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Nitrous Oxide -- Summary
In the words of Cantarel et al. (2011), nitrous oxide (N2O) is "an influential greenhouse gas," with a per-molecule global warming potential "approximately 300 times that of CO2 (IPCC, 2001)," and they say it "has shown linear increases of 0.2-0.3% per year over the last few decades, largely as a result of changes in agricultural practices and direct emissions from agricultural soils (IPCC, 2007)." As a result, understanding the factors that control the concentration of N2O the atmosphere, and how the sources and sinks of N2O vary with changes in climate and other factors, has long been an important concern among the scientific community. And in this Summary, therefore, we review important research that has been conducted on this topic, beginning with a discussion of studies examining how increases in atmospheric CO2 might modify the release of N2O into the atmosphere.

One of the main sources of nitrous oxide is agriculture, which accounts for almost half of its emissions in some countries (Pipatti, 1997). And with N2O originating from microbial N cycling in soil -- mostly from aerobic nitrification or from anaerobic denitrification (Firestone and Davidson, 1989) -- there is a concern that CO2-induced increases in carbon input to soil, together with increasing N input from other sources, will increase substrate availability for denitrifying bacteria and may result in higher N2O emissions from agricultural soils as the air's CO2 content continues to rise.

In a study designed to investigate this possibility, Kettunen et al. (2007a) grew mixed stands of timothy (Phleum pratense) and red clover (Trifolium pratense) in sandy-loam-filled mesocosms at low and moderate soil nitrogen levels within greenhouses maintained at either 360 or 720 ppm CO2, while measuring harvestable biomass production and N2O evolution from the mesocosm soils over the course of three crop cuttings. This work revealed that the total harvestable biomass production of P. pratense was enhanced by the experimental doubling of the air's CO2 concentration by 21 percent and 26 percent, respectively, in the low and moderate soil N treatments, while corresponding biomass enhancements for T. pratense were 22 percent and 18 percent. In addition, the researchers found that after emergence of the mixed stand and during vegetative growth before the first harvest and N fertilization, N2O fluxes were higher under ambient CO2 in both the low and moderate soil N treatments. In fact, it was not until the water table had been raised and extra fertilization given after the first harvest that the elevated CO2 seemed to increase N2O fluxes. The four Finnish researchers thus concluded that the mixed stand of P. pratense and T. pratense was "able to utilize the increased supply of atmospheric CO2 for enhanced biomass production without a simultaneous increase in the N2O fluxes," thereby raising "the possibility of maintaining N2O emissions at their current level, while still enhancing the yield production [via the aerial fertilization effect of elevated CO2] even under low N fertilizer additions."

In a similar study, Kettunen et al. (2007b) grew timothy (Phleum pratense) in monoculture within sandy-soil-filled mesocosms located within greenhouses maintained at atmospheric CO2 concentrations of either 360 or 720 ppm for a period of 3.5 months at moderate (standard), low (half-standard), and high (1.5 times standard) soil N supply, while they measured the evolution of N2O from the mesocosms, vegetative net CO2 exchange, and final above- and below-ground biomass production over the course of three harvests. In this experiment the elevated CO2 concentration increased the net CO2 exchange of the ecosystems (which phenomenon was primarily driven by CO2-induced increases in photosynthesis) by about 30 percent, 46 percent and 34 percent at the low, moderate, and high soil N levels, respectively, while it increased the above-ground biomass of the crop by about 8 percent, 14 percent, and 8 percent at the low, moderate and high soil N levels, and its below-ground biomass by 28 percent, 27 percent, and 41 percent at the same respective soil N levels. And once again, Kettunen et al. report that "an explicit increase in N2O fluxes due to the elevated atmospheric CO2 concentration was not found."

In another CO2-enrichment study, while working at the Nevada Desert FACE Facility northwest of Las Vegas, Nevada (USA), McCalley et al. (2011) measured soil fluxes of reactive N gases (NO, NOX, NH3) and N2O in plots receiving long-term fumigation with ambient (380 ppm) and elevated (550 ppm) CO2. These treatments were begun in April 1997; and reactive N gas flux measurements were made under these conditions several years later in April 2005, July 2005, July 2006, January 2007 and March 2007, as well as after the termination of CO2 fumigation in July 2007, October 2007, January 2008 and April 2008.

