What impact does the ongoing rise in the air's CO2 content have on the atmosphere's methane concentration? The implications of this question are huge, in light of the fact that, molecule for molecule, methane is a much more powerful greenhouse gas than is carbon dioxide. Hence, we here consider this question as it applies to methane emissions associated with agricultural operations.
Encouraging indications that atmospheric CO2 enrichment might significantly reduce methane emissions associated with the production of rice were provided by Schrope et al. (1999), who studied batches of rice growing in large vats filled with topsoil and placed within greenhouse tunnels maintained at atmospheric CO2 concentrations of 350 and 700 ppm, each of which tunnels was further subdivided into four sections that provided temperature treatments ranging from ambient to as much as 5°C above ambient. As would be expected, doubling the air's CO2 content significantly enhanced rice biomass production in this system, increasing it by up to 35% above-ground and by up to 83% below-ground. However, in a truly unanticipated development, methane emissions from the rice grown at 700 ppm CO2 were found to be 10 to 45 times less than emissions from the plants grown at 350 ppm. As Schrope et al. describe it, "the results of this study did not support our hypothesis that an effect of both increased carbon dioxide and temperature would be an increase in methane emissions." Indeed, they report that "both increased carbon dioxide and increased temperatures were observed to produce decreased methane emissions," except for the first 2°C increase above ambient, which produced a slight increase in methane evolution from the plant-soil system.
In checking for potential problems with their experiment, Schrope et al. could find none. They thus stated that their results "unequivocally support the conclusion that, during this study, methane emissions from Oryza sativa [rice] plants grown under conditions of elevated CO2 were dramatically reduced relative to plants gown in comparable conditions under ambient levels of CO2," and to be doubly sure of this fact, they went on to replicate their experiment in a second year of sampling and obtained essentially the same results. Four years later, however, a study of the same phenomenon by a different set of scientists yielded a far different result in a different set of circumstances.
Inubushi et al. (2003) grew a different cultivar of rice in 1999 and 2000 in paddy culture at Shizukuishi, Iwate, Japan in a FACE study where the air's CO2 concentration was increased 200 ppm above ambient. They found that the extra CO2 "significantly increased the CH4 [methane] emissions by 38% in 1999 and 51% in 2000," which phenomenon they attributed to "accelerated CH4 production as a result of increased root exudates and root autolysis products and to the increased plant-mediated CH4 emission because of the higher rice tiller numbers under FACE conditions." With such a dramatically different result from that of Schrope et al., many more studies likely will be required to clarify this issue and determine which of these two contrasting results is the more typical of rice culture around the world.
A somewhat related study was conducted by Kruger and Frenzel (2003), who note that "rice paddies contribute approximately 10-13% to the global CH4 emission (Neue, 1997; Crutzen and Lelieveld, 2001)," and 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)." Because of these facts they further 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."
To help promote that discussion, Kruger and Frenzel 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. They 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)," similar to the polarized findings of Schrope et al. and Inubushi et al. with respect to the effects of elevated CO2 on methane emissions. In the mean, therefore, there may well be little to no change in overall CH4 emissions from rice fields in response to both elevated CO2 and increased N-fertilization. With respect to their own study, for example, Kruger and Frenzel say that "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 that "N-fertilisation had only a transient influence and was counter-balanced in the field by an elevated CH4 production." The implication of these findings is well articulated in the concluding sentence of their paper: "neither positive nor negative consequences for the overall global warming potential could be found."
Another agricultural source of methane is the fermentation of feed in the rumen of cattle and sheep. Fievez et al. (2003) studied the effects of various types and levels of fish-oil feed additives on this process by means of both in vitro and in vivo experiments with sheep, observing a maximal 80% decline in the ruminants' production of methane when using fish-oil additives containing n-3-eicosapentanoic acid. Likewise, New Zealand scientists, as described in our Editorial of 7 August 2002, have demonstrated that the presence of condensed tannins found in certain pasture plants can also reduce methane emissions from sheep and cattle, which account for approximately 90% of that island country's methane emissions. And, as we report in our Subject Index Topic for Tannins, enriching the air with CO2 can greatly increase the tannin concentrations of many different forage plants.
In view of these several observations, it is very possible that increasing condensed tannin concentrations in pasture crops by genetic engineering and allowing the air's CO2 content to continue to rise could result in significant decreases in methane emissions from cattle and sheep. The outlook is also good for accomplishing this feat by including fish-oil feed additives in their diets. In addition, it is possible that elevated CO2 concentrations may lead directly to reduced methane emissions from rice culture, although more work must be done to confirm or refute this hypothesis. Nevertheless, the balance of evidence suggests we can be cautiously optimistic about our agricultural intervention capabilities and their capacity to help stem the tide of earth's historically-rising atmospheric methane concentration, which could take a huge bite out of methane-induced global warming.
References
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Cai, Z., Xing, G., Yan, X., Xu, H., Tsuruta, H., Yogi, K. and Minami, K. 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., Ladha, J.K., Reichardt, W., Doberman, A. and Singh, U. 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.
Fievez, V., Dohme, F., Danneels, M., Raes, K. and Demeyer, D. 2003. Fish oils as potent rumen methane inhibitors and associated effects on rumen fermentation in vitro and in vivo. Animal Feed Science and Technology 104: 41-58.
Inubushi, K., Cheng, W., Aonuma, S., Hoque, M.M., Kobayashi, K., Miura, S., Kim,. H.Y. and Okada, M. 2003. Effects of free-air CO2 enrichment (FACE) on CH4 emission from a rice paddy field. Global Change Biology 9: 1458-1464.
Kruger, M. and Frenzel, P. 2003. Effects of N-fertilisation on CH4 oxidation and production, and consequences for CH4 emissions from microcosms and rice fields. Global Change Biology 9: 773-784.
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.
Schrope, M.K., Chanton, J.P., Allen, L.H.. and Baker, J.T. 1999. Effect of CO2 enrichment and elevated temperature on methane emissions from rice, Oryza sativa. Global Change Biology 5: 587-599.
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.