Chapter 2
Effects of Long-Term Phenoxyalkanoic Acid Herbicide Field Applications on the Rate of Microbial Degradation 1
2
Allan E. Smith and Guy P. Lafond 1
Agriculture Canada, Research Station, Regina, Saskatchewan S4P 3A2, Canada Agriculture Canada, Experimental Farm, Indian Head, Saskatchewan S0G 2K0, Canada
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2
There is evidence from a relatively few field studies to indicate that repeated treatments of 2,4-D and MCPA result in enhanced herbicide degradation rates as a result of adaptation by soil microorganisms. There is also some evidence that cross-enhancement can occur under field conditions, whereby previous applications of either 2,4-D or MCPA, result in enhanced breakdown of both herbicides.
The p h e n o x y a l k a n o i c a c i d h e r b i c i d e s 2,4-D (2,4d i c h l o r o p h e n o x y a c e t i c a c i d ) and MCPA ( 4 - c h l o r o - 2 m e t h y l p h e n o x y a c e t i c a c i d ) , d e v e l o p e d d u r i n g t h e Second World War (1, 2 ) , were t h e f i r s t t r u l y s e l e c t i v e h e r b i c i d e s f o r t h e c o n t r o l of weeds i n c r o p s and h e r a l d e d t h e s t a r t o f modern c h e m i c a l weed control. Success was immediate and t h e i r use i n c r e a s e d d r a m a t i c a l l y w i t h t h e a n n u a l p r o d u c t i o n o f 2,4-D i n t h e U n i t e d S t a t e s r i s i n g from 6 m i l l i o n kg i n 1950 t o o v e r 20 m i l l i o n kg by 1964 {2) . Other h e r b i c i d e s b a s e d on p h e n o x y a c e t i c , p h e n o x y p r o p i o n i c , and p h e n o x y b u t y r i c a c i d s were soon d e v e l o p e d f o r a g r i c u l t u r a l use i n Europe and N o r t h America. Thus, 2,4,5-T ( 2 , 4 , 5 - t r i c h l o r o p h e n o x y a c e t i c a c i d ) and s i l v e x ( 2 - ( 2 , 4 , 5 - t r i c h l o r o p h e n o x y ) p r o p i o n i c a c i d ) were i n t r o d u c e d i n 1945 and 1953, r e s p e c t i v e l y , f o r shrub and brushwood c o n t r o l . Mecoprop (2-(4c h l o r o - 2 - m e t h y l p h e n o x y ) p r o p i o n i c a c i d ) became a v a i l a b l e i n 1956 and d i c h l o r p r o p ( 2 - ( 2 , 4 - d i c h l o r o p h e n o x y ) p r o p i o n i c a c i d ) i n 1961, b o t h p r o v i n g a c t i v e a g a i n s t many weed s p e c i e s n o t c o n t r o l l e d by 2,4-D o r MCPA. The p h e n o x y b u t y r i c a c i d s 2,4-DB ( 4 - ( 2 , 4 - d i c h l o r o p h e n o x y ) b u t y r i c a c i d ) and MCPB ( 4 - ( 4 - c h l o r o - 2 - m e t h y l p h e n o x y ) b u t y r i c a c i d ) , i n t r o d u c e d d u r i n g t h e l a t e 1950s, were a b l e t o c o n t r o l weeds i n legume c r o p s n o r m a l l y damaged by low c o n c e n t r a t i o n s o f o t h e r phenoxy h e r b i c i d e s . Because o f t h e i r e f f e c t i v e n e s s a g a i n s t a n n u a l and p e r e n n i a l weeds and o t h e r unwanted v e g e t a t i o n , and because o f t h e i r r e l a t i v e low c o s t , t h e s e e i g h t c h e m i c a l s , known as 0097-6156/90/0426-0014$06.00/0 Published 1990 American Chemical Society
Racke and Coats; Enhanced Biodegradation of Pesticides in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
2. SMITH & LAFOND
Effects of Long- Term Phenoxyalkanoic Acid Applications
s u b s t i t u t e d phenoxyalkanoic a c i d h e r b i c i d e s are s t i l l e x t e n s i v e l y used throughout t h e world. Of t h e s e c h e m i c a l s , 2,4-D and MCPA a r e used t o t h e g r e a t e s t e x t e n t .