In discussing the flux measurements, the five researchers report that "long-term exposure to elevated CO2 decreased reactive N gas emissions from Mojave Desert soils," and that it did so primarily "in islands of fertility created by the dominant shrub Larrea tridentata," and especially "in the spring and fall when recent precipitation, either natural or artificial, created soil conditions that are optimal for biological activity." Emissions of N2O, on the other hand, were "a very small component" of gaseous N loss and were "largely insensitive to elevated CO2." In addition, the five researchers state that the greater-than-60% reductions in reactive N gas fluxes during periods of peak N demand imply that elevated CO2 is "increasing the retention of biologically available N during critical growth periods," which is a major benefit for desert ecosystems.

Writing as background for their work, Livesley et al. (2009) state that "soils provide the largest terrestrial carbon store, the largest atmospheric CO2 source, the largest terrestrial N2O source and the largest terrestrial CH4 sink, as mediated through root and soil microbial process," and that "a change in land use or management can alter these soil processes such that net greenhouse gas exchange may increase or decrease." Seeking to determine how the emission and absorption of these three greenhouse gases differ between forests and pastures, the authors "measured soil-atmosphere exchange of CO2, N2O and CH4 in four adjacent land-use systems (native eucalypt woodland, clover-grass pasture, Pinus radiata and Eucalyptus globulus plantation) for short, but continuous, periods between October 2005 and June 2006 using an automated trace gas measurement system near Albany in southwest Western Australia."

With respect to nitrous oxide, the six scientists discovered that soil N2O emissions were more than an order of magnitude greater in the pasture than in the natural and managed forests," and given the authors' findings with respect to CO2 and CH4, it was concluded that "there is a triple greenhouse-gas benefit from afforestation of pasture systems," where in addition to carbon sequestration via tree biomass, "there is a decrease in N2O emissions because of lower nitrogen inputs and a tighter nutrient cycling, and an increase in CH4 uptake by forest soils." Such findings demonstrate the important role of land use change and land management in the release of nitrous oxide to the atmosphere.

In a different type of study -- driven by the possibility that the climate of the Amazon Basin may gradually become drier due to a warming-induced increase in the frequency and/or intensity of El Niņo events that have historically brought severe drought to the region -- Davidson et al. (2004) devised an experiment to determine the consequences of the drying of the soil of an Amazonian moist tropical forest for the net surface-to-air fluxes of both N2O and methane (CH4). This they did in the Tapajos National Forest near Santarem, Brazil, by modifying a one-hectare plot of land covered by mature evergreen trees so as to dramatically reduce the amount of rain that reached the forest floor (throughfall), while maintaining an otherwise similar one-hectare plot of land as a control for comparison.

Prior to making this modification, the three researchers measured the gas exchange characteristics of the two plots for a period of 18 months; then, after initiating the throughfall-exclusion treatment, they continued their measurements for an additional three years. This work revealed that the "drier soil conditions caused by throughfall exclusion inhibited N2O and CH4 production and promoted CH4 consumption." In fact, they report that "the exclusion manipulation lowered annual N2O emissions by >40 percent and increased rates of consumption of atmospheric CH4 by a factor of >4," which results they attributed to the "direct effect of soil aeration on denitrification, methanogenesis, and methanotrophy."

Consequently, if global warming would indeed increase the frequency and/or intensity of El Niņo events as some claim it will, the results of this study suggest that the anticipated drying of the Amazon Basin would initiate a strong negative feedback via (1) large drying-induced reductions in the evolution of both N2O and CH4 from its soils, and (2) a huge drying-induced increase in the consumption of CH4 by its soils. Although Davidson et al. envisage a more extreme second phase response "in which drought-induced plant mortality is followed by increased mineralization of C and N substrates from dead fine roots and by increased foraging of termites on dead coarse roots" (an extreme response that would be expected to increase N2O and CH4 emissions), it should also be noted that the projected rise in the air's CO2 content would likely prohibit such a thing from ever occurring, due to the documented tendency for atmospheric CO2 enrichment to greatly increase the water use efficiency of essentially all plants, which would enable the forest to continue to flourish under significantly drier conditions than those of the present.

