Invasive "polluting plant" contributes to ozone levels.
Hickman, JE, S Wu, LJ Mickley and MT Lerdau. 2010. Kudzu (Pueraria montana) invasion doubles emissions of nitric oxide and increases ozone pollution. Proceedings of the National Academy of Sciences http://dx.doi.org/10.1073/pnas.0912279107.
Kudzu – an invasive plant common in the southeastern United States – contributes to the production of ozone, and at its worst, may add as much as a week to the number of days when ozone levels exceed pollution limits in the region. Kudzu releases two key ingredients – nitric oxide and isoprene – that are important to making ozone, which is an air pollutant with known health effects. When researchers looked, kudzu-invaded areas had higher levels of nitric oxide compared to uninvaded areas.
Kudzu is a vine native to Asia. It was first introduced in the United States in the late 19th century in order to prevent soil erosion. Kudzu flourished in the climate of the southeastern United States and now covers more than seven million acres. Kudzu can grow as much as seven feet per week.
Ozone is an air pollutant that forms from nitrogen oxides (NOx) and volatile organic carbons (VOCs) in the presence of sunlight and heat. Ozone is the primary component of smog. Exposure to ozone can cause airway irritation, wheezing, asthma attacks and even permanent lung damage in susceptible people, including the elderly, children and people with asthma.
Air pollution from power plants, factories, cars, trucks and chemical solvents are usually thought of as the major sources of NOx and VOCs, especially in urban areas. In the southeastern United States, especially rural and nonagricultural areas, levels of ozone in the air are tied to how much NOx is available. In these areas, Kudzu may be an important determinant of ozone levels because of its ability to produce nitric oxide.
Soil and air were studied at three sites with and without kudzu invasions in Madison County, Georgia. Several nitrogen indicators were measured in the soil and air. To determine the plant's impacts on nitric oxide levels, the invaded areas were compared to uninvaded areas.
Researchers plugged their data into a computer simulation model – called GEOS-Chem – to estimate future effects of kudzu invasion on regional ozone levels. GEOS-Chem combines weather data from NASA and potential climate change scenarios proposed to occur by 2050. In this study, researchers assumed a moderate climate change scenario put forward by the Intergovernmantal Panel on Climate Change.
Nitric oxide emitted from soil were significantly higher – more than twice as high – in invaded areas as in uninvaded areas. The soil samples from areas with kudzu also had increased nitrogen cycling rates. Nitrogen cycling rates describe the speed at which nitrogen is transferred from the plant to the soil and from the soil back and forth to the ecosystem. Nitrogen mineralization and nitrification, both of which are important steps in the nitrogen cycle in soil, were also increased in areas invaded by kudzu.
According to the computer model, the most extreme case of widespread kudzu growing over nonurban and nonagricultural soil would result in a greater than 25 percent increase in nitric oxide emissions from soil.
The increase in nitric oxide would then contribute to an increase in ozone levels. In areas of the country most vulnerable to changes in nitric oxide levels – Mississippi, Arkansas and Tennessee – the kudzu-related increase in ozone could add as many as seven additional high ozone episodes during the summer when ozone levels are highest.
The natural process of nitrogen transformation used by an invasive, fast-growing plant may affect human health by increasing levels of a type of nitrogen that contributes to the formation of harmful air pollution. Certain rural areas where the Asian native is overgrowing and expanding its range could be most affected.
While the kudzu invasion in the southeastern United States has been long regarded as an ecological issue, this study is the first to link the plant to increased ozone with potential respiratory health implications. In this study, the researchers found that kudzu produces significant amounts of nitric oxide that can lead to the formation of ozone.
In the southeastern United States, the amount of ozone that can be produced is determined by the available levels of nitric oxide in the air. This study shows that kudzu is another source of the raw material and is able to produce nitric oxide in amounts large enough to result in up to seven additional high ozone episodes in certain regions. Therefore, in non-urban agricultural areas, kudzu may be a significant factor impacting human health.
Burning fossil fuels is one of the biggest sources of NOx. In the United States, these emissions from cars, trucks and industries have decreased by one-third since 1990 as a result of increased regulations and new technologies.
In the future, as climate change alters where plant species grow, kudzu may expand into new regions and influence air pollution levels with undetermined human health consequences.