Location: Contaminant Fate and Transport ResearchTitle: Irrigation, organic matter addition, and tarping as methods of reducing emissions of methyl iodide from agricultural soil) Author
Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2010
Publication Date: 1/7/2011
Publication URL: www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2353.pdf
Citation: Ashworth, D.J., Luo, L., Xuan, R., Yates, S.R. 2011. Irrigation, organic matter addition, and tarping as methods of reducing emissions of methyl iodide from agricultural soil. Environmental Science and Technology. 45(4):1384-1390. Interpretive Summary: Methyl iodide (MeI) is a fumigant pesticide used to kill plant pests in soil prior to planting crops. Because fumigants are gaseous, they can rapidly move from the soil into the atmosphere where they can become inhaled by humans and cause health problems. It is necessary to determine how rapidly, and to what extent, MeI is transferred from soil to the atmosphere, and to assess strategies for reducing this transfer. Using stainless steel columns filled with soil, we found that MeI very rapidly moved from soil to the atmosphere (more rapidly than other soil fumigants). The total amount of MeI reaching the atmosphere was 83 % of the total amount injected into the soil column at a depth of 30 cm below the surface. Adding irrigation water to the soil surface before and after application of the MeI did not reduce this total amount of emissions. However, increasing the amount of organic matter in the soil reduced the total emissions to 63 %. This occurred because the organic matter can induce the chemical degradation of the MeI. By covering the soil surface with a plastic film which is highly impermeable to MeI, the emissions were reduced to just 0.04 % of the total added to the soil. This is therefore considered an excellent strategy for reducing the potential for MeI pollution of the atmosphere. In an agricultural setting, the plastic film would ultimately need to be ripped to allow for crop planting. In our experiments, ripping the film after 2 weeks increased the total MeI loss to 6 %. This research will benefit farmers, growers and stakeholders with interests in alternative methods of reducing methyl iodide emissions.
Technical Abstract: Methyl iodide (MeI) is set to become increasingly used as a highly effective alternative to the soil fumigant methyl bromide. Due to its physical properties, its emission from soil to air is likely to be high and may become a human health risk. Using soil columns that make it possible to determine emissions and gas phase distribution of soil fumigants, we studied MeI behavior in two soils of differing organic matter content. Additionally, the effectiveness of surface irrigation and tarping with virtually impermeable film (VIF) was assessed. In the lower organic matter, bare soil (control), emissions of MeI were very rapid and very high (83 % of total). Although the peak emission flux was reduced by irrigation, the total loss was very similar to the control (82 %). Tarping with VIF dramatically reduced emissions (0.04 % total emissions). In the higher organic matter soil, degradation rate of MeI was increased around 4-fold, leading to a significant reduction in emissions (63 % total emissions). The work suggests that surface tarping with VIF would be highly effective as an emissions reduction strategy, and would also result in the maintenance of high soil gas concentrations (important for pest control). Ripping of the tarp after two weeks led to an immediate spike release of MeI but, even so, the flux rate at this time was almost 20 times lower than the peak flux rate in the control. Even with tarp ripping, the total emission loss from the VIF treatment remained low (6 %).