o r y and n o n h i s t o r y s o i l s was e v i d e n t , but the b e h a v i o r o f f o n o f o s was much more v a r i a b l e i n the f o n o f o s - h i s t o r y s o i l s . T h e r e seemed t o be two groups o f h i s t o r y s o i l s : t h o s e i n which the f o n o f o s m i n e r a l i z a t i o n r a t e was q u i t e r a p i d (5 s o i l s , 24.2-46.6%) and o t h e r s i n w h i c h i t was comparable t o the n o n - h i s t o r y s o i l s (8 s o i l s , 8.3-14.9%). The m i n e r a l i z a t i o n o f t e r b u f o s and p h o r a t e was not s i g n i f i c a n t l y g r e a t e r i n the c o r r e s p o n d i n g h i s t o r y s o i l s . Ethoprop e x h i b i t e d q u i t e v a r i a b l e m i n e r a l i z a t i o n b e h a v i o r i n b o t h h i s t o r y and n o n h i s t o r y s o i l s , and from the s m a l l number o f s o i l s a v a i l a b l e w i t h e t h o p r o p h i s t o r y i t i s not p o s s i b l e t o draw any m e a n i n g f u l c o n c l u s i o n s . This c o r r o b o r a t e s e a r l i e r s t u d i e s t h a t have r e p o r t e d no e v i d e n c e o f enhanced t e r b u f o s o r p h o r a t e d e g r a d a t i o n . Other r e p o r t s have i n v e s t i g a t e d the p o s s i b l e enhanced d e g r a d a t i o n o f t e r b u f o s and p h o r a t e and a l s o have f o u n d no e v i d e n c e o f an a c c e l e r a t e d r a t e o f d e g r a d a t i o n (21.22). T h e r e f o r e , more i n t e n s i v e s o i l d e g r a d a t i o n s t u d i e s f o c u s e d on c a r b o f u r a n , i s o f e n p h o s , f o n o f o s , and s e v e r a l related insecticides. C u m u l a t i v e p l o t s o f c a r b o f u r a n m i n e r a l i z a t i o n i n a h i s t o r y and a n o n h i s t o r y s o i l a r e p r e s e n t e d i n F i g u r e 1. The i n i t i a l l y accelerating rate of C02 p r o d u c t i o n , i n d i c a t i v e o f a m i c r o b i a l r e s p o n s e (e.g., enzyme i n d u c t i o n , p o p u l a t i o n growth), i s c h a r a c t e r i s t i c o f enhanced d e g r a d a t i o n . Comparison o f t h e mineralization of C - c a r b o n y l and C-ring labelled carbofuran d e m o n s t r a t e s an important c o n s i d e r a t i o n i n t h i s t y p e o f a s s a y : the l o c a t i o n o f the C l a b e l i n the i n s e c t i c i d e i s c r i t i c a l f o r t h i s type o f assay to provide u s e f u l i n f o r m a t i o n . Although the c a r b o n y l C was almost c o m p l e t e l y e v o l v e d as CO2, the r i n g C was o n l y slowly mineralized. T h e r e a r e s e v e r a l complementary r a p i d d e g r a d a t i o n s c r e e n i n g a s s a y s t h a t have been e f f e c t i v e l y used i n the l a b o r a t o r y . These i n c l u d e b i o a s s a y (16.23). gas c h r o m a t o g r a p h i c a s s a y o f p e r s i s t e n c e (17), c o l o r i m e t r i c a s s a y (24), and l i q u i d medium a s s a y (15.25). All c a n be u s e f u l i n s c r e e n i n g l a r g e numbers o f s o i l s f o r g r o s s d i f f e r e n c e s i n i n s e c t i c i d e - d e g r a d i n g behavior. However, i t i s e x t r e m e l y important t h a t s u c h a s s a y s a r e not used t o p r o v i d e t h e s o l e b a s i s f o r e x p l a i n i n g p o t e n t i a l i n s t a n c e s o f enhanced d e g r a d a t i o n (26). In o r d e r f o r enhanced d e g r a d a t i o n t o be t r u l y c o n f i r m e d f o r a g i v e n p e s t i c i d e a more t h o r o u g h l a b o r a t o r y and/or f i e l d i n v e s t i g a t i o n i n which the m i c r o b i o l o g i c a l a s p e c t s a r e c l a r i f i e d i s r e q u i r e d . Only t h e n c a n enhanced d e g r a d a t i o n be d i s t i n g u i s h e d from n a t u r a l v a r i a t i o n s i n s o i l i n s e c t i c i d e d e g r a d a t i o n due t o v a r i a t i o n i n s o i l p r o p e r t i e s (e.g., biomass, pH, o r g a n i c c o n t e n t ) . The most u s e f u l aspect of these r a p i d degradation assays i s i n i d e n t i f y i n g suspect s o i l s and p e s t i c i d e s f o r f u r t h e r i n v e s t i g a t i o n . P e s t i c i d e Degradation

i n Companion S o i l s

The s e c o n d component o f the l a b o r a t o r y methodology f o r s t u d y o f enhanced d e g r a d a t i o n i n v o l v e d a more i n t e n s e i n v e s t i g a t i o n o f t h e s o i l d e g r a d a t i o n o f i n s e c t i c i d e s i d e n t i f i e d as s u s p e c t from t h e r a p i d d e g r a d a t i o n a s s a y ( c a r b o f u r a n , i s o f e n p h o s , f o n o f o s ) as w e l l as 2 additional i n s e c t i c i d e s (cloethocarb, c h l o r p y r i f o s ) . The d e g r a d a t i o n



