Evaluating phytochemical and microbial contributions to atrazine degradation

Authors: HATCH, KATHLEEN - Lindenwood University; LERCH, ROBERT - Retired ARS Employee; KREMER, ROBERT - Retired ARS Employee; WILLETT, CAMMY - University Of Missouri; ROBERTS, CRAIG - University Of Missouri; GOYNE, KEITH - Virginia Polytechnic Institution & State University

Submitted to: Journal of Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2022
Publication Date: 8/19/2022
Citation: Hatch, K.M., Lerch, R.N., Kremer, R.J., Willett, C.D., Roberts, C.A., Goyne, K.W. 2022. Evaluating phytochemical and microbial contributions to atrazine degradation. Journal of Environmental Management. 321. Article 115840. https://doi.org/10.1016/j.jenvman.2022.115840.
DOI: https://doi.org/10.1016/j.jenvman.2022.115840

Interpretive Summary: Grass buffer strips have been demonstrated to reduce herbicide losses in surface runoff and increase degradation in soil. However, the basis for increased herbicide degradation in grass buffer soils remained unclear and has been attributed to increased microbial activity and to plant defense compounds that have been shown to degrade herbicides, such as atrazine. Atrazine remains one of the most commonly used herbicides for weed control in corn, but it has been shown to contaminate water resources, especially streams in agricultural watersheds. This work included three separate studies to assess how grasses affect atrazine degradation and to determine the relative contributions of microbial activity and plant defense compounds to atrazine degradation in soil. In the first study, we extracted root compounds from seven switchgrass cultivars and tested their ability to degrade atrazine under laboratory conditions. In 16 hours, these root extracts degraded 44 to 85% of the atrazine, demonstrating that switchgrass roots contained one or more compounds capable of degrading atrazine, but attempts to identify these root compound(s) failed. In the next study, we used a known atrazine-degrading compound, DBG, that was isolated from eastern gamagrass roots and studied the effect of DBG and atrazine binding to clay and soil on the rate of atrazine degradation. Results of this study showed that if either DBG or atrazine were bound to clay or soil, there was no atrazine degradation. This result indicated that root compounds would have little or no impact on atrazine degradation in the root zones of eastern gamagrass and switchgrass. In the last study, we added atrazine to large soil containers with four treatments: bare ground (control), eastern gamagrass, Kanlow switchgrass cultivar, and Cimarron switchgrass cultivar. The experiment was conducted in a greenhouse to control temperature and light; soil samples were collected periodically over 112 days. Results showed that atrazine degraded rapidly in all treatments, with half of the atrazine gone in 8 to 11 days for all treatments. All treatments showed significant increases in populations of specific soil microbial groups, such as actinomycetes, fungi, and bacteria. These results indicated that microbial response to added atrazine was the dominant way in which atrazine degrades in soil, and plant root compounds contribute little to atrazine degradation in the root zones of grasses. This work will benefit land managers and growers to better understand how grass buffers can mitigate herbicide contamination in runoff.

Technical Abstract: Inclusion of warm-season grasses like switchgrass (Panicum virgatum) and eastern gamagrass (Tripsacum dactyloides) in vegetated buffer strips has been shown to mitigate herbicide contamination in runoff and increase their degradation in soil. The mode of action by which buffer strip rhizospheres enhance herbicide degradation remains unclear but has been attributed to microbial and phytochemical contributions. The objectives of this study were to: 1) screen root extracts from seven switchgrass cultivars for the ability to degrade the herbicide atrazine in solution; 2) determine sorption coefficients (Kd) of the known atrazine degrading phytochemical, DBG, to soil and Ca-montmorillonite, and investigate if DBG or atrazine sorption alters degradation processes; and 3) quantify atrazine degradation rates and soil microbial response to atrazine application in mesocosms containing soil and select warm-season grasses. Phytochemicals extracted from the roots of seven switchgrass cultivars degraded 44 to 85% of added atrazine in 16-h laboratory assays, demonstrating that some switchgrass cultivars could rapidly degrade atrazine under laboratory conditions. However, attempts to isolate atrazine-degrading phytochemicals from the most rapidly degrading cultivars were unsuccessful. Sorption studies revealed that DBG was strongly sorbed to soil (Kd = 87.2 L/kg) and Ca-montmorillonite (Kd = 31.7 L/kg), and DBG driven atrazine hydrolysis was entirely inhibited when either atrazine or DBG were sorbed to Ca-montmorillonite. Atrazine degradation rates in the mesocosms were rapid (half-life = 8.2 - 11.2 d), but not significantly different between two switchgrass cultivars, an eastern gamagrass cultivar, and bare-ground control. However, significant changes in five phospholipid fatty acid biomarkers were observed among treatments. These changes indicated that different atrazine degrading microbial consortia resulted in equivalent atrazine degradation rates between treatments. Results demonstrated that soil microbial response was the dominant mechanism controlling atrazine degradation in soil studied, rather than root phytochemicals.