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Methane is involved in a number of chemical and physical processes in the
Earth's atmosphere, including global warming. Atmospheric methane
originates mainly from biogenic sources, such as rice paddies and natural
wetlands; the former account for at least 30% of the global annual emission
of methane to the atmosphere. As an increase of rice production
by 60% is the most appropriate way to sustain the estimated increase of the
human population during the next three decades, intensified
global fertilizer application will be necessary: but it is
known that an increase of the commonly used ammonium-based fertilizers can
enhance methane emission from rice agriculture. Approximately 10-30%
of the methane produced by methanogens in rice paddies is consumed by methane-oxidizing
bacteria associated with the roots of rice; these bacteria
are generally thought to be inhibited by ammonium-based fertilizers, as was
demonstrated for soils and sediments.
In contrast, we show here that the activity and growth of such bacteria in
the root zone of rice plants are stimulated after fertilization. Using a combination
of radioactive fingerprinting and molecular biology
techniques, we identify the bacteria responsible for this effect. We expect
that our results will make necessary a re-evaluation of the link between fertilizer
use and methane emissions, with effects on global warming studies.