R A C K E & COATS


25 ι —

Isofenphos

Isof. History

20 —

ί Isof. History Cyclohex. C

/ /

//

— j

+

Nonhistory Isof. History Chloramph.

{τ — '

χ-' 0

1

2

3

4

isIsof. History Autoclaved

)+

5

6

7

Days of Incubation 14

Figure 1. Mineralization of C-labeled carbofuran and isofenphos to one-week laboratory assay.

1 4

C0

2

during a


74


o f t h e s e i n s e c t i c i d e s was examined i n 'companion' s o i l s . For a given i n s e c t i c i d e t h i s included a s o i l with a h i s t o r y of a p p l i c a t i o n of t h a t i n s e c t i c i d e and a s i m i l a r s o i l from an u n t r e a t e d a d j a c e n t experimental p l o t or f i e l d . The h i s t o r y s o i l s f o r c a r b o f u r a n , i s o f e n p h o s , and f o n o f o s had d i s p l a y e d e v i d e n c e o f r a p i d i n s e c t i c i d e d e g r a d a t i o n i n t h e r a p i d d e g r a d a t i o n assay. The s t r a t e g y was t o examine t h e r a t e s and p r o d u c t s o f d e g r a d a t i o n i n t r e a t e d and u n t r e a t e d companion s o i l s t o d e t e r m i n e i f a s h i f t i n dégradâtive b e h a v i o r had o c c u r r e d . F o r t h i s s t u d y 100 ^ s a m p l e s o f s o i l were p l a c e d i n g l a s s j a r s and t r e a t e d w i t h e i t h e r C - r i n g - c a r b o f u r a n , C - r i n g - c l o e t h o c a r b , C-ring-isofenphos, C-ring-fonofos, or C-ring-chlorpyrifos at 5 μg/g. S o i l s were moistened t o 0.3 b a r s o i l m o i s t u r e t e n s i o n and i n c u b a t e d f o r 4 weeks a t 25°C by u s i n g a f l o w - t h r o u g h i n c u b a t i o n s y s t e m i n which a i r was p e r i o d i c a l l y p a s s e d from t h e j a r s and t h r o u g h 0. 1 Ν NaOH t r a p s f o r m o n i t o r i n g o f e v o l v e d C 0 2 and maintenance o f a e r o b i c c o n d i t i o n s (27). S o i l s were t h e n e x t r a c t e d w i t h o r g a n i c s o l v e n t s t o remove e x t r a c t a b l e m e t a b o l i t e s and t h e e x t r a c t a n a l y z e d by t h i n - l a y e r o r h i g h - p r e s s u r e l i q u i d chromatography f o r d e t e r m i n i n g d i s t r i b u t i o n of metabolites. Samples o f e x t r a c t e d s o i l were combusted t o d e t e r m i n e s o i l - b o u n d C-residues. R e s u l t s o f the s o i l p e s t i c i d e metabolism study are presented i n T a b l e I I , and t h e companion s o i l s a r e l i s t e d f i r s t f o r e a c h p e s t i c i d e investigated. F o r c a r b o f u r a n , t h e r e was a tremendous d i f f e r e n c e i n d e g r a d a t i o n r a t e between t h e h i s t o r y and n o n h i s t o r y s o i l s . As oppposed t o t h e m i n e r a l i z a t i o n assay, i n which C-carbonylc a r b o f u r a n was used, t h i s s t u d y employed C-ring-carbofuran. T h e r e f o r e , a l t h o u g h l i t t l e m i n e r a l i z a t i o n o f the c a r b o f u r a n r i n g o c c u r r e d i n any s o i l , t h e r e was a tremendous a c c u m u l a t i o n o f s o i l - b o u n d r e s i d u e s i n the s o i l i n which c a r b o f u r a n was r a p i d l y degraded. The enhanced d e g r a d a t i o n o f c a r b o f u r a n has been e x t e n s i v e l y documented (18). The b e h a v i o r o f c l o e t h o c a r b was s i m i l a r t o t h a t o f c a r b o f u r a n , and i t t o o was e x t e n s i v e l y d e g r a d e d i n t h e h i s t o r y v e r s u s t h e n o n h i s t o r y s o i l w i t h s u b s t a n t i a l accompanying p r o d u c t i o n o f s o i l - b o u n d r e s i d u e s . C l o e t h o c a r b i s a carbamate i n s e c t i c i d e t h a t was under development f o r t h e rootworm i n s e c t i c i d e market but was withdrawn about the same time t h a t d e c r e a s e d p e r s i s t e n c e i n s o i l a f t e r r e p e a t e d use was n o t e d ( 2 8 ) . B o t h i s o f e n p h o s and f o n o f o s were much l e s s p e r s i s t e n t i n t h e h i s t o r y versus the n o n h i s t o r y s o i l s . A major d i f f e r e n c e between t h e s e i n s e c t i c i d e s and the carbamates was t h a t c o n s i d e r a b l e m i n e r a l i z a t i o n o f t h e a r o m a t i c r i n g p o r t i o n o f t h e organophosphorus compounds o c c u r r e d . F o r i s o f e n p h o s , c o n s i d e r a b l e q u a n t i t i e s (15.2%) o f i s o f e n p h o s oxon a c c u m u l a t e d o n l y i n t h e n o n h i s t o r y , whereas no s u c h a c c u m u l a t i o n was n o t e d i n t h e i s o f e n p h o s - h i s t o r y s o i l . Several r e p o r t s o f the r e d u c e d p e r s i s t e n c e o f i s o f e n p h o s i n f i e l d p l o t s f o l l o w i n g r e p e a t e d use have appeared (29.30). and i s o f e n p h o s was withdrawn from t h e rootworm i n s e c t i c i d e market a f t e r w i d e s p r e a d e x p e r i e n c e s o f c o n t r o l f a i l u r e f o l l o w i n g second y e a r a p p l i c a t i o n s . T h i s i s e s p e c i a l l y i r o n i c because upon f i r s t a p p l i c a t i o n t o s o i l i s o f e n p h o s behaves as one o f t h e most p e r s i s t e n t organophosphorus i n s e c t i c i d e s (31). In c o n t r a s t t o t h e o t h e r i n s e c t i c i d e s i n t e n s e l y i n v e s t i g a t e d , c h l o r p y r i f o s showed no d e c r e a s e i n p e r s i s t e n c e i n s o i l w i t h a h i s t o r y o f c h l o r p y r i f o s use. T h i s c o r r o b o r a t e s e a r l i e r l a b o r a t o r y and f i e l d


