Background
In the words of the authors, "rice paddies contribute approximately 10-13% to the global CH4 [methane] emission (Neue, 1997; Crutzen and Lelieveld, 2001)."  They also note that "during the next 30 years rice production has to be increased by at least 60% to meet the demands of the growing human population (Cassman et al., 1998)."  Hence, they note that "increasing amounts of fertilizer will have to be applied to maximize yields [and] there is ongoing discussion on the possible effects of fertilization on CH4 emissions."

What was done
The authors investigated the effects of N-fertiliser (urea) on CH4 emission, production and oxidation in rice culture in laboratory, microcosm and field experiments they conducted at the Italian Rice Research Institute in northern Italy.

What was learned
The authors report that in some prior studies "N-fertilisation stimulated CH4 emissions (Cicerone and Shetter, 1981; Banik et al., 1996; Singh et al., 1996)," while "methanogenesis and CH4 emission was found to be inhibited in others (Cai et al., 1997; Schutz et al., 1989; Lindau et al., 1990)," which suggests that, in the mean, there may be little to no change in overall CH4 emissions from rice fields in response to increased N-fertilization.  This was also the conclusion they reached in their own study.  As they describe their findings, "combining our field, microcosm and laboratory experiments we conclude that any agricultural praxis improving the N-supply to the rice plants will also be favourable for the CH4 oxidising bacteria," noting, however, that "N-fertilisation had only a transient influence and was counter-balanced in the field by an elevated CH4 production."

What it means
The implication of these findings is well articulated in the concluding sentence of the authors' paper: "neither positive nor negative consequences for the overall global warming potential could be found."

References
Banik, A., Sen, M. and Sen, S.P.  1996.  Effects of inorganic fertilizers and micronutrients on methane production from wetland rice (Oryza sativa L.).  Biology and Fertility of Soils 21: 319-322.

Cai, Z.C., Xing, G.X., Yan, X.Y. et al.  1997.  Methane and nitrous oxide emissions from rice paddy fields as affected by nitrogen fertilizers and water management.  Plant and Soil 196: 7-14.

Cassman, K.G., Peng, S., Olk, D.C. et al.  1998.  Opportunities for increased nitrogen-use efficiency from improved resource management in irrigated rice systems.  Field Crops Research 56: 7-39.

Cicerone, R.J. and Shetter, J.D.  1981.  Sources of atmospheric methane.  Measurements in rice paddies and a discussion.  Journal of Geophysical Research 86: 7203-7209.

Crutzen, P.J. and Lelieveld, J.  2001.  Human impacts on atmospheric chemistry.  Annual Review of Earth and Planetary Sciences 29: 17-45.

Lindau, C.W., DeLaune, R.D., Patrick Jr., W.H. et al.  1990.  Fertilizer effects on dinitrogen, nitrous oxide, and methane emission from lowland rice.  Soil Science Society of America Journal 54: 1789-1794.

Neue, H.U.  1997.  Fluxes of methane from rice fields and potential for mitigation.  Soil Use and Management 13: 258-267.

Schutz, H., Holzapfel-Pschorrn, A., Conrad, R. et al.  1989.  A 3-year continuous record on the influence of daytime, season, and fertilizer treatment on methane emission rates from an Italian rice paddy.  Journal of Geophysical Research 94: 16405-16416.

Singh, J.S., Singh, S., Raghubanshi, A.S. et al.  1996.  Methane flux from rice/wheat agroecosystem as affected by crop phenology, fertilization and water level.  Plant and Soil 183: 323-327.