Submitted to: Weed Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/6/2012
Publication Date: 8/15/2012
Citation: Krutz, L.J., Zablotowicz, R.M., Reddy, K.N. 2012. Selection pressure, cropping system and rhizosphere proximity affect atrazine degrader populations and activity in s-triazine adapted soil. Weed Science. 60:516-524. Interpretive Summary: Enhanced atrazine degradation is a phenomenon whereby soil borne bacteria have developed the ability to rapidly degrade this economically important herbicide to the point that residual weed control is reduced. We postulated that atrazine persistence in affected soils could be restored to baseline levels by eliminating atrazine applications and switching from corn to soybean production. Our results indicate that even after four years of not applying atrazine that soil bacteria retained the ability to rapidly degrade the herbicide. Modeling efforts indicate that atrazine would need to be excluded for perhaps 40 yrs before the compounds persistence was similar to that observed prior to bacteria developing the ability to rapidly degrade this pesticide. This means that alternative chemistries should be used in place of or in conjunction with atrazine in affected soils to maintain acceptable weed control
Technical Abstract: Atrazine degrader populations and activity in s-triazine adapted soils are likely affected by interactions among and (or) between s-triazine application frequency, crop production system, and proximity to the rhizosphere. A field study was conducted on an s-triazine adapted soil to determine the effect(s) of s-triazine exclusion interval (1, 2, 3 or 4 yrs), 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 to bulk soil, regardless of crop. The time required for 50% mineralization of ring-labeled 14C-atrazine (DT50min) following exclusion of s-triazine herbicides increased linearly at a rate of 1.5 d yr-1. In contrast, the DT50min for this site prior to a known s-triazine application was 90 d and declined exponentially over five years of successive atrazine applications: 24.5 d after 1 yr, 10.8 d after 2 successive yrs, and 3.8 d after 5 successive atrazine applications. Omitting s-triazines can reduce degrader populations and activity in adapted soils, but more than four years is required to return mineralization kinetics to non-adapted levels, regardless of crop or rhizosphere proximity.