4

1 4

14


6. R A C K E & COATS Table

75


I I . R e c o v e r y o f P a r e n t I n s e c t i c i d e s and D e g r a d a t i o n P r o d u c t s From Companion I n s e c t i c i d e - H i s t o r y and U n t r e a t e d S o i l s A f t e r a 4-Week L a b o r a t o r y I n c u b a t i o n *

Insecticide/ Soil History

Parent Pesticide

Extractable Metabolites

Soil-Bound

CO2


C Recovered i n % o f A p p l i e d Carbofuran Untreated S o i l Carbofuran S o i l Cloethocarb S o i l Isofenphos S o i l

56.0 0.8 1.6 46.4

6.5 1.9 2.2 3.0

29.5 71.8 76.9 39.6

5.8 14.0 11.0 9.3

Cloethocarb Untreated S o i l Cloethocarb S o i l Carbofuran S o i l Isofenphos S o i l

67.0 6.4 24.0 45.6

2.0 0.1 2.1 1.2

22.8 73.9 43.9 35.8

8.1 18.8 15.3 10.2

Isofenphos Untreated S o i l Isofenphos S o i l Fonofos S o i l Carbofuran S o i l

62.8 12.9 76.3 74.9

16.1 3.6 13.5 12.4

9.2 23.6 8.0 7.3

10.0 52.4 4.7 4.5

Fonofos Untreated S o i l Fonofos S o i l Isofenphos S o i l Carbofuran S o i l

50.3 2.1 35.3 60.4

1.8 1.2 3.0 1.7

27.2 33.7 37.3 20.8

19.3 51.2 25.6 15.0

Chlorpyrifos Untreated S o i l Chlorpyrifos Soil Fonofos S o i l Carbofuran S o i l

57.1 55.2 33.9 55.8

6.0 5.2 32.6 13.4

17.1 8.1 18.6 15.4

15.2 26.7 9.1 10.9

* A d a p t e d i n p a r t from

(22.27.28)

