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Effects of Elevated CO2 on Yield and N2O Emissions of a Mixed Stand of Timothy and Red Clover
Kettunen, R., Saarnio, S., Martikainen, P.J. and Silvola, J. 2007. Can a mixed stand of N2-fixing and non-fixing plants restrict N2O emissions with increasing CO2 concentration? Soil Biology & Biochemistry 39: 2538-2546.

The authors note that nitrous oxide (N2O) is an important greenhouse gas that also destroys stratospheric ozone, and that "agriculture is one of the main anthropogenic sources of N2O, accounting e.g. in Finland for almost half of the national 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) - they say there is concern that CO2-induced increases in C input to the soil, together with increasing N input from other sources, will "increase substrate availability for denitrifying bacteria and may result in higher N2O emission from agricultural soils in the future."

What was done
In a study designed to investigate this hypothesis, Kettunen et al. grew mixed stands of timothy (Phleum pratense) and red clover (Trifolium pratense) in sandy-loam-filled mesocosms at low and moderate soil nitrogen levels (5 and 10 g N m-2, respectively) within thermo-controlled 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 cuttings of the crop.

What was learned
The four Finnish researchers found that the total harvestable biomass production of P. pratense was enhanced by their doubling of the air's CO2 concentration by 21 and 26%, respectively, in the low and moderate N treatments, while corresponding values for T. pratense were 22 and 18%. In addition, they found that "after emergence of the mixed stand and during vegetative growth before the first harvest and N fertilization, the N2O fluxes were significantly higher under ambient CO2 at low (P = 0.001, ca. 55%) and at the moderate (P < 0.0005, ca. 72%) N treatments." Indeed, it was not until the water table had been raised and extra fertilization given after the first harvest that the elevated CO2 concentration seemed to increase N2O fluxes.

What it means
Kettunen et al. concluded from their observations 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 elevated CO2] even under low N fertilizer additions."

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.

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

Reviewed 17 October 2007