What was done
In a FACE study conducted near Zurich, Switzerland, on three types of grassland (monocultures and mixed cultures of Trifolium repens L. and Lolium perenne L.) receiving two different nitrogen treatments (low N = 14 g N m^-2 year^-1; high N = 56 g N m^-2 year^-1) that were maintained at atmospheric CO2 concentrations of either 350 or 600 ppm, the authors measured soil microbial biomass and the activities of associated enzymes covering cycles of the most important elements (C, N and P) over a period of nine years.

What was learned
The researchers report that "the enrichment in CO2 increased soil microbial biomass (+48.1%) as well as activities of invertase (+36.2%), xylanase (+22.9%), urease (+23.8%), protease (+40.2%) and alkaline phosphomonoesterase (+54.1%) in spring," and that "in autumn, the stimulation of microbial biomass was 25% less and that of enzymes 3-12% less than in spring," all of which increases, in their words, "were most likely stimulated by increase in roots under enriched CO2."

What it means
Certain scientists, such as Hungate et al. (2003), have claimed that the future availability of nitrogen will likely be too low to support large increases in plant growth over the long term, primarily because of their contention that when CO2 enrichment increases soil C:N ratios, "decomposing microorganisms require more nitrogen," and they contend that "this effect can reduce nitrogen mineralization," which they say is "the main source of nitrogen for plants." However, Drissner et al. found that in their 9-year-long FACE experiment, "stimulation of enzyme activities in the enriched CO2 indicated enhanced C, N and P cycling and greater availabilities of nutrients for microbial and plant growth [our italics]." And they go on to say that their results "support the hypothesis of positive feedback proposed by Zak et al. (1993), who stated that additional C stimulates microbial decomposition and thus leads to more available N under enriched CO2 [our italics]."

For more on this topic, see the many items archived under Nitrogen (Progressive Limitation Hypothesis) in our Subject Index.

References
Hungate, B.A., Dukes, J.S., Shaw, M.R., Luo, Y. and Field, C.B. 2003. Nitrogen and climate change. Science 302: 1512-1513.

Zak, D.R., Pregitzer, K.S., Curtis, P.S., Teeri, J.A., Fogel, R. and Randlett, D.L. 1993. Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles. Plant and Soil 151: 105-117.