s t u d i e s i n which t h e p e r s i s t e n c e o f c h l o r p y r i f o s has n o t been a f f e c t e d b y r e p e a t e d f i e l d a p p l i c a t i o n s (21.32). I t i s a t t h i s p o i n t t h a t t o o many s t u d i e s o f enhanced i n s e c t i c i d e degradation conclude. E v i d e n c e has been g e n e r a t e d t h a t i n s o i l s , p l o t s , o r f i e l d s s e l e c t e d f o r study the p e s t i c i d e e x h i b i t s d e c r e a s e d p e r s i s t e n c e when i t has been r e p e a t e d l y a p p l i e d . These companion s o i l s have o f t e n been s e l e c t e d a s a r e s u l t o f o b s e r v e d f i e l d f a i l u r e s , w i t h ' c o n t r o l * s o i l c o l l e c t e d from a n a d j a c e n t fencerow, p l o t , o r f i e l d . But does t h e c o m p a r i s o n o f r a t e s o f i n s e c t i c i d e d e g r a d a t i o n i n h i s t o r y and n o n - h i s t o r y s o i l s r e p r e s e n t s u f f i c i e n t e v i d e n c e f o r p r o v i n g t h a t a n i n s e c t i c i d e has undergone enhanced d e g r a d a t i o n ? We b e l i e v e not f o r s e v e r a l r e a s o n s . First, i t i s r e c o g n i z e d t h a t t h e r e i s a tremendous amount o f n a t u r a l v a r i a t i o n i n t h e r a t e o f p e s t i c i d e d e g r a d a t i o n between s o i l s due t o s u c h e n v i r o n m e n t a l f a c t o r s a s o r g a n i c m a t t e r c o n t e n t , pH, t e x t u r e , a n d moisture. F o r example, Laskowski e t a l . (33) r e p o r t e d t h a t t h e r e was



76


between a 2- and 3 6 - f o l d d i f f e r e n c e i n t h e d e g r a d a t i o n r a t e o f 12 p e s t i c i d e s i n d i f f e r e n t s o i l types. M e r e l y because s o i l from one f i e l d d i s p l a y s a much more r a p i d r a t e o f d e g r a d a t i o n t h a n an a d j a c e n t one does not mean t h a t the m i c r o b i a l a d a p t a t i o n f o r enhanced d e g r a d a t i o n has o c c u r r e d . Second, by d e f i n i t i o n , enhanced m i c r o b i a l d e g r a d a t i o n i s a m i c r o b i a l p r o c e s s , and u n l e s s i t i s d e m o n s t r a t e d t h a t the m i c r o b i a l community i s i n v o l v e d i n the r a p i d d e g r a d a t i o n o b s e r v e d , i t i s i m p o s s i b l e t o d i s t i n g u i s h i t from a b i o t i c mechanisms of p e s t i c i d e d degradation. T h i r d , c u r r e n t u n d e r s t a n d i n g o f enhanced d e g r a d a t i o n r e c o g n i z e s t h a t the mechanism o f enhanced m i c r o b i a l d e g r a d a t i o n i n v o l v e s an i n d u c i b l e m i c r o b i a l a d a p t a t i o n f o r p e s t i c i d e c a t a b o l i s m (18.34). I t has been d e m o n s t r a t e d t h a t the l e v e l o f t o t a l m i c r o b i a l biomass i n s o i l c a n sometimes g r e a t l y i n f l u e n c e t h e r a t e o f p e s t i c i d e d e g r a d a t i o n (35). S h o u l d the term 'enhanced d e g r a d a t i o n ' be used t o d e s c r i b e the i n c r e a s e d r a t e o f d e g r a d a t i o n s e e n i n s o i l s from f i e l d s t h a t have g r e a t e r l e v e l s o f m i c r o b i a l biomass (e.g. manured f i e l d s ) ? I t i s e v i d e n t from t h e s e c o n s i d e r a t i o n s t h a t f o r enhanced d e g r a d a t i o n t o be t r u l y c h a r a c t e r i z e d , t h e m i c r o b i o l o g i c a l a s p e c t s o f the phenomenon must be i n v e s t i g a t e d . M i c r o b i o l o g y o f Enhanced I n s e c t i c i d e

