Submitted to: North American Agroforestry Conference
Publication Type: Proceedings
Publication Acceptance Date: 5/31/2007
Publication Date: 6/10/2007
Citation: Lin, C.H., Lerch, R.N., Kremer, R.J., Garrett, H.E., George, M.F. 2007. Stimulated Rhizodegradation of Atrazine by Selected Plant Species. North American Agroforestry Conference, June 10-13, Quebec City, Quebec. p. 95-105.
Interpretive Summary: Vegetative buffer strips (VBS) composed of forage grass species reduce herbicide transport by enhancing water infiltration of surface runoff from adjacent croplands. Once deposited, the fate of the herbicide in the VBS soil is critical to the long-term effectiveness of the VBS at preventing herbicides from entering surface or ground waters. If the herbicide remains intact within the VBS soil, it may be transported from the VBS to nearby water resources. On the other hand, if the VBS promotes degradation of the herbicide, then it will not contaminate water resources and it will generally result in less toxic breakdown products. Therefore, we conducted a study with the corn herbicide, atrazine, to determine if forage grasses affect the soil degradation of atrazine. Seven forage grass species were tested: 1) switchgrass, 2) eastern gammagrass, 3) tall fescue, 4) orchardgrass, 5) smooth bromegrass, 6) ryegrass, and 7) Illinois bundle flower. All plant treatments were grown in pots containing Mexico silt loam, a common claypan soil. Pots containing soil without plants were used as controls. Forages were grown to maturity, and the rhizosphere soil (i.e., the surface soil zone in contact with plant roots) was separated by hand from the plants and roots. Atrazine was then applied to the rhizosphere soil and incubated in the dark for 100 days. Rates of atrazine degradation in plant rhizospheres were significantly enhanced by 84 to 260% compared to the control. Eastern gammagrass showed the highest capability for promoting atrazine degradation in the rhizosphere, with more than 90% of the atrazine degraded compared to 24% in the control. Of the species tested, eastern gammagrass, orchardgrass, smooth bromegrass, and switchgrass showed the greatest ability to enhance soil degradation of atrazine, and these forages are recommended for inclusion in VBS to decrease atrazine losses in corn production areas. This research will benefit conservation agencies and policy makers interested in developing effective practices for reducing herbicide transport from cropland. It will benefit farmers because well designed VBS will require the least amount of land be taken out of production to achieve conservation goals.
Technical Abstract: The efficacy of vegetative buffer strips (VBS) in removing herbicides from surface runoff is related to the ability of plant species to promote rapid herbicide degradation. A growth chamber study was conducted to investigate the rhizodegradation of 14C-atrazine and the relationship of degradation with soil enzyme activities in the rhizosphere of seven selected plant species. The plant species included: 1) switchgrass, 2) eastern gammagrass, 3) tall fescue, 4) orchardgrass, 5) smooth bromegrass, 6) ryegrass, and 7) Illinois bundle flower. All plant treatments were grown in pots containing Mexico silt loam. Pots containing soil without plants were used as controls. Forages were grown to maturity (~3 months), and the rhizosphere soil was separated by hand from the plants. Radio labeled atrazine was then applied to the rhizosphere soil and incubated in the dark for 100 days. Rates of atrazine degradation in plant rhizospheres were significantly enhanced by 84 to 260% compared to the control. All plant species significantly enhanced atrazine degradation compared to the control, but eastern gammagrass showed the highest capability for promoting bio-degradation of atrazine in the rhizosphere. More than 90% of atrazine was degraded in the eastern gammagrass rhizosphere compared to 24% in the control. Biological dealkylation of atrazine strongly correlated with increased enzymatic activities of Beta-glucosidase (GLU) (r = 0.96), dehydrogenase (DHG) (r = 0.842) and fluorescein diacetate (FDA) hydrolysis (r =0.702). The incorporation of forage species, into VBS designs will significantly promote the degradation of atrazine transported to the VBS. Microbial parameters widely used for assessment of soil quality (e.g., DHG and GLU activities) are promising as useful tools for evaluating the overall degradation potential of various vegetative buffer designs for atrazine remediation.