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 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-fertilization 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."
Additional understanding of CO2-induced impacts on methane emissions from rice was gained from the study of Cheng et al. (2008), who examined well watered (flooded) and fertilized rice (Oryza sativa L.) plants. In their experiment, plants were fumigated with air containing either 380 or 680 ppm CO2 from the panicle formation stage at 59 days after transplanting (DAT, from seedling trays into pots) within controlled-environment chambers maintained at either high (32°C) or low (22°C) night temperatures, with day temperature held constant at 32°C, until either 107 or 114 DAT. During this latter period, they measured the flux of methane between the pots and the atmosphere each day at 10:00 and 22:00 hours; and at the conclusion of the experiment they determined the dry weight of each organ of all of the plants employed in the study.
Results indicated that the extra 300 ppm of CO2 increased CH4 emissions by 32.2% in the low night temperature treatment, but by only 3.5% in the high night temperature treatment. Likewise, they found that the elevated CO2 increased the dry weight gained by the plants in the low night temperature treatment by 38.4%, but by a smaller 12.7% in the high night temperature treatment.
In considering the above findings, an interesting metric that can be derived from these data is the ratio of the percent increase in CO2-induced biomass production (a positive effect) to the percent increase in CO2-induced CH4 emissions (a negative effect) as the air's CO2 concentration rose from 380 to 680 ppm. This benefit/cost ratio was 1.19 in Cheng et al.'s low-night-temperature treatment and 3.63 in their high-night-temperature treatment, which indicates that in transiting from the low-night-temperature to the high-night-temperature environment in their experiment as the air's CO2 concentration rose by 300 ppm, the benefit/cost ratio rose by a little over 200%. Consequently, because night temperatures rose significantly faster than day temperatures throughout most of the world over the last several decades, this phenomenon may well have had a net two-pronged positive effect on the biosphere; and it could well have a similar positive effect in the future, increasing the magnitude of the aerial fertilization effect of atmospheric CO2 enrichment at a considerably faster relative rate than it increases the relative rate of CO2-induced methane emissions to the atmosphere.
Introducing their study of the subject, Qaderi and Reid (2011) report that the release of aerobic methane by vegetation has been indirectly confirmed by the field studies of Braga do Carmo et al. (2006), Crutzen et al. (2006) and Sanhueza and Donoso (2006), as well as by the satellite studies of Frankenberg et al. (2005, 2008). In addition, they note that CH4 emissions from plants can be stimulated by higher air temperatures (Vigano et al., 2008; Qaderi and Reid, 2009) and water stress (Qaderi and Reid, 2009). And since "methane is the second most important long-lived greenhouse gas after carbon dioxide and is thought to be ~25 times more potent than CO2 in its ability to act as a greenhouse gas," as they describe it, they decided to see what effect the ongoing rise in the air's CO2 content might possibly have on this phenomenon. More specifically, Qaderi and Reid "examined the combined effects of temperature, carbon dioxide and watering regime on CH4 emissions from six commonly cultivated crop species: faba bean, sunflower, pea, canola, barley and wheat" in an experiment where "plants were grown from seeds in controlled-environment growth chambers under two temperature regimes (24°C day/20°C night and 30°C day/26°C night), two CO2 concentrations (380 and 760 ppm) and two watering regimes (well watered and water stressed)," where the "plants were first grown under 24/20°C for one week from sowing, and then placed under experimental conditions for a further week," after which "plant growth, gas exchange and CH4 emission rates were determined." So what did the researchers learn?
First of all, they say they found "no detectable CH4 from [a] control treatment (without plant tissue), indicating that CH4 from the experimental treatments was emitted only from plant tissues." Second, they found that the plants grown under higher temperature and water stress emitted more CH4 than those grown under lower temperature and no water stress. And third, they found that "elevated CO2 had the opposite effect," so that it "partially reverses" the effects of the other two factors.
In commenting on their findings, Qaderi and Reid say that "although rising atmospheric CO2 reduces plant CH4 emissions, it may not fully reverse the effects of temperature and drought," which they assume will increase in tandem with the ongoing rise in the air's CO2 content. Nevertheless, this result is still a positive finding. In addition, it may well be much more positive than they make it out to be, especially if temperatures and drought do not increase with the passage of time while the air's CO2 content does.
Another agricultural source of methane is the fermentation of feed in the rumen of cattle and sheep. Within this context, Boadi et al. (2004) reviewed methods for reducing CH4 emissions from dairy cows. At the time of their review, existing mitigation strategies for reducing emissions from dairy cows included the addition of ionophores and fats to their food, as well as the use of high-quality forages and grains in their diet, while newer mitigation strategies included "the addition of probiotics, acetogens, bacteriocins, archaeal viruses, organic acids, [and] plant extracts (e.g., essential oils) to the diet, as well as immunization, and genetic selection of cows." In fact, they provided a table of 20 such strategies, where the average maximum potential CH4 reduction that may result from the implementation of each strategy is 30% or more.
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 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.
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