Degradation

A f i r s t s t e p i n t h e l a b o r a t o r y methodology r e g a r d i n g t h e m i c r o b i o l o g y o f enhanced d e g r a d a t i o n i n v o l v e d d e t e r m i n a t i o n o f m i c r o b i a l i n v o l v e m e n t i n the r a p i d i n s e c t i c i d e d e g r a d a t i o n n o t e d i n some history soils. The s t r a t e g y employed was t o expose s o i l s w i t h s u s p e c t e d enhanced d e g r a d a t i o n t o v a r i o u s a n t i m i c r o b i a l t r e a t m e n t s and m o n i t o r any i n h i b i t i o n o f i n s e c t i c i d e d e g r a d a t i o n . The r a p i d i n s e c t i c i d e d e g r a d a t i o n a s s a y i n which e v o l u t i o n o f C02 was used as an i n d i c a t o r o f d e g r a d a t i o n p r o v e d i d e a l f o r s u c h an i n v e s t i g a t i o n . Samples o f s o i l f o r the m i n e r a l i z a t i o n a s s a y were e i t h e r a u t o c l a v e d t o d e s t r o y t o t a l m i c r o b i a l a c t i v i t y , t r e a t e d w i t h 100 μg/g o f c h l o r a m p h e n i c o l t o i n h i b i t s o i l b a c t e r i a , o r t r e a t e d w i t h 100 μg/g o f cycloheximide to i n h i b i t s o i l fungi p r i o r to C-insecticide application. To i n v e s t i g a t e r e p o r t s t h a t t h e i n s e c t i c i d e e t h o p r o p i n t e r f e r e d w i t h enhanced c a r b o f u r a n d e g r a d a t i o n and t h a t s o i l m i c r o o r g a n i s m s might be u t i l i z i n g c a r b o f u r a n as a n i t r o g e n s o u r c e , s o i l samples p r e t r e a t e d w i t h 25 μg/g o f e t h o p r o p o r sodium n i t r a t e were a l s o i n c u b a t e d . R e s u l t s o f t h e s e a s s a y s a r e p r e s e n t e d i n F i g u r e 1. With c a r b o f u r a n , i s o f e n p h o s , and f o n o f o s (not shown), s t e r i l i z a t i o n by autoclaving t o t a l l y inhibited rapid pesticide degradation i n h i s t o r y soils. By i t s e l f t h i s seems t o i n d i c a t e t h a t the r a p i d d e g r a d a t i o n i s m i c r o b i a l l y mediated. However, a u t o c l a v i n g a l s o d e s t r o y s i m m o b i l i z e d enzymes p r e s e n t i n s o i l , and i t has been shown t h a t t h e s e enzyme systems a r e r e s p o n s i b l e f o r the e x t r e m e l y r a p i d organo-phosphorus i n s e c t i c i d e d e g r a d a t i o n o b s e r v e d i n some s o i l s (36). In o r d e r t o b e t t e r demonstrate the involvement o f t h e s o i l m i c r o b i a l community i n p e s t i c i d e d e g r a d a t i o n , i t i s d e s i r a b l e t o use a n t i m i c r o b i a l t r e a t m e n t s t h a t a r e more s e l e c t i v e t h a n a u t o c l a v i n g . A l t h o u g h the a n t i b a c t e r i a l compound c h l o r a m p h e n i c a l g r e a t l y i n h i b i t e d the d e g r a d a t i o n o f t h e s e i n s e c t i c i d e s , the a n t i f u n g a l compound c y c l o h e x i m i d e d i d not. Thus, t h i s type o f a s s a y c a n p r o v i d e i n f o r m a t i o n not o n l y on t h e m i c r o b i a l n a t u r e o f t h e phenomenon but a l s o on t h e m i c r o b i a l groups i n v o l v e d . The p r i m a r y r o l e o f t h e s o i l



6.

