Accelerated triazine degradation in soil

Dale L. Shaner, USDA-ARS (retired), USA

Introduction

The symmetrical triazines, including atrazine, simazine, cyanazine and ametryn, have been a mainstay in weed management in multiple crops.  One of the advantages of the triazines is their long term soil residual activity.  However, in the last 10 years growers have noticed that atrazine is not lasting as long as it used to.  One reason for this is the selection of triazine resistant weeds, but there is another phenomenon that was first recognized in the mid-1990s, which is accelerated degradation.  This paper will describe this phenomenon and the implications of accelerated triazine degradation on the efficacy of this class of herbicides.

Discovery of accelerated degradation

Atrazine has been used since the 1950s and has been a very effective soil-residual herbicide. Early research on the degradation of atrazine in the field indicated that the herbicide is primarily degraded by a combination of microbial and chemical hydrolysis, both of which occur very slowly.  In addition, it was shown that atrazine degraded more slowly under basic soil conditions (> pH 7).   The half-life (the time it takes for half of the herbicide to be degraded) of atrazine ranged from 28 to 178 d depending on soil type, soil pH and the rate applied.  Carryover of atrazine to sensitive follow crops was of great concern.  Current weed control programs function under this historic paradigm, that is, s-triazine herbicides are relatively persistent, provide season long residual weed control, and are highly susceptible to off-site transport.  Data now indicate, however, that a recent bacterial adaptation has occurred thereby enabling rapid mineralization of highly substituted s-triazine herbicides.   

In the mid-1990s researchers isolated a soil microbe from an atrazine contaminated soil collected from an agricultural chemical dealership in the USA and another from a site in Canada.  These microbes were capable of degrading atrazine completely to CO₂ and nitrogen.  Subsequent work showed that other microbes also had this ability and that they contained a plasmid (a small piece of DNA) which encoded for six enzymes that completely degrade atrazine and other closely related triazines.  The genes encoding these enzymes occur in diverse phyla, are highly conserved and are globally ubiquitous.  For example, bacterial isolates from four phyla originating from six continents bear homologs of s-triazine catabolism genes.  These genes are contained on a plasmid which can be easily shared among soil microbes.  It is not known how the plasmid containing the triazine degrading genes spread, but since there is such homology in the sequence of the plasmid throughout the world, it is likely that the plasmid spreads with dust blown throughout the world.

In 2002 I discovered that atrazine was being rapidly degraded in fields that had a history of atrazine use.  An example of this is shown in Figure 1.  In this case, the farmer applied 1.5 kg/ha of atrazine on 30 April, and then applied another 1 kg/ha on 30 May.  The level of atrazine in the soil rapidly dissipated after the first application and then dissipated even more rapidly after the second application.  This farmer had not planted glyphosate-resistant corn and could not manage the weeds due to the rapid dissipation of atrazine.  He suffered severe crop loss.  A survey of other fields throughout the USA showed that accelerated atrazine degradation occurs in approximately 80% of the fields which have a history of atrazine or simazine use.  This accelerated degradation has been found in corn, sugarcane, orchard crops, and chemical fallow fields.  Accelerated triazine degradation has also been documented in sorghum fields in Australia (Rattray et al. 2007).  A good review on the phenomenon of accelerated triazine degradation is Krutz et al. 2010.

Detection of accelerated degradation

I developed a lab assay that can be used to detect accelerated atrazine degradation (Shaner et al. 2007).  In this assay soil from the field is treated with atrazine and then the rate of dissipation of the herbicide is measured.  In soils with accelerated degradation, atrazine will have a half-life in the soil of 1 to 3 days. I had one soil where the half-life is 9 hours.  Atrazine’s half-life in a soil without accelerated degradation will be greater than 10 days.  I did develop an assay using pesticide test strips which are used to detect atrazine levels in tap water.  This strips will detect any level higher than 3 ppb in water.  Water extracts taken from soil with accelerated degradation will not have any detectable atrazine by 7 days after treatment.

Figure 1: Dissipation of atrazine in a corn field in eastern Colorado, USA.

Factors affecting presence of accelerated triazine degradation

We have attempted to determine what factors are associated with accelerated triazine degradation.  There are three main factors: 1} Recent atrazine use (within the last 3 years); 2) Soil pH greater than 6.2, and 3) Soil organic matter greater than 1%..  What is interesting about these results is that accelerated triazine degradation occurs in soil with high pH, which is directly opposite of the old paradigm which says that triazines degraded more rapidly in acidic soils.  If a field has not been treated for several years with triazine, accelerated degradation can still occur, there will just be a lag phase before degradation happens.  In a field test on a soil which had not been treated with atrazine for more than 5 years, we found that there was very little degradation for the first 3-4 weeks, and then the herbicide dissipated very quickly (Figure 2). 

Other environmental factors affect the rate of accelerated degradation.  Since it is a biological phenomenon, cold temperatures (below 45^o C) and dry soil conditions will slow down the rate of degradation.  In addition, high nitrogen levels in the soil inhibit the rate of degradation.  It has been shown that the soil microbes are using the triazine as a nitrogen source and they do not metabolize the atrazine until the soil nitrogen levels decrease.

Figure 2: Dissipation of atrazine in field corn

Implications of accelerated triazine degradation

There are several implications of accelerated atrazine degradation.  The most obvious is that the triazines residual activity will be greatly shortened.  This means that the grower cannot rely on season long control with the triazine.  In my experience, atrazine at labelled use rates will provide about 3 to 4 weeks of control and then the more tolerant species will start to break through.  In one case, the grower that I was working with liked the phenomenon in his field.  He applied atrazine post to 2-4 leaf corn and then applied another application about 4 weeks later. By the time the herbicide had dissipated, the canopy had closed and there were few weeds.  After harvest, he planted the field in an annual rye cover crop with no injury from residual atrazine. 

A second implication of accelerated triazine degradation is a greatly reduced risk of the herbicide leaching to the ground water.  We compared the movement of atrazine in the top 30 cm of the soil profile in a plot that had accelerated degradation with an adjoining plot without the accelerated degradation.  In the plot with the accelerated degradation, atrazine did not move below the top 15 cm, whereas it moved down to at least the 30 cm level in the plot without the accelerated degradation (Figure 3).

Figure 3: Comparison of the movement of atrazine in the soil profile over time in a plot with accelerated degradation (continuous corn) with a plot without accelerated degradation (wheat-fallow-wheat-corn)

Conclusion

Accelerated triazine degradation has been found throughout the world where atrazine has been heavily used.  This accelerated degradation results in reduced residual activity and can lead to unsatisfactory weed control.  The factors that are related to accelerated triazine degradation are 1)recent triazine application; 2)soil pH greater than 6.2; and 3)soil organic matter. 

Literature

D. J. Rattray, et al.  2007 Australian Journal of Soil Research, 45:598–606.

Krutz et al. 2010. Pest Management Science 66:461–481.

Shaner et al. 2007. Weed Science. 5:528–535.

Contact details

Dale Shaner
USDA-ARS
01-970-556-4826
daleshaner@aol.com