Bioremediation capacity of five forage grasses for atrazine, Balance (isoxaflutole) and nutrient removal

Well-designed, multi-species riparian buffer strips are recognized as one of the most cost-effective bioremediation approaches to alleviating nonpoint sources of agricultural pollutants from croplands. Thirty-six, 1 m wide and 0.5 m deep, fluorinated lysimeters with six different ground covers (bare ground, orchardgrass, tall fescue, timothy, smooth bromegrass, and switchgrass) were established in 1998 to evaluate the effect of forages on the fate and transport of atrazine (ATR), Balance (isoxalflutole; IXF) and nitrate. Concentrations of ATR, Balance and their metabolites in the leachate, soil and plant tissues were determined by solid phase or liquid-liquid extraction followed by high performance liquid chromatography UV spectroscopy (HPLC-UV), HPLC mass spectrometry (HPLC-MS), HPLC tandem mass spectrometry (MS/MS), or gas chromatography tandem mass spectrometry (GC-MS/MS). The results suggested that the total Balance (parent + metabolites) showed higher mobility than the atrazine and its metabolites. Differences in the timing of transport reflect the rapid degradation of IXF to the more soluble, stable and biologically active diketonitrile (DKN) metabolite in the system. Grass treatments significantly enhanced ATR degradation in the leachates and soils, especially through N-dealkylation reaction, but they did not reduce total ATR present in the leachates. Among the forage-treated lysimeters, the greatest ATR degradation was found in switchgrass-treated (80.7%), timothy-treated (74.7%) and smooth bromegrass-treated (72.2%) soils. In contrast, grass treatments did not promote the hydrolysis of DKN in soils and leachates, but they significantly reduced the total quantity in the leachates through enhanced evapotranspiration. The degradation of DKN appears to be an abiotic process under the conditions tested in this study. Switchgrass, smooth bromegass and tall fescue exhibited the highest microbial denitrification capacity and could remove up to 99.7% of nitrate from leachates and 91.2% of nitrate from the lysimeter soils. For ATR-treated forages, growth inhibition of each forage coincided with its hydroxylation capacity. For Balance-treated grasses, growth inhibition is strongly associated with their capacity to convert DKN to a nonphytotoxic benzoic acid metabolite. In a bioremediation system utilizing the forages tested here, the enhanced biological degradation by the forage will play an essential role in the removal of ATR. On the other hand, the apparent abiotic nature of DKN degradation in the soil implies that physical trapping is likely to be the primary remediation mechanism for Balance removal. Based on this lysimeter study, switchgrass, tall fescue and smooth bromegrass are good candidates for incorporation in multi-species riparian buffer practices designed for the bioremediation of ATR, Balance and nitrate.