MICROBIAL ADAPTION FOR ACCELERATED ATRAZINE MINERALIZATION/DEGRADATION IN MISSISSIPPI DELTA SOILS

Authors: Zablotowicz, Robert; Weaver, Mark; Locke, Martin

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2006
Publication Date: 6/19/2006
Citation: Zablotowicz, R.M., Weaver, M.A., Locke, M.A. 2006. Microbial adaption for accelerated atrazine mineralization/degradation in mississippi delta soils. Weed Science 54:538-547.

Interpretive Summary: Most well-drained Mississippi Delta soils have been historically used for cotton production, but corn has recently become a desirable alternative crop. A survey of 21 soils twenty-one collected from grower and experimental sites in Leflore, Sunflower, and Washington counties, Mississippi, with known management histories was conducted in 2000 and 2001. Atrazine degradation was assessed in 30-day laboratory studies using radiolabelled herbicide. All soils with a history of atrazine application were capable of extensive and rapid degradation of atrazine to carbon dioxide. Soils with a history of two atrazine applications had a half-life of 6 days or less, while soils with no atrazine exposure had half-lives of 14 to over 60 d. Atrazine degrading-microorganisms were observed in all atrazine exposed soils, and none were detected in soils with no history of atrazine use. These results indicate that microbial populations capable of accelerated atrazine degradation have developed in Mississippi Delta soils. This may reduce the weed control efficacy of atrazine, but reduce the potential for off-site movement.

Technical Abstract: Most well-drained Mississippi Delta soils have been historically used for cotton production, but corn has recently become a desirable alternative crop. Between 2000 and 2001 twenty-one surface soils (0 to 5 cm depth) were collected from grower and experimental sites in Leflore, Sunflower, and Washington counties, Mississippi, with known management histories. Atrazine degradation was assessed in 30-day laboratory studies using 14C-ring labeled herbicide. Mineralization was extensive in all soils with a history of one to three atrazine applications with cumulative mineralization over 30 d ranging from 45 to 72%. In contrast, cumulative mineralization of atrazine from three soils with no atrazine history was only 5 to 10%. However, one soil with no history of atrazine application mineralized 54% and 29% of the atrazine added in soils collected in 2000 and 2001, respectively. Methanol extracted 15 to 23% of the 14C-atrazine 7 d. after treatment in soils having two applications within the past six years, while 65 to 70% was extracted from no history soils. Based on first order kinetic models, soils that have had two years of atrazine exposure exhibited a half-life of less than 6 d. Most Probable Number estimates of atrazine-ring-mineralizing microorganisms ranged from 452 to 5,899 propagules g-1 in atrazine exposed soils, and none were detected in soils with no history of atrazine use. Although most soils exhibited rapid atrazine mineralization, analysis of DNA isolated from these soils by direct PCR or nested PCR failed to amplify DNA sequences with primers for the atzA atrazine chlorohydrolase gene. These results indicate that microbial populations capable of accelerated atrazine degradation have developed in Mississippi Delta soils. This may reduce the weed control efficacy of atrazine, but reduce the potential for off-site movement. Studies are continuing to identify the genetic basis of atrazine degradation in these soils.