Evaluating phytochemical contributions to atrazine degradation

Abstract

Surface water contamination from the herbicide atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) has been a concern for many years. The use of vegetated buffer strips (VBS) is known to improve water quality but the use of specific vegetation has the potential to substantially improve VBS efficacy. This research revealed significant differences in microbial response, based on phospholipid fatty acid (PLFA) biomarkers, to atrazine application specific to treatments of eastern gamagrass, Kanlow switchgrass, Cimarron switchgrass, and bare ground Control in mesocosm studies. Interestingly, microbial community changes measured with PLFA were not reflected in rates of atrazine degradation which were similar for all treatments. Atrazine degradation rates were especially surprising with regard to the eastern gamagrass treatment, as these plants are known producers of the atrazine degrading benzoxazinone (Bx) compound DIBOA-Glc. Additional research revealed atrazine degradation from switchgrass root extracts of eight varieties under laboratory conditions. Identification of atrazine degrading phytochemicals in switchgrass root extract was not completed due to seasonal variation in phytochemical production and instability of root extracts. Sorption studies of DIBOA-Glc revealed partitioning coefficients of 31.65 for Ca saturated montmorillonite (Ca-montmorillonite) clay and 87.22 for Mexico silt loam soil. Studies of the atrazine: DIBOA-Glc reaction in the presence of Ca-montmorillonite equilibrated with atrazine, as well as Ca-montmorillonite equilibrated with DIBOA-Glc, revealed that atrazine hydrolysis was inhibited by clay sorption. These results indicated that phytochemically mediated atrazine degradation from Bx compounds contributes minimally to atrazine degradation in soil. The driving force behind atrazine degradation within the soil environment appears to be microbially mediated. Lastly, throughout attempts to identify the atrazine metabolite formed from switchgrass root extract, it was discovered that the biocide sodium azide (NaN3) was capable of producing a previously undocumented aminated atrazine metabolite as well as significantly different atrazine degradation rates between switchgrass varieties. These results serve as a warning against the use of NaN3 as a biocide with atrazine degradation studies. Selection of vegetation for VBS based on the support of soil microbial communities has the potential to enhance VBS efficacy with regard to minimizing atrazine contamination of surface water.