Other researchers have also examined the relationship between nitrous oxide emissions and soil water status. Goldberg and Gebauer (2009), for example, investigated the influence of drying and rewetting events on N2O emissions from the soil of a mature Norway spruce forest in Northeast Bavaria, Germany. Writing as background for their work they state that "the only sink for N2O considered in global models is the destruction of atmospheric N2O in the stratosphere through photolysis and photooxidation (IPCC, 2007)," and also citing the 2007 IPCC report, they say that "soils have been identified as the main sources for atmospheric N2O."

In conducting their analysis to learn more about the emission of N2O from wet vs. dry soils, Goldberg and Gebauer induced an artificial summer drought of 46 days duration (which was accompanied by a natural drought) via throughfall exclusion (TE) that was provided by special roof installations, which they followed with an experimental rewetting of 66 mm over two days, during which periods (and before and after them) they closely monitored N2O fluxes from the soils of the TE and unaltered control (C) plots that were exposed to the elements.

According to the two researchers, their results indicated that "before the drought, both the C and TE plots showed slightly positive N2O fluxes from the soil to the atmosphere," in harmony with the sentiment of the IPCC. During the drought, on the other hand, they report that "the soil of both the throughfall exclusion and control plots served as an N2O sink," contrary to what might have been expected in light of IPPC statements; and they say that "the sink strength of the throughfall exclusion plots was doubled [italics added] compared with the control plots." Rewetting, however, "turned the soil into a source for atmospheric N2O again," but they found that "it took almost four months [italics added] to turn the cumulative soil N2O fluxes from negative (sink) to positive (source) values." Thus, on the basis of their observations, Goldberg and Gebauer concluded that "long drought periods can lead to drastic decreases of N2O fluxes from soils to the atmosphere or may even turn forest soils temporarily to N2O sinks [italics added]," which situation may in some places persist for years at a time. It is also entirely possible, however, that over the entire terrestrial surface of the planet, the net result is indeed that "soils are the main sources for atmospheric N2O," as stated by the IPCC. Nevertheless, the two scientists conclude that the fact that there is what they call an unbalanced global N2O budget "underlines the likelihood of a hitherto unconsidered sink function of soils."

In combining the effects of soil water status and atmospheric CO2 on N2O emissions, Welzmiller et al. (2008) measured N2O and denitrification emission rates in a C4 sorghum [Sorghum bicolor (L.) Moench] production system with ample and limited flood irrigation rates under Free-Air CO2 Enrichment (seasonal mean = 579 ppm) and control (seasonal mean = 396 ppm) conditions during the 1998 and 1999 summer growing seasons at the experimental FACE site near Maricopa, Arizona (USA). In doing so, they found that "elevated CO2 did not result in increased N2O or N-gas emissions with either ample or limited irrigation," which findings they describe as being "consistent with findings for unirrigated western U.S. ecosystems reported by Billings et al. (2002) for Mojave Desert soils and by Mosier et al. (2002) for Colorado shortgrass steppe." Discussing the implications of their findings, Welzmiller et al. say their results suggest that "as CO2 concentrations increase, there will not be major increases in denitrification in C4 cropping environments such as irrigated sorghum in the desert southwestern United States," which further suggests there will not be an increased impetus for global warming due to this phenomenon.

Adding temperature to the mix, Cantarel et al. (2011) monitored N2O fluxes in an in situ ecosystem manipulation experiment simulating the climate predicted for the study area (an upland temperate grassland in the French Massif Central region), making use of the Clermont Climate Change Experiment facility, where Bloor et al. (2010) were conducting "a long-term grassland study of multiple climate changes applied in an additive experimental design." Thus, over a two-year period, they monitored N2O fluxes under conditions "simulating the climate predicted for the study area in 2080 (3.5°C temperature increase, 20% reduction in summer rainfall and atmospheric CO2 levels of 600 ppm)."

"Overall," as the four researchers described the results of their study, "experimental warming had a positive effect on the annual N2O emissions." However, and "contrary to expectations," as they put it, "combined summer drought and warming had no significant effect on mean N2O fluxes recorded at any time," nor did "elevated CO2 in combination with warming and drought." Thus, it would appear that if climate change proceeded as predicted for this particular part of the world, it would likely not be magnified to any significant degree by climate-change-induced increases in ecosystem N2O emissions.

