TO THE dismay of environmentalists, a potent ozone-depleting pesticide that
was to have been phased out could be saved鈥攂y composted manure.
Researchers in the US have found that emissions of the agricultural fumigant
gas methyl bromide could be reduced to what they claim is insignificant levels
by applying composted manure and a novel plastic cover to soil before the gas is
injected. The manure speeds up the breakdown of the gas to harmless products
like methyl alcohol and bromide ions dissolved in water.
Methyl bromide gas is used to kill pests like nematodes, fungi and weeds
before planting crops such as strawberries, tomatoes and flowers. Although
widely used around the world, the gas is scheduled to be phased out in
industrialised countries by 2005 because it depletes stratospheric ozone. The
phase-out date was recently pushed back by Congress from 2001 to 2005 because of
the lack of effective alternatives.
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Now researchers at the US Department of Agriculture (USDA) say farmers may
not need to stop using methyl bromide at all. While strawberry farmers welcome
the fact that they may not now lose what they regard as an important tool,
environmentalists are adamant that methyl bromide should go. 鈥淚t鈥檚 unrealistic
to think that there would be the level of control to reduce emissions to zero,鈥�
says a spokesperson for Pesticide Action Network in San Francisco. 鈥淭he chemical
is going to be phased out and research efforts should be focused on
补濒迟别谤苍补迟颈惫别蝉.鈥�
But USDA researchers are pressing ahead with their new plan. 鈥淚 think it is
possible to use methyl bromide with nearly zero emissions,鈥� says Scott Yates of
the USDA鈥檚 Agricultural Research Service in Riverside, California.
Earlier this year, he and his colleagues reported that they could reduce
methyl bromide emissions by an order of magnitude by using a new, highly
impermeable plastic instead of the high-density polyethylene tarpaulins that
farmers use to cover soil (Geophysical Research Letters, vol 25, p
1633).
They have now found that by simply applying composted manure鈥攐r the
inexpensive fertiliser ammonium thiosulphate鈥攖o soil, they can reduce
emissions even more (Environmental Science & Technology, vol 32, p
2438 and p 3094). Both the manure and fertiliser quicken methyl bromide鈥檚
decomposition in soil, producing harmless by-products, according to Yates. 鈥淚n
combination with tarpaulins, you can effectively eliminate emissions to the
atmosphere,鈥� he says.
Yates says the soil additives and new plastic tarpaulins could make the
effect of methyl bromide鈥檚 use on farms negligible globally. There are many
natural sources of methyl bromide, and synthetic methyl bromide is also used to
fumigate buildings. 鈥淎bout 25 per cent of the sources are from soil fumigation
globally. We think you could get that down to about 1 per cent,鈥� Yates says.
鈥淭hat, of course, assumes that every place you go would use the new plastic
and would have the same results we had. You鈥檇 need to test it all over the
world,鈥� he cautions.