RACKE & COATS


77

b a c t e r i a i n the enhanced dégradâtive p r o c e s s has been w e l l documented (15.27.34). I t i s worth n o t i n g t h a t a n e x c e s s s u p p l y o f n i t r a t e d i d not a f f e c t c a r b o f u r a n d e g r a d a t i o n , but t h e i n s e c t i c i d e e t h o p r o p a p p a r e n t l y i n h i b i t e d the r a p i d d e g r a d a t i o n o f c a r b o f u r a n t h r o u g h some unknown mechanism. Use o f s t e r i l i z e d s o i l a s s a y s has commonly been used t o d i s t i n g u i s h enhanced d e g r a d a t i o n f r o m a b i o t i c d e g r a d a t i o n (15.29). A s e c o n d s t e p i n the s t u d y o f the m i c r o b i o l o g y o f enhanced d e g r a d a t i o n i n v o l v e d a n attempt t o enumerate the m i c r o b i a l p o p u l a t i o n i n s o i l capable of b e n e f i c i a l l y c a t a b o l i z i n g these i n s e c t i c i d e s . T h i s i s a c r i t i c a l s t e p because i t d i s t i n g u i s h e s between mere c o m e t a b o l i s m and m i c r o b i a l l y b e n e f i c i a l c a t a b o l i s m . A mostprobable-number (MPN) assay, by which m i c r o b i a l numbers a r e s t a t i s t i c a l l y e s t i m a t e d from a s e r i e s o f d i l u t i o n s , was used t o determine l e v e l s o f microorganisms capable of m e t a b o l i z i n g carbofuran, isofenphos, or fonofos i n s o i l s d i s p l a y i n g r a p i d p e s t i c i d e degradation. MPN t e c h n i q u e s have been e x t e n s i v e l y u t i l i z e d f o r the s t u d y o f m i c r o o r g a n i s m s i n a q u a t i c systems and t h e b a s i c methodology d e s c r i b e d by S o m e r v i l l e e t a l . (37) was used. The b a s a l s a l t s medium i n which m i c r o b i a l growth was a s s a y e d c o n t a i n e d e i t h e r i s o f e n p h o s o r f o n o f o s a s a s o l e c a r b o n s o u r c e , o r c a r b o f u r a n as a s o l e c a r b o n and n i t r o g e n s o u r c e . R e s u l t s w i t h i s o f e n p h o s were q u i t e c o n c l u s i v e . In n o n h i s t o r y s o i l s t h e r e were no m i c r o o r g a n i s m s d e t e c t e d t h a t c o u l d c a t a b o l i z e i s o f e n p h o s a s a s o l e c a r b o n s o u r c e , whereas i n s o i l s w i t h a n enhanced r a t e o f i s o f e n p h o s d e g r a d a t i o n between 6,000 and 12,000 i s o f e n p h o s d e g r a d i n g m i c r o o r g a n i s m s were p r e s e n t p e r gram o f s o i l . A similar p o p u l a t i o n o f i s o f e n p h o s d e g r a d e r s c o u l d be i n d u c e d i n u n t r e a t e d s o i l by prêtreatment w i t h 100 ppm o f i s o f e n p h o s . At h i g h e r i s o f e n p h o s p r e t r e a t m e n t l e v e l s i n c r e a s e s i n the t o t a l number o f s o i l b a c t e r i a a s measured b y s t a n d a r d p l a t e count methods c o u l d be d e t e c t e d . Thus, the numbers o f t o t a l b a c t e r i a i n i s o f e n p h o s - h i s t o r y and u n t r e a t e d s o i l s i n c r e a s e d 2 0 - f o l d and 7 - f o l d i n r e s p o n s e t o a 5,000 μg/g isofenphos a p p l i c a t i o n , r e s p e c t i v e l y . In the c a s e o f f o n o f o s , no p o p u l a t i o n o f f o n o f o s - c a t a b o l i z i n g m i c r o o r g a n i s m s c o u l d be d e t e c t e d i n the s o i l s w i t h r a p i d r a t e s o f f o n o f o s d e g r a d a t i o n . This indicates t h a t t h e r a p i d d e g r a d a t i o n o f f o n o f o s i n t h e s e s o i l s may be u n r e l a t e d t o any a c t u a l m i c r o b i a l a d a p t a t i o n and t h u s may not i n v o l v e enhanced degradation. In the c a s e o f c a r b o f u r a n , b o t h s o i l s d i s p l a y i n g a n enhanced r a t e o f c a r b o f u r a n d e g r a d a t i o n and u n t r e a t e d s o i l s c o n t a i n e d s i g n i f i c a n t p o p u l a t i o n s of carbofuran-metabolizing microorganisms. I n g e n e r a l , the numbers o f c a r b o f u r a n d e g r a d e r s was g r e a t e r i n c a r b o f u r a n - h i s t o r y s o i l s d i s p l a y i n g a n enhanced r a t e o f c a r b o f u r a n d e g r a d a t i o n (150,000-458,000/g s o i l ) t h a n i n u n t r e a t e d s o i l s (23,000-183,000/g s o i l ) . T h i s i m p l i e s t h a t t h e r e may be q u a l i t a t i v e d i f f e r e n c e s i n the r a t e a t which s o i l m i c r o o r g a n i s m s c a n degrade a given i n s e c t i c i d e as well as q u a n t i t a t i v e d i f f e r e n c e s i n populations o f degraders. A l t h o u g h work o n e s t i m a t i n g the p o p u l a t i o n s o f p h e n o x y a l k a n o i c h e r b i c i d e d e g r a d e r s and c a r b a m o t h i o a t e h e r b i c i d e d e g r a d e r s i n s o i l has shown h i g h e r p o p u l a t i o n l e v e l s i n s o i l s d i s p l a y i n g enhanced r a t e s o f h e r b i c i d e d e g r a d a t i o n (38.39). i n some c a s e s no s i g n i f i c a n t d i f f e r e n c e i n p o p u l a t i o n l e v e l s has been n o t e d (40). A t h i r d s t e p i n the s t u d y o f the m i c r o b i o l o g y o f enhanced d e g r a d a t i o n i n v o l v e d i s o l a t i o n o f pure c u l t u r e s o f i n s e c t i c i d e -



