Nitrous oxide
From ClimateWiki
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
One of the main sources of nitrous oxide (N2O) is agriculture, which accounts for almost half of N2O emissions in some countries (Pipatti, 1997). 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.”
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) CO2 during the 1998 and 1999 summer growing seasons at the experimental FACE site near Maricopa, Arizona (USA). The study found “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.”
In 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.
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 Niño 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 Niño 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), we note 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.
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.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/n/nitrousoxide.php
References
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.
Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html
Davidson, E.A., Ishida, F.Y. and Nepstad, D.C. 2004. Effects of an experimental drought on soil emissions of carbon dioxide, methane, nitrous oxide, and nitric oxide in a moist tropical forest. Global Change Biology 10: 718-730.
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.
Kettunen, R., Saarnio, S., Martikainen, P.J. and Silvola, J. 2007a. Can a mixed stand of N2-fixing and non-fixing plants restrict N2O emissions with increasing CO2 concentration? Soil Biology & Biochemistry 39: 2538-2546.
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.
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.
Pipatti, R. 1997. Suomen metaani-ja dityppioksidipaastojen rajoittamisen mahdollisuudet ja kustannustehokkuus. VTT tiedotteita. 1835, Espoo, 62 pp.
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.
