Water Week

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Nasty “little ozone factory”: Earth’s fastest-growing plant kudzu is leading source of hydrocarbon isoprene to form ozone with NO, and to a lesser extent nitrous oxide

Posted by waterweek on 22 September 2007

Among the fastest-growing plants on Earth, kudzu (Pueraria montana) was introduced to the US from Japan in 1876 as an ornamental plant. Subsequently planted to control soil erosion, it has become a serious pest across much of the nation, reported New Scientist (11/8/2007, p.13).

Nitrogen emission fuse: New research, revealed at the Ecological Society of America’s meeting in San Jose, California, shows that the vine is also pumping huge amounts of nitrogen compounds into the soil, water and atmosphere. This could aggravate damage to forests, choke waterways with algal blooms, increase concentrations of smog – even quicken the pace of global warming.

NO emitting plant: Kudzu has long been known to be one of the leading plant sources of the hydrocarbon isoprene. In the presence of sunlight, isoprene reacts with oxides of nitrogen to form ozone. These oxides – mostly N2O – were thought to come from human activities such as fertilising cropland and burning fossil fuels. Kudzu, however, maintains a population of soil-dwelling bacteria in its roots that turn atmospheric nitrogen into ammonia (NH3), which it needs to make chlorophyll. Like other plants, as leaves fall off and decay, the nitrogen fertilises the soil, from where it can pollute the atmosphere with NO, and to a lesser extent nitrous oxide (N2O).

Plant emitters on the rampage: Jonathan Hickman of the State University of New York at Stony Brook and Manuel Lerdau of the University of Virginia in Charlottesville studied patches of forest and meadow in Georgia and Maryland. They compared nitrogen levels in leaf litter and soil in areas kudzu had invaded with those where it was left absent. Kudzu leaf litter was found to contain nearly twice the amount of nitrogen as litter from native tree species. “In effect the plant is fertilising the ecosystem,” says Lerdau, who fears the excess nitrogen will make it easier for other fast growing invasing plants to take over.

Nutrient depletes rivers of oxygen, kills fish: When rains come, the excess nutrient would also be washed into rivers, causing algal blooms that deplete waters of oxygen and lead to the death of fish. As soil levels of nitrogen rise, so too will emissions of NO and N2O. NO readily converts to NO2 in the atmosphere, and so could lead to spikes in low-level ozone. Hickman’s initial findings show that NO fluxes from the soil in invaded areas are twice those from uninvaded patches.

“Little ozone factory”: “Kudzu is a little ozone factory,” Lerdau said. Researchers are still unsure just how extensive this atmospheric pollution will be, but Thomas Sharkey, a botanist at the University of Wisconsin-Madison, believes there is a danger to human health. “Childhood asthma is at an all-time high,” he notes. Hickman has also found that kudzu causese small increases in emissions of N2O which is 300 times more potent a greenhouse gas than carbon dioxide.

New Scientist, 11/8/2007, p. 13

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