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 Earths atmosphere, including global warming(1), 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(2). 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(3), intensified global fertilizer application will be necessary(3): 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(4,5); these bacteria are generally thought to be inhibited by ammonium-based fertilizers, as tvas demonstrated for soils(6-8) and sediments(9,10). 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 fingerprinting(11) and molecular biology(12) 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.