Ability of Forage Grasses Exposed to Atrazine and Isoxaflutole to Reduce Nutrient Levels in Soils and Shallow Groundwater

Abstract

Successful implementation of vegetative buffers requires inclusion of plant species that facilitate rapid dissipation of deposited contaminants before they have a chance to be transported in surface runoff or to shallow groundwater. Thirty‐six field lysimeters with six different ground covers [bare ground, orchardgrass (Dactylis glomerata L.), tall fescue (Festuca arundinacea Schreb.), smooth bromegrass (Bromus inermis Leyss.), timothy (Phleum pratense L.), and switchgrass (Panicum virgatum L.)] were established to evaluate the ability of grasses to reduce nutrient levels in soils and shallow groundwater. Nitrate (NO₃ ^?) and orthophosphate (PO₄ ^3?) were uniformly applied to each lysimeter. In addition, half of the lysimeters received an application of atrazine, and the other half received isoxaflutole (Balance?) at levels indicative of surface runoff from cropland. The leachate from each lysimeter was collected after major rainfall events during a 25‐day period, and soil was collected from each lysimeter at the end of the 25‐day period. Water samples were analyzed for NO₃‐N and PO₄‐P, and soil samples were analyzed for NO₃‐N. Grass treatments reduced NO₃‐N levels in leachate by 74.5 to 99.7% compared to the bare ground control, but timothy was significantly less effective at reducing NO₃‐N leaching than the other grasses. Grass treatments reduced residual soil NO₃‐N levels by 40.9 to 91.2% compared to the control, with tall fescue, smooth bromegrass, and switchgrass having the lowest residual levels. Switchgrass decreased PO₄‐P leaching to the greatest extent, reducing it by 60.0 to 74.2% compared to the control. The ability of the forage grasses to reduce nutrient levels in soil or shallow groundwater were not significant between herbicide treatments. Quantification of microbial NO₃ ^? dissipation rates in soil suggested that denitrification was greatest in switchgrass, smooth bromegrass, and tall fescue treatments. The overall performance of these three grasses indicated that they are the most suitable for use in vegetative buffers because of their superior ability to dissipate soil NO₃ ^? and reduce nutrient transport to shallow groundwater.