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Effects of Elevated CO2 on Nitrogen Cycling in Four European Grasslands
Barnard, R., Barthes, L., Le Roux, X., Harmens, H., Raschi, A., Soussana, J.-F., Winkler, B. and Leadley, P.W. 2004. Atmospheric CO2 elevation has little effect on nitrifying and denitrifying enzyme activity in four European grasslands. Global Change Biology 10: 488-497.

The authors say "it has often been suggested that elevated CO2 could substantially alter soil microbial nitrogen and nitrification and denitrification processes in natural and semi-natural ecosystems." What is more, it has periodically been claimed that these CO2-induced changes may reduce the availability of the nitrogen forms that are suitable for use by plants to the point where they may be present in insufficient quantities to allow for significant CO2-induced increases in carbon storage in the world's soils in response to the projected long-term increase in atmospheric CO2 concentration (see our Editorial of 10 Dec 2003).

What was done
In a study designed to further explore this complex subject, Barnard et al. investigated the effects of elevated CO2 on nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), soil concentrations of extractable NH4+ and NO3-, as well as microbial biomass N, in four long-term CO2-enrichment experiments of the MEGARICH project (Managing European Grasslands as a Sustainable Resource in a Changing Climate) at sites located in Clermont-Ferrand (France), Neuherberg (Germany), Rapolano Terme (Italy) and Bangor (United Kingdom).

What was learned
The researchers say their study revealed that "microbial biomass N, NEA, DEA and extractable soil NH4+ and NO3- were generally not affected [our italics] by elevated CO2 in these grassland ecosystems after several years of treatment, nor by cutting frequency or temperature at the sites that included these treatments."

What it means
Although it has been claimed that the ongoing rise in the atmosphere's CO2 concentration could significantly alter soil microbial N and nitrification and denitrification processes in natural and semi-natural ecosystems (which could squash any hope of CO2-induced increases in soil carbon sequestration in a CO2-accreting atmosphere), Banard et al. say they "find little evidence of this in four grassland sites in Europe." Consequently, there is reason to be optimistic about the ability of the biosphere to enlarge its carbon storage capacity in response to the ongoing rise in the air's CO2 content.

Reviewed 1 March 2006