Plants: Guilty of Producing and Releasing MeBr Into the Atmosphere?

It is generally understood that anything which destroys stratospheric ozone molecules thins the ozone layer, allowing potentially harmful ultraviolet light to reach the earth. It is also known that methyl bromide, the fumigant now widely used in agriculture throughout the world, has been declared an ozone depletor. Scientists have even estimated the amount of methyl bromide emissions that can be attributed to agricultural use of the chemical.

What is not known is how much of the total amount of methyl bromide emitted into the atmosphere comes from plants. Jay Gan, a soil chemist with USDA's Agricultural Research Service, has just completed a study that sheds some light on this uncertainty. He is with the U.S. Salinity Laboratory, an ARS research facility in Riverside, California.

In collaboration with J. Sims at the University of California-Riverside, Gan and S.R. Yates found that Brassica plants take bromide from the soil, convert it to methyl bromide, and then release it into the atmosphere. Brassica species include both wild and domestic plants such as mustard, kale, cabbage, broccoli, rapeseed, turnips, and radishes.

"We selected Brassica because Canadian scientists have proven that leaf parts of this species produce methyl bromide," Gan reports. "But whole plants had not been tested before to see if they produce and release methyl bromide. There is a missing source of atmospheric methyl bromide. Unlike completely manmade ozone depletors, methyl bromide comes from manmade and natural sources. We know that methyl bromide is put into the air by the oceans, biomass burning, emissions from leaded gasolines, and agricultural fumigation. But these sources don't account for the total amount of methyl bromide emitted into the atmosphere. Maybe green plants could be responsible for a significant amount of the missing source."

Gan and colleagues first measured methyl bromide emitted from plants grown in soil containing different levels of bromide. "It is known that most soils contain bromide, but we added differing amounts to soil and transplanted seedlings in the greenhouse," he says. "We used broccoli, cabbage, mustard, Chinese cabbage, and pak-choi, all cultivated Brassica species. We also planted alyssum and wild mustard, both wild species."

After 2 to 6 weeks, they put the plants in a closed glass container for 24 hours to let the methyl bromide accumulate to a measurable amount. Gan measured the methyl bromide by taking samples of the air from the jars and analyzing them using gas chromatography.

"We found that the initial rates of methyl bromide produced correlated to the amounts of bromide we added to the soil," Gan reports. He says that the average amount of bromide in soil is about 1 ppm (part per million), depending on where the soil is sampled. "As you get closer to the ocean, soils contain more bromide; farther inland, they contain less."

In their second experiment, Gan and his collegues used field-grown Brassica plants. "We used broccoli, rapeseed, cabbage, and wild mustard plants. We took the plants from the field with soil still attached to the roots. Again, we placed the plants in sealed containers in the lab to capture methyl bromide emissions," Gan explains.

Although the bromide level was less than 0.6 ppm in any of the soils tested, significantly, methyl bromide emissions were still measured. Thus, under natural conditions, Brassica plants were able to extract bromide from the soil, convert it to methyl bromide, and release it.

Since they knew that the soil or the plants themselves could degrade the methyl bromide produced in the 24 hours the plants were in the jars, Gan and colleagues did separate degradation experiments to derive the actual methyl bromide production rates.

After they measured methyl bromide production, they separately measured the bromide content in the plants and soil to determine the plant uptake of bromide from the soil.

"We recovered 95 to100 percent of the spiked bromide from plant tissues," Gan says. Using the rates at which methyl bromide is produced by cabbage and rapeseed—for which the global bio-mass is known—and a linear relationship between this rate and soil bromide, Gan estimates that, globally, rapeseed alone could produce 6.6 million kilograms of methyl bromide each year. And cabbage, 0.4 million kilograms, more or less.

The preliminary study also showed that radishes and turnips can produce methyl bromide. "We found that the Brassicaceae family as a whole is capable of producing methyl bromide," Gan says.

Worldwide, the Brassicaceae family is an important part of the terrestrial plant biomass, made up of vegetables, oil crops, pastures, ornamental crops, weeds, and wild species.

"We think it is possible that the amount of methyl bromide produced by this family of plants may be substantial," Gan says. "And although these were the only plants in our study, there are probably many other terrestrial plants out there producing methyl bromide."

Since methyl bromide has manmade and natural origins, Gan thinks it's critical to understand the sources. We may be able to control the manmade sources but are not likely able to do much with the natural ones. Right now, he points out, there are important facts that aren't being considered in estimating the amount of methyl bromide in the atmosphere. First, we haven't considered the fact that soil contains a certain amount of bromide and that nonextractable bromine can be released as bromide. Second, the earth's crust of soil is a large bromide reservoir.

"Our research suggests that land plants may be a link between soil bromide and stratospheric methyl bromide," Gan says. "Our findings also agree with the ratio of 1.2 to 1.4 northern to southern hemispheric ratio of atmospheric methyl bromide. Because the northern hemisphere has a greater landmass than the southern, it would follow that it also has a greater capacity for plants to produce methyl bromide."

"We think more study is needed on the terrestrial ecosystem as it relates to depleting the ozone layer," Gan says.

Last Updated: April 9, 1999