78

ENHANCED BIODEGRADATION O F PESTICIDES IN T H E ENVIRONMENT

degrading s o i l microorganisms. The s t r a t e g y was t o i n o c u l a t e a minimal m i c r o b i a l medium w i t h e i t h e r i s o f e n p h o s s u p p l i e d a s t h e s o l e c a r b o n s o u r c e , o r c a r b o f u r a n s u p p l i e d a s t h e s o l e c a r b o n and n i t r o g e n s o u r c e , and t h e n i s o l a t e i n d i v i d u a l c o l o n i e s from t h e medium f o r f u r t h e r c h a r a c t e r i z a t i o n and i d e n t i f i c a t i o n . Two b a c t e r i a l s t r a i n s were i s o l a t e d from s o i l d i s p l a y i n g a n enhanced r a t e o f i s o f e n p h o s d e g r a d a t i o n , a n Arthrobacter sp. and a Pseudomonas sp. (22.27). The Arthrobacter p r o v e d t o be t h e most e f f e c t i v e dégrader o f i s o f e n p h o s and c o u l d u t i l i z e i t o r i t s h y d r o l y s i s p r o d u c t s , i s o p r o p y l s a l i c y l a t e and s a l i c y l a t e , a s s o l e c a r b o n s o u r c e s f o r growth w h i l e m i n e r a l i z i n g t h e a r o m a t i c r i n g ( 2 2 ) . An e a r l i e r s t u d y o f enhanced i s o f e n p h o s d e g r a d a t i o n had r e s u l t e d i n t h e i s o l a t i o n o f a Corynebacterium sp. from i s o f e n p h o s - h i s t o r y s o i l s t h a t c o u l d m i n e r a l i z e i s o f e n p h o s ( 4 1 ) . Repeated e f f o r t s t o i s o l a t e c a r b o f u r a n - d e g r a d e r s f a i l e d t o i d e n t i f y m i c r o o r g a n i s m s t h a t c o u l d grow on a minimal medium c o n t a i n i n g carbofuran. A l t h o u g h an e a r l y s t u d y r e s u l t e d i n i s o l a t i o n o f a Pseudomonas sp. t h a t had some c a p a b i l i t y t o degrade c a r b o f u r a n i n c u l t u r e (15), the o n l y c a r b o f u r a n - c a t a b o l i z i n g microorganism s u c c e s s f u l l y i s o l a t e d from c a r b o f u r a n - h i s t o r y s o i l was a n Achromobacter sp. t h a t u t i l i z e d c a r b o f u r a n a s a s o l e n i t r o g e n s o u r c e (42). I t seems l i k e l y t h a t t h e r e a r e a v a r i e t y o f s o i l microorganisms, p r i m a r i l y b a c t e r i a , t h a t c a n adapt f o r i n s e c t i c i d e c a t a b o l i s m and t h u s mediate enhanced d e g r a d a t i o n . By i t s e l f , i s o l a t i o n o f a n i n s e c t i c i d e - c a t a b o l i z i n g m i c r o o r g a n i s m from s o i l i s n o t s u f f i c i e n t e v i d e n c e o f enhanced d e g r a d a t i o n , but c o u p l e d w i t h d a t a on i n c r e a s e d r a t e s o f d e g r a d a t i o n i n h i s t o r y s o i l s i t does complete t h e e v i d e n c e n e c e s s a r y t o s u b s t a n t i a t e t h i s phenomenon. One o f t h e a d d i t i o n a l bonuses t h a t r e s u l t s from i s o l a t i o n o f t h e s e i n s e c t i c i d e - d e g r a d i n g m i c r o o r g a n i s m s i s t h e o p p o r t u n i t y t o s t u d y t h e enzymology and g e n e t i c s o f the process. C r o s s - A d a p t a t i o n s f o r Enhanced

Degradation

A f i n a l c o n s i d e r a t i o n o f enhanced d e g r a d a t i o n o f s o i l i n s e c t i c i d e s i n the c o r n b e l t i s the s p e c i f i c i t y o f the a d a p t a t i o n t h a t leads t o enhanced m i c r o b i a l d e g r a d a t i o n . I t has been n o t e d t h a t i n some c a s e s an i n s e c t i c i d e i s degraded r a p i d l y i n s o i l from a f i e l d t o w h i c h i t has n e v e r been a p p l i e d b e f o r e , but a s i m i l a r i n s e c t i c i d e has. The s p e c i f i c i t y o f enhanced d e g r a d a t i o n c a n be s t u d i e d a t a l l l e v e l s from f i e l d p e r s i s t e n c e b e h a v i o r t o i n t e r a c t i o n s w i t h m i c r o b i a l enzymes. C r o s s - a d a p t a t i o n s seem t o be e s p e c i a l l y c h a r a c t e r i s t i c o f enhanced carbamate i n s e c t i c i d e d e g r a d a t i o n . Carbofuran degraded r a p i d l y i n a s o i l p r e v i o u s l y exposed t o c l o e t h o c a r b , w h i l e t h e r a t e o f c l o e t h o c a r b d e g r a d a t i o n was somewhat enhanced i n s o i l w i t h c a r b o f u r a n h i s t o r y ( T a b l e I I ) . A number o f r e l a t e d carbamate i n s e c t i c i d e s were d e g r a d e d i n s o i l r e p e a t e d l y t r e a t e d w i t h c a r b o f u r a n ( 1 7 ) , and t h e degree o f c r o s s - a d a p t a t i o n f o r carbamate i n s e c t i c i d e d e g r a d a t i o n i n s o i l has been shown t o depend on s t r u c t u r a l s i m i l a r i t y . Likewise, the c a r b o f u r a n - c a t a b o l i z i n g Achromobacter sp. i s o l a t e d from c a r b o f u r a n - h i s t o r y s o i l c o u l d u t i l i z e a number o f carbamate i n s e c t i c i d e s a s s o l e n i t r o g e n s o u r c e s ( 4 2 ) . The a d a p t a t i o n f o r enhanced d e g r a d a t i o n o f organophosphorus i n s e c t i c i d e s , however, a p p e a r s t o be much more s p e c i f i c . Neither fonofos nor c h l o r p y r i f o s was r a p i d l y d e g r a d e d i n i s o f e n p h o s - h i s t o r y s o i l , and o n l y f o n o f o s was