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S h o r t l y a f t e r t h e i r i n t r o d u c t i o n , i t was r e p o r t e d t h a t t h e p h e n o x y a c e t i c a c i d s were b i o l o g i c a l l y degraded i n s o i l w i t h t h e c a r b o n atoms o f t h e h e r b i c i d e s b e i n g u s e d as e n e r g y s o u r c e s by t h e degrading organisms. S o i l microorganisms capable o f degrading p h e n o x y a l k a n o i c a c i d h e r b i c i d e s were s u b s e q u e n t l y i s o l a t e d and identified. Over t h e y e a r s , a c o n s i d e r a b l e s c i e n t i f i c l i t e r a t u r e has appeared on t h e d e g r a d a t i o n o f p h e n o x y a l k a n o i c a c i d h e r b i c i d e s , e s p e c i a l l y 2,4-D and MCPA, by s o i l m i c r o o r g a n i s m s and on t h e various factors that affect t h e i r persistence. S o i l s p r e v i o u s l y t r e a t e d with c e r t a i n h e r b i c i d e s often e x h i b i t an i n c r e a s e d a b i l i t y t o degrade t h e s e c h e m i c a l s , s e e m i n g l y as a r e s u l t o f a p r o l i f e r a t i o n o f microorganisms adapted t o the h e r b i c i d e s i n q u e s t i o n . Such s o i l s a r e s a i d t o have become "enriched". The enhanced d e g r a d a t i o n o f h e r b i c i d e s i n s o i l as a r e s u l t o f r e p e a t e d a p p l i c a t i o n s has r e c e n t l y been r e v i e w e d (3_) . The phenomenon o f cross-enhancement, or the a b i l i t y of s o i l s p r e t r e a t e d w i t h s p e c i f i c h e r b i c i d e s t o degrade o t h e r s t r u c t u r a l l y r e l a t e d h e r b i c i d e s more r a p i d l y t h a n i n u n t r e a t e d c o n t r o l s o i l s , has been known f o r n e a r l y 40 y e a r s . Both enhanced d e g r a d a t i o n and c r o s s - a d a p t a t i o n have been o b s e r v e d under f i e l d c o n d i t i o n s f o r phenoxyalkanoic a c i d h e r b i c i d e s ( 3 - 5 ) . The o b j e c t i v e s o f t h i s r e v i e w a r e t o b r i e f l y summarize t h e v a r i o u s f a c t o r s i n f l u e n c i n g t h e breakdown o f t h e p h e n o x y a l k a n o i c a c i d h e r b i c i d e s i n s o i l s o f N o r t h America and Europe, and t h e n t o d i s c u s s t h e e f f e c t s o f t h e i r r e p e a t e d f i e l d usage on t h e r a t e o f t h e i r microbial degradation. Mechanisms R e s u l t i n g i n S o i l
Degradation
Abiotic. With a b s o r p t i o n maxima i n t h e range 2 8 0 t o 2 90 nM (6) phenoxyalkanoic a c i d s a r e a b l e t o absorb r a d i a t i o n found i n s u n l i g h t and undergo p h o t o c h e m i c a l d e c o m p o s i t i o n . T h e i r breakdown i n i r r a d i a t e d aqueous s o l u t i o n s i s w e l l s t u d i e d (_5) . However, p h o t o c h e m i c a l breakdown o f p h e n o x y a l k a n o i c a c i d s by s u n l i g h t a t t h e s o i l s u r f a c e i s n o t c o n s i d e r e d t o be a major s o u r c e o f h e r b i c i d a l l o s s (5) , s i n c e t h e c h e m i c a l s a r e a p p l i e d as post-emergence a p p l i c a t i o n s t o growing c r o p s , and r