One of the main sources of N2O is agriculture, which in Finland accounts for almost half of that nations's N2O emissions (Pipatti, 1997). Moreover, 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 and 26%, respectively, in the low and moderate soil N treatments, while corresponding biomass enhancements for T. pratense were 22 and 18%. 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%, 46% and 34% at the low, moderate and high soil N levels, respectively, while it increased the above-ground biomass of the crop by about 8%, 14% and 8% at the low, moderate and high soil N levels, and its below-ground biomass by 28%, 27% and 41% 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 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% 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 -- but which real-world data suggest will not occur (see El Niño (Relationship to Global Warming) in our Subject Index) -- 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 (see Water Use Efficiency in our Subject Index), which would enable the forest to continue to flourish under significantly drier conditions than those of the present.

In concluding this review of soil N2O emissions, we turn our attention to the study of Crutzen et al. (2007), who calculated the amount of N2O that would be released to the atmosphere as a result of using nitrogen fertilizer to grow crops to be converted to biofuels. As they describe it, "all past studies have severely underestimated the release rates of N2O to the atmosphere, with great potential impact on climate warming," and they found that when the extra N2O emission from biofuel production is properly calculated, "the outcome is that the production of commonly used biofuels, such as biodiesel from rapeseed and bioethanol from corn (maize), can contribute as much or more to global warming by N2O emissions than cooling by fossil fuel savings."

As a result of these observations, Crutzen et al. conclude that "on a globally averaged basis the use of agricultural crops for energy production can readily be detrimental for climate due to the accompanying N2O emissions." In addition, they note that "increased emissions of N2O will also lead to enhanced NOX concentrations and ozone loss in the stratosphere." As a result, they conclude that the relatively large emission of N2O associated with biofuel production actually "exacerbates the already huge challenge of getting global warming under control."

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, while hopes for biofuel production to mitigate global warming are little more than wishful -- and errant! -- thinking.

References
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. 2007. 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.

Pipatti, R. 1997. Suomen metaani-ja dityppioksidipaastojen rajoittamisen mahdollisuudet ja kustannustehokkuus. VTT tiedotteita 1835, Espoo, 62 pp.