Stimulation by ammonium-based fertilizers of methane oxidation in soil around rice roots

Methane is involved in a number of chemical and physical processes in the Earth's atmosphere, including global warming1. 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 atmosphere2. 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 decades3, intensified global fertilizer application will be necessary3: 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 rice4,5; these bacteria are generally thought to be inhibited by ammonium-based fertilizers, as was demonstrated for soils6,7,8 and sediments9,10. 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 fingerprinting11 and molecular biology12 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.