Ammonium Thiosulfate Fertilizer Reduces Methyl Bromide Emissions From Soil

J. Gan, pesticide chemist, and S.R. Yates, soil scientist, USDA–ARS, U.S. Salinity Laboratory, Riverside, CA, and J.O. Becker, plant pathologist, Department of Nematology, University of California, Riverside, CA

The excessive emission of methyl bromide (MeBr) after soil injection is caused by the chemical's rapid diffusion and relatively slow degradation in soil. In most soils, it takes less than 1 hour for MeBr to move from the injection point to the soil surface, but more than a few days for 50 percent of the applied MeBr to degrade. Currently known strategies to reduce MeBr emissions are based on the suppression or delay of the transport process. Conceivably, MeBr emission can also be reduced if its degradation in soil is enhanced. This approach, however, has not been well investigated.

We have identified ammonium thiosulfate (ATS) as a powerful MeBr degrader in soil. Applying ATS to the soil surface drastically reduces MeBr emissions, and preliminary data on efficacy against nematodes and weeds show that this approach is promising for field application. ATS is currently used as a sulfur and nitrogen fertilizer, and is available at low cost. For instance, Thio-Sul, a liquid fertilizer containing 60 percent ATS, is sold at $1.46 a gallon at a local dealership. Second, thiosulfate has been found to enhance N utilization efficiency of fertilizers such as urea. And third, thiosulfate degrades to sulfate in soil, and poses little threat to groundwater.

As the first step, we quantitatively correlated the dependence of MeBr degradation in soil to ATS application rates. We found that as the initial ATS:MeBr molar ratio was increased, the degradation rate of MeBr rapidly increased, or the corresponding persistence rapidly decreased. For instance, the half-life of MeBr degradation in an unamended sandy loam was 133 hours (h), but was shortened to less than 5 h when 4 parts of ATS were added for 1 part of MeBr in the soil. As MeBr was degraded in ATS-amended soil, equal molar concentrations of Br- were concurrently produced, indicating that ATS-enhanced MeBr degradation is a complete transformation. Degradation was similarly enhanced in different soils, indicating that ATS amendment should be equally effective for reducing MeBr emissions from different soils.

Secondly, we evaluated the effectiveness of ATS for reducing MeBr emissions by using polyethylene plastic tarped, packed soil columns (60 cm long by 12.5 cm inside diameter). After liquid MeBr was injected at the 30-cm depth into an unamended column, a total of 61 percent of the applied MeBr was lost via volatilization. When Thio-Sul was applied with water to the soil surface either before or immediately after MeBr injection at 660 kg per ha (or 90 gal per acre), less than 10 percent emission occurred. At the end of experiment, most of the MeBr in the ATS-amended columns was recovered as Br- near the soil surface. This indicated that extensive MeBr degradation had occurred near the soil surface, just before MeBr was able to escape into the air.

Next, we evaluated the applicability of ATS to reduce MeBr emissions and the effect on pest control efficacy in field plots. MeBr was applied by "hot-gas" injection through subsurface drip tubing into plastic tarped, 2-ft-wide raising beds at 100 lb per acre. The field was infested with root-knot nematodes, Meloidogyne incognita, and volunteer Lima beans (used as a weed substitute). Three treatments were considered: non-fumigated control, standard fumigation, and standard fumigation + ATS application. In ATS-treated plots, we sprayed Thio-Sul onto the soil surface before fumigation, at 660 kg ha-1 or 90 gal a-1. Compared to the non-fumigated plots, MeBr fumigation, with or without ATS amendment, provided effective control against root-knot nematodes and Lima beans. There was no statistical difference between standard fumigation and ATS-amended fumigation for nematode control, but efficacy against volunteer Lima beans was significantly (p = 0.05) reduced. This indicates that under field conditions, surface ATS application may not greatly affect the efficacy of MeBr fumigation, justifying further studies to expand the evaluation for other pests or pathogens (e.g., fungi), and for other MeBr fumigation methods (e.g., shank injection).

Using an existing fertilizer to minimize MeBr emissions represents a novel risk-mitigation approach. The potential of using ATS to reduce MeBr emission is of particular interest because 1) fertilizers such as Thio-Sul are very inexpensive, and 2) fertilizer application can be easily incorporated into current MeBr fumigation procedures. Assuming that 100 gallons of Thio-Sul per acre are used, the additional cost for the fertilizer itself is only $150.

The significant reduction in MeBr emission and the limited impact on pest control efficacy caused by ATS amendment suggest that additional effort should be made to further develop this approach. Even if effective, this technology could only be used if legislative changes were made in existing regulations.

Last Updated: July 24, 1998