2015职称英语理工类考试阅读理解文章及练习Plant Gas：文章

　　Plant Gas

　　Scientists have been studying natural sources of methane for decades but hadnt regarded plants as a producernotes Frank Kepplera geochemist at the Max Planck Institute for Nuclear Physics in HeldelbergGermany.Now Keppler and his colleagues find that plantsfrom grasses to treesmay also be sources of the greenhouse gas.This is really surprisingbecause most scientists assumed that methane production requires an oxygen-free environment.

　　Previouslyresearchers had thought that it was impossible for plants to make significant amounts of the gas.They had assumed that microbes2 need to be in environments without oxygen to produce methane.Methane is a greenhouse gaslike carbon dioxide.Gases such as methane and carbon dioxide trap heat in Earths atmosphere and contribute to global warming.

　　In its experimentsKepplers team used sealed chambers that contained the same concentration of oxygen that Earths atmosphere has.They measured the amounts of methane that were released by both living plants and dried plant materialsuch as fallen leaves.

　　With the dried plantsthe researchers took measurement at temperatures ranging from 30 degrees Celsius to 70 degrees C.At 30 degrees Cthey founda gram of dried plant material released up to 3 nanograms of methane per hour.(One nanogram is a billionth of a gram.) With every 10-degree rise in temperaturethe amount of methane released each hour roughly doubled.

　　Living plants growing at their normal temperatures released as much as 370 nanograms of methane per gram of plant tissue per hour.Methane emissions tripled when living and dead plant was exposed to sunlight.

　　Because there was plenty of oxygen availables unlikely that the types of bacteria that normally make methane were involved.Experiments on plants that were grown in water rather than soil also resulted in methane emissions.Thats another strong sign that the gas came from the plants and not soil microbes.

　　The new finding is an interesting observation, says Jennifer Y.Kinga biogeochemist at the University of Minnesota in St.Paul3.Because some types of soil microbes consume methanethey may prevent plant-produced methane from reaching the atmosphere.Field tests will be needed to assess the plants influenceshe notes.(367 words)