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1 July 2012 Selection Pressure, Cropping System, and Rhizosphere Proximity Affect Atrazine Degrader Populations and Activity in s-Triazine–Adapted Soil
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Abstract

A field study was conducted on an s-triazine–adapted soil to determine the effects of s-triazine exclusion interval (1, 2, 3, or 4 yr), crop production system (continuous corn or continuous soybean), and rhizosphere proximity (bulk or rhizosphere soil) on atrazine degrader populations and activity. Atrazine degrader populations were quantified by a radiological Most Probable Number technique, while degrader activity was assessed via mineralization of ring-labeled 14C-atrazine. As the s-triazine exclusion interval increased, atrazine degrader populations declined exponentially, regardless of crop or rhizosphere proximity. Crop and exclusion interval interacted to affect degrader populations (P  =  0.0043). Pooled over rhizosphere and bulk soil, degrader populations were 1.5-fold higher and declined 2.8-fold faster in soybean than corn. An interaction between rhizosphere proximity and exclusion interval was also noted (P  =  0.0021), whereby degrader populations were 1.9-fold higher and declined 2.8-fold slower in rhizosphere compared with bulk soil, regardless of crop. The time required for 50% mineralization of ring-labeled 14C-atrazine (DT50) following exclusion of s-triazine herbicides increased linearly at a rate of 2.2 d yr−1. In contrast, the DT50 for this site prior to a known s-triazine application was 85 d and declined exponentially over 5 yr of successive atrazine applications: 24.5 d after 1 yr, 10.8 d after two successive years, and 3.8 d after five successive atrazine applications. Omitting s-triazines can reduce degrader populations and activity in adapted soils, but more than 4 yr is required to return mineralization kinetics to nonadapted levels, regardless of crop or rhizosphere proximity.

Nomenclature: Atrazine; s-triazine; corn, Zea mays L.; soybean, Glycine max (L.) Merr.

Weed Science Society of America
L. Jason Krutz, Robert M. Zablotowicz, and Krishna N. Reddy "Selection Pressure, Cropping System, and Rhizosphere Proximity Affect Atrazine Degrader Populations and Activity in s-Triazine–Adapted Soil," Weed Science 60(3), 516-524, (1 July 2012). https://doi.org/10.1614/WS-D-11-00104.1
Received: 22 June 2011; Accepted: 1 March 2012; Published: 1 July 2012
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