6. R A C K E & COATS

79


d e g r a d e d r a p i d l y i n f o n o f o s - h i s t o r y s o i l ( T a b l e I I ) . No c r o s s - a d a p t a t i o n s f o r enhanced d e g r a d a t i o n o f organophosphorus i n s e c t i c i d e s were n o t e d i n s o i l d i s p l a y i n g enhanced i s o f e n p h o s d e g r a d a t i o n , and t h e Arthrobacter sp. i s o l a t e d c o u l d m e t a b o l i z e o n l y i s o f e n p h o s i n pure c u l t u r e and d i d n o t m e t a b o l i z e o r c o m e t a b o l i z e o t h e r organophosphorus i n s e c t i c i d e s ( 2 2 ) . I t i s l i k e l y t h a t t h e s p e c i f i c i t y o f enhanced d e g r a d a t i o n depends o n t h e m i c r o b i a l m e t a b o l i c use o f t h e i n s e c t i c i d e . H y d r o l y s i s o f any N-methylcarbamate i n s e c t i c i d e y i e l d s methylamine a s one p r o d u c t , and i f t h e b a c t e r i a a r e u t i l i z i n g t h i s p r o d u c t a s a n i t r o g e n s o u r c e , t h e a b i l i t y t o h y d r o l y z e carbamate i n s e c t i c i d e s i n g e n e r a l would be advantageous (42). However, m i c r o b i a l d e g r a d a t i o n o f organophosphorus i n s e c t i c i d e s a s c a r b o n s o u r c e s p r o c e e d s v i a i n i t i a l h y d r o l y s i s and s e c o n d a r y m e t a b o l i s m o f t h e a r o m a t i c p h e n o l i c metabolites. Because t h e h y d r o l y t i c m e t a b o l i t e s o f organophosphorus i n s e c t i c i d e s t e n d t o be somewhat unique, t h i s might e x p l a i n t h e h i g h s p e c i f i c i t y o f t h e enhanced d e g r a d a t i o n o f b o t h i s o f e n p h o s (27) and d i a z i n o n (43). Towards a n E x p e r i m e n t a l

D e f i n i t i o n o f Enhanced

Degradation

In summary, t h e enhanced d e g r a d a t i o n o f a n i n s e c t i c i d e i n v o l v e s i t s r a p i d d e g r a d a t i o n b y a p o p u l a t i o n o f s o i l m i c r o o r g a n i s m s t h a t has adapted t o b e n e f i c i a l l y c a t a b o l i z e i t f o l l o w i n g exposure t o i t o r a similar insecticide. T h i s enhanced r a t e o f d e g r a d a t i o n may o r may not r e s u l t i n f a i l u r e o f t h e compound t o c o n t r o l t h e t a r g e t p e s t d e p e n d i n g o n e n v i r o n m e n t a l c o n d i t i o n s . F o r enhanced d e g r a d a t i o n t o be t h o r o u g h l y i n v e s t i g a t e d s t u d i e s must be c a r r i e d o u t t o d e m o n s t r a t e an i n c r e a s e d r a t e o f d e g r a d a t i o n i n s o i l s w i t h p r i o r i n s e c t i c i d e exposure, t o i d e n t i f y t h e r a t e s and p r o d u c t s o f d e g r a d a t i o n i n s i m i l a r s o i l s under c o n t r o l l e d c o n d i t i o n s , and t o e l u c i d a t e t h e m i c r o b i o l o g i c a l a s p e c t s o f t h e phenomenon i n c l u d i n g t h e i d e n t i f i c a t i o n o f a n adapted, i n s e c t i c i d e - c a t a b o l i z i n g m i c r o b i a l population. U s i n g t h e s e c r i t e r i a , o n l y c a r b o f u r a n and i s o f e n p h o s have been s u f f i c i e n t l y e x p e r i m e n t a l l y s c r u t i n i z e d s o a s t o c h a r a c t e r i z e t h e i r enhanced d e g r a d a t i o n i n s o i l . Some e v i d e n c e s u g g e s t s t h e p o s s i b i l i t y t h a t o t h e r i n s e c t i c i d e s may a l s o have undergone enhanced d e g r a d a t i o n , but f u r t h e r s t u d y i s r e q u i r e d t o provide s u f f i c i e n t evidence.

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Enhanced Biodegradation of Pesticides in the Environment - American

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