Introducing their study of the subject, Carter et al. (2011) write that "in temperate regions, climate change is predicted to increase annual mean temperature and intensify the duration and frequency of summer droughts, which together with elevated atmospheric carbon dioxide concentrations, may affect the exchange of nitrous oxide (N2O) and methane (CH4) between terrestrial ecosystems and the atmosphere." Working in a dry temperate heathland with a nutrient-poor sandy soil located about 50 km northwest of Copenhagen, Denmark -- the vegetation of which was dominated by Scotch Heather (Calluna vulgaris), Wavy Hairgrass (Deschampsia flexuosa) and various mosses -- Carter et al. set out to investigate "the effects of future climatic and atmospheric conditions on the biosphere-atmosphere exchange of N2O and CH4."

With respect to N2O emissions, the researchers found that "as single experimental factors, elevated CO2, temperature and summer drought had no major effect on the N2O fluxes, but the combination of CO2 and warming stimulated N2O emission, whereas the N2O emission ceased when CO2 was combined with drought." Given such observations, Carter et al. say that their study "highlights the importance of evaluating climate change parameters in multifactor treatments as the response of CH4 and N2O flux rates to different two- and three-factor combinations may not be predicted from the responses to the individual treatments." And in summing up their work they add that "overall, our study suggests that in the future, CH4 uptake may increase slightly, while N2O emission will remain unchanged in temperate ecosystems on well-aerated soils."

In summation, it would appear that concerns about additional global warming arising from enhanced N2O emissions from agricultural soils in a CO2-enriched atmosphere of the future are not well founded.

Billings, S.A., Schaeffer, S.M. and Evans, R.D. 2002. Trace N gas losses and mineralization in Mojave Desert soils exposed to elevated CO2. Soil Biology and Biochemistry 34: 1777-1784.

Bloor, J.M.G., Pichon, P., Falcimagne, R., Leadley, P. and Soussana, J.F. 2010. Effects of warming, summer drought and CO2 enrichment on aboveground biomass production, flowering phenology and community structure in an upland grassland ecosystem. Ecosystems 13: 888-900.

Cantarel, A.A.M., Bloor, J.M.G., Deltroy, N. and Soussana, J.-F. 2011. Effects of climate change drivers on nitrous oxide fluxes in an upland temperate grassland. Ecosystems 14: 223-233.

Carter, M.S., Ambus, P., Albert, K.R., Larsen, K.S., Andersson, M., Prieme, A. van der Linden, L. and Beier, C. 2011. Effects of elevated atmospheric CO2, prolonged summer drought and temperature increase on N2O and CH4 fluxes in a temperate heathland. Soil Biology & Biochemistry 43: 1660-1670.

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Firestone, M.K. and Davidson, E.A. 1989. Microbiological basis of NO and N2O production and consumption in soil. In: Andreae, M.O. and Schimel, D.S. (Eds.) Exchange of Trace Gases Between Terrestrial Ecosystems and the Atmosphere. Wiley, Chichester, pp. 7-21.

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Kettunen, R., Saarnio, S. and Silvola, J. 2007b. N2O fluxes and CO2 exchange at different N doses under elevated CO2 concentration in boreal agricultural mineral soil under Phleum pratense. Nutrient Cycling in Agroecosystems 78: 197-209.

Livesley, S.J., Kiese, R., Miehle, P., Weston, C.J., Butterbach-Bahl, K. and Arndt, S.K. 2009. Soil-atmosphere exchange of greenhouse gases in a Eucalyptus marginata woodland, a clover-grass pasture and Pinus radiata and Eucalyptus globulus plantations. Global Change Biology 15: 425-440.

McCalley, C.K., Strahm, B.D., Sparks, K.L., Eller, A.S.D. and Sparks, J.P. 2011. The effect of long-term exposure to elevated CO2 on nitrogen gas emissions from Mojave Desert soils. Journal of Geophysical Research 116: 10.1029/2011JG001667.

Mosier, A.R., Morgan, J.A., King, J.Y., LeCain, D. and Milchunas, D.G. 2002. Soil-atmosphere exchange of CH4, CO2, NOX, and N2O in the Colorado shortgrass steppe under elevated CO2. Plant and Soil 240: 201-211.

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Welzmiller, J.T., Matthias, A.D., White, S. and Thompson, T.L. 2008. Elevated carbon dioxide and irrigation effects on soil nitrogen gas exchange in irrigated sorghum. Soil Science Society of America Journal 72: 393-401.

Last updated 12 June 2013