Microbial biodegradation of the herbicide 2,4-D in soils and subsurface sediments

Much of the row crop production agriculture in the Midwest occurs on soils developed from glacial till, but little research has addressed herbicide biodegradation in subsurface till. This study investigated the ability of indigenous microorganisms to mineralize the herbicide 2,4-D (2,4-dichlorophenoxyacetic acid) at sites in Iowa and Indiana. Microorganisms were able to mineralize 2,4-D, with rates of mineralization ranging from 0.02 to 1.0% d"1 over the first 30 d. Rates of mineralization and total 2,4-D mineralized generally declined with depth in these profiles. Totai2,4-D mineralized ranged from 31 to 42% in surface soils and 2 to 47% in subsurface sediments between 150 to 180 d after addition. Populations of 2,4-D-degraders in soils and glacial sediments ranged from non-detectable to 59,000 degraders g·1 soil. The adaptation of 2,4-D metabolizing microorganisms in the subsurface impacted the mineralization rates and totai2,4-D mineralized. Sandy subsurface sediments ranging in depth from 2.9 to 3.5 m from one Indiana core exhibited greater mineralization rates before and after adaptation with more extensive 2,4-D mineralization than till from approximately 1 m depth. Lag periods in these two subsurface samples ranged from 42 to 80 d prior to increased mineralization. The population of 2,4-D-degraders in these deeper sandy sediments were greater than the populations in the overlying till. INTRODUCTION Ground water pollution by agricultural chemicals is a concern because ground water supplies a significant portion of the drinking water used in the United States (Hallberg, 1986). The U.S. Environmental Protection Agency (EPA) found pesticide detections in 23.5% of drinking water wells tested in 1992. Normal field usage of pesticides was suspect for 20.8% of all wells with detections (Jacoby, 1992). Atrazine, alachlor, and cyanazine movement from glacial till soils and agricultural watershed