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Title: Transport and fate of methyl iodide a its pest control in soils

item LUO, LIFANG - University Of California
item Ashworth, Daniel
item Dungan, Robert - Rob
item XUAN, RICHENG - University Of California
item Yates, Scott

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2010
Publication Date: 8/15/2010
Citation: Luo, L., Ashworth, D.J., Dungan, R.S., Xuan, R., Yates, S.R. 2010. Transport and fate of methyl iodide a its pest control in soils. Environmental Science and Technology. 44(16):6275-6280.

Interpretive Summary: The use of volatile pesticides has resulted in adverse human and environmental health risks, especially air pollution. To minimize air pollution from soil fumigation, various approaches have been developed to reduce fumigant emissions to the atmosphere. However, few studies have provided quantitative information on both fumigant movement and its effect on pest control efficacy. The objective of this study was to describe a protocol for determining fumigant volatilization, degradation, spatio-temporal distribution and pest control in the 2-D soil column. A representative weed seed, citrus nematode, and fungi were used to provide a range of pest control information. An experiment was conducted where a soil was fumigated for 24 hours at 1/3 the normal fumigant application rate. The fraction volatilized was about 25.8 % of applied mass; about 44% degraded in the soil; and residual fumigant in the soil was about 6.8%. The 24-hour fumigation effectively eliminated citrus nematodes but had little influence on the fungi (i.e., Fusarium oxysporum). To control barnyardgrass weed seeds, a higher fumigant application rate (i.e., normal field application rate) or plastic tarps would be needed to achieve control. Using the approach presented in this study, the emission rate, soil gas-phase concentration and pest control in the soil column can be tested under various treatments. With such information, mathematical models can be verified and optimized fumigant dose and emission reduction strategies can be developed that have the least human and environmental health risks, while offering adequate pest control. Currently, this research would be valuable to the scientific community studying new methods to fumigant agricultural soils. In the future, this research could be incorporated into predictive models that would allow end-users (i.e., commodity groups, growers, regulators) to predict both emissions and pest control for a given fumigation practice.

Technical Abstract: For fumigants, information on transport and fate, as well as pest control, is needed to develop management practices with the fewest human and environmental health risks while offering sufficient pest control efficacy. For this purpose, a 2-D soil chamber (60 cm wide, 60 cm long, and 6 cm thick) with a surface-mounted flux chamber was designed to determine volatilization, spatial and temporal distribution of soil gas-phase concentration, degradation and organism survivability after methyl iodide (MeI) fumigation. Three types of pests (barnyardgrass seed [Echinochloa crus-galli], citrus nematode [Tylenchulus semipenetrans], and fungi [Fusarium oxysporum]) were used to give a broad spectrum of pest control information. After MeI fumigation at a rate of 56.43 kg ha-1 for 24 hr, about 25.8 % of MeI was emitted into air, 6.8 % remained in the soil, and 43.6% degraded in the soil (based on the residual iodide concentration). The uncertainty in the measured MeI degradation using iodide concentration was thought to contribute to the unrecovered MeI (about 23%). Based on the spatial and temporal distribution of soil gas-phase concentration, the concentration-time index (CT) and its distribution was quantified. The citrus nematodes were effectively eliminated even at low CT values (< 30 µg hr ml-1) but all Fusarium oxysporum survived at the applied rate. The response of barnyardgrass seeds spatially varied with the concentration-time index (CT) values in the 2-D soil chamber. To fully control barnyardgrass seeds, CT of greater than 300 µg hr ml-1 was required. Using this experimental approach, different fumigant emission reduction strategies can be tested and mathematical models can be verified to determine which strategies produce least emission to atmosphere while maintaining sufficient pest control efficacy.