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Title: Water and methyl isothiocyanate distribution in soil after drip fumigation

item NELSON, SHAD - Texas A&M University
item AJWA, HUSEIN - University Of California
item Trout, Thomas
item STROMBERGER, MARY - Colorado State University
item Yates, Scott
item SHARMA, SHANKAR - University Of California

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2014
Publication Date: 6/25/2014
Citation: Nelson, S.D., Ajwa, H.A., Trout, T.J., Stromberger, M., Yates, S.R., Sharma, S. 2014. Water and methyl isothiocyanate distribution in soil after drip fumigation. Journal of Environmental Quality. 42:1555-1564.

Interpretive Summary: Soil fumigants are highly volatile pesticides used to kill soil-borne pests and pathogens before planting crops. Research has shown that a significant fumigant fraction (about 20-90%) can be emitted into the atmosphere after soil fumigation, which increases health and environmental risks. With the ban on use of methyl bromide, effective alternatives are needed that do not pollute the atmosphere and are effective in controlling soil-borne pests and pathogens. One alternative is metam sodium, which produces a fumigant methyl isothiocyanate. However, this fumigant does not always provide sufficient pest control. Therefore, to develop effective application methodologies, it is necessary to understand its fate under a variety of soil and application conditions. This study was conducted to evaluate the effect of water application on fumigant distribution in soil, determine the fumigant disappearance rate after application, and to compare simulated soil fumigant concentrations to measurements. The best fumigant distribution in soil occurred when metam sodium was applied with moderate amounts of water. Soil disappearance followed first-order kinetics with a half-life of about 24–30 hours. The predictive model Hydrus 2D/3D provided adequate predictions of fumigant concentrations. This research would be of interest to scientists, regulators, cooperative extension, growers and film manufacturers.

Technical Abstract: Methyl isothiocyanate (MITC) generators, such as metam sodium (Met-Na), are used for soil fumigation of agricultural land. The ban on the fumigant methyl bromide (MBr) has resulted in greater use of MITC generators. In order to understand the efficacy of MITC, it is necessary to assess its generation and disappearance kinetics when Met-Na is applied to soil. This study evaluated the movement of water and distribution and dissipation of MITC in soil after application of Met-Na through surface drip irrigation systems. The effects of varying water application volume (25, 50 and 75 mm) and rate (1.9, 5.0, and 7.5 L h/1 m/1) were evaluated in a sandy loam soil. Good fumigant distribution within the sandy loam soil was observed under medium water application amount (50 mm) with slow to intermediate drip application rates (1.9-5.0 L h/1 m/1). Low water application amount (25 mm) or high application rate (7.5 L h/1 m/1) did not provide adequate MITC distribution throughout the soil bed width and rooting depth. Dissipation patterns of MITC in soil in all water application amounts and rates followed first order kinetics with a rate constant of 0.025±0.004 h/1 and a half-life of 27±3 h. Simulated water distribution through the soil profile using HYDRUS 2D/3D fitted measured field data well and the model accurately simulated MITC fumigant distribution in the soil.