The study and modeling of atrazine fate in buffer strips soil
Rameshwar S. Kanwar
Study on degradation and sorption of atrazine within different species of grass-hedges Vegetative Buffer Strips (VBS) (big bluestem (Andropogon geradi), switchgrass (Panicum virgatum L.), and eastern gamagrass (Tripsacum dactyloides L.)) was conducted. Over seven years eastern gamagrass appeared to be accumulating organic C in both the surface 15 and 15--30 cm soil depths. Soil under eastern gamagrass had the highest mineralization rate of atrazine and the highest atrazine-degrader populations. Atrazine was degraded rapidly in eastern gamagrass soil with the atrazine half-life of 10 days. Atrazine adsorbed to thatch more readily than VBS soil. These data suggested that eastern gamagrass should be used preferential to big bluestem grass and switchgrass for establishing grass-hedges VBS;Preferential flow of bromide and atrazine in the 5 and 9-year-old switchgrass Riparian Buffer Strips (RBS) and their adjacent cropped areas were observed. However, there was little difference in transport characteristics between these two RBS soils and their adjacent cropped soils. Despite similar texture and organic C contents, atrazine sorption was greater in RBS soils than adjacent cropped area soils. Greater sorption would lessen the potential leaching of atrazine to groundwater. Degradation of atrazine was slower under switchgrass RBS compared to the adjacent cropped soil. The rapid degradation of atrazine in the corn-soybean soil adjacent to the 5-year-old RBS was due to a larger population of atrazine-degrading microorganisms, which were also responsible for rapid mineralization of atrazine;Simulation results of atrazine fate after entry into the RBS using the Root Zone Water Quality Model (RZWQM) indicated that the RBS would retain about 79--94% of atrazine in runoff from the adjacent cornfield. The RZWQM predicted very low atrazine concentrations in seepage (<3 mug L --1). Our results suggested that RZWQM underpredicted leaching in the soil profile. Further modifications of the leaching subroutine are needed to correctly predicted pesticide loss in the seepage. Atrazine loss in runoff leaving the RBS was mostly sensitive to macropore size and plant residue load, but less sensitive to organic matter content. However, further study of the relationship between macroporosity and runoff losses of pesticides effectively are needed to verify the model sensitivity to macropores.