e s i d u e s r e a c h i n g t h e s o i l w i l l t h u s be p r o t e c t e d from t h e sun's r a d i a t i o n by t h e c r o p canopy. P h e n o x y a l k a n o i c a c i d s a r e f o r m u l a t e d as e s t e r s and amine s a l t s and c h e m i c a l h y d r o l y t i c mechanisms a r e c o n s i d e r e d r e s p o n s i b l e f o r the r a p i d c o n v e r s i o n o f t h e p h e n o x y a l k a n o i c e s t e r s t o t h e c o r r e s p o n d i n g p h e n o x y - a l k a n o i c a n i o n i n m o i s t s o i l s (_5) . The d i m e t h y l a m i n e s a l t o f 2,4-D s i m i l a r l y undergoes c h e m i c a l c o n v e r s i o n i n t h e s o i l , by d i s s o c i a t i o n , t o t h e a c i d a n i o n (_5) . However, a b i o t i c p r o c e s s e s do n o t appear t o r e s u l t i n any s i g n i f i c a n t d e g r a d a t i o n o f p h e n o x y a l k a n o i c a c i d s i n t h e s o i l (_5) . Microbiological. The importance o f s o i l m i c r o o r g a n i s m s f o r t h e d e g r a d a t i o n o f 2,4-D, MCPA, and 2 , 4 , 5 - T was demonstrated, s h o r t l y a f t e r t h e i r i n t r o d u c t i o n , by s o i l p e r f u s i o n e x p e r i m e n t s 8) . S o i l columns were c o n t i n u o u s l y p e r c o l a t e d w i t h aqueous s o l u t i o n s
Racke and Coats; Enhanced Biodegradation of Pesticides in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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ENHANCED BIODEGRADATION OF PESTICIDES IN T H E ENVIRONMENT
c o n t a i n i n g h i g h c o n c e n t r a t i o n s o f the h e r b i c i d e and t h e e l u a t e was m o n i t o r e d on a r e g u l a r b a s i s f o r h e r b i c i d e r e m a i n i n g . An i n i t i a l s l i g h t r e d u c t i o n i n h e r b i c i d e c o n c e n t r a t i o n due t o a d s o r p t i o n by s o i l c o l l o i d s was f o l l o w e d by a l a g - p h a s e , d u r i n g which t h e r e was l i t t l e apparent h e r b i c i d e l o s s . The l a g - p h a s e , r a n g i n g from 2 weeks f o r 2,4-D t o 40 weeks f o r 2,4,5-T, was f o l l o w e d by r a p i d breakdown (2/ .8) . F u r t h e r a p p l i c a t i o n s o f the p h e n o x y a c e t i c acid t o the s o i l columns were r a p i d l y degraded w i t h no l a g - p h a s e . It was c o n s i d e r e d 8) t h a t d u r i n g the l a g - p h a s e t h e r e was a b u i l d up o f m i c r o o r g a n i s m s c a p a b l e o f m e t a b o l i z i n g the h e r b i c i d e s and u s i n g them as energy s o u r c e s . Once a s u f f i c i e n t l y l a r g e p o p u l a t i o n had been r e a c h e d r a p i d breakdown o f t h e p h e n o x y a c e t i c a c i d s o c c u r r e d , so t h a t a d d i t i o n a l a p p l i c a t i o n s o f the c h e m i c a l s t o the e n r i c h e d s o i l s were a l s o r a p i d l y m e t a b o l i z e d . Subsequent s t u d i e s c o n f i r m e d t h a t m i c r o b i o l o g i c a l d e g r a d a t i o n was the mechanism by which p h e n o x y a l k a n o i c a c i d s a r e b r o k e n down i n the s o i l , and many s o i l b a c t e r i a and a c t i n o m y c e t e s c a p a b l e o f e f f e c t i n g t h e i r breakdown have been i s o l a t e d . These e a r l y s t u d i e s have been e x t e n s i v e l y r e v i e w e d (7-11). Over the y e a r s much has been made o f the s o - c a l l e d lag-phase f i r s t r e p o r t e d by Audus 8) from s o i l p e r f u s i o n e x p e r i m e n t s . Many o f t h e h e r b i c i d e - d e g r a d i n g organisms c o u l d have been i n i t i a l l y k i l l e d w i t h the h i g h h e r b i c i d e c o n c e n t r a t i o n s and w a t e r - s a t u r a t e d c o n d i t i o n s used i n t h e p e r f u s i o n s t u d i e s . A l s o a n a l y t i c a l p r o c e d u r e s used f o r m e a s u r i n g the h e r b i c i d e c o n c e n t r a t i o n s i n t h e p e r f u s a t e s , b a s e d on u l t r a - v i o l e t a b s o r p t i o n and p l a n t b i o a s s a y , were n o n s p e c i f i c and i n s e n s i t i v e . Much o f the e a r l y m i c r o b i o l o g i c a l work on the breakdown o f the p h e n o x y a l k a n o i c acid h e r b i c i d e s was c a r r i e d out w i t h i s o l a t e d m i c r o o r g a n i s m s i n c u l t u r e solutions using high substrate concentrations. Such c o n d i t i o n s a r e v a s t l y d i f f e r e n t from t h o s e o c c u r r i n g under f i e l d s i t u a t i o n s and i n l a b o r a t o r y s o i l s where much lower h e r b i c i d e c o n c e n t r a t i o n s are used. Thus, the l a g - p h a s e was most l i k e l y t h e r e s u l t o f e x p e r i m e n t a l and a n a l y t i c a l c o n d i t i o n s o f t h e e a r l y i n v e s t i g a t o r s . The e f f e c t s o f c o n c e n t r a t i o n on t h e d e g r a d a t i o n o f 2,4-D in s o i l have been s t u d i e d u s i n g ( 1 4 C ) l a b e l e d h e r b i c i d e w i t h the r e l e a s e o f (14C)carbon d i o x i d e b e i n g used as a measure o f the r a t e of breakdown o f t h e 2,4-D. In some o f t h e s e s t u d i e s , the l o s s o f 2,4-D was c o n s i d e r e d t o b e . b i p h a s i c w i t h a slow i n i t i a l e v o l u t i o n o f (14C)carbon d i o x i d e b e i n g f o l l o w e d by a more r a p i d r e l e a s e (1214). In c o n t r a s t , o t h e r s t u d i e s have shown the e v o l u t i o n o f (14C)carbon d i o x i d e from (14C)2,4-D t r e a t e d systems t o be u n i f o r m w i t h time (L3, JL5, 16) . A s i m i l a r phenomenon has been r e p o r t e d f o r the breakdown o f (14C)MCPA i n s o i l U 7 ) . In a l l t h e above e x p e r i m e n t s no attempts were made t o s p e c i f i c a l l y a n a l y z e f o r (14C)2,4-D or (14C)MCPA a c t u a l l y r e m a i n i n g , and i t has been c a u t i o n e d (_4, _5) t h a t e v o l u t i o n o f (14C) c a r b o n d i o x i d e i s not a t r u e measure o f t h e d e c o m p o s i t i o n r a t e o f ( 1 4 C ) p h e n o x y a l k a n o i c acids i n s o i l s . R a d i o a c t i v e s t u d i e s i n which s o l v e n t - e x t r a c t a b l e (14C) was c h a r a c t e r i z e d by c h r o m a t o g r a p h i c a s s a y have r e v e a l e d no lag-phase when low c o n c e n t r a t i o n s (
