What was done
The authors enriched the air above plots of a nutrient-poor species-rich calcareous grassland in northwestern Switzerland with an extra 235 ppm of CO2 via a set of novel windscreens (Leadley et al., 1997) that operated 24 hours per day, except during mid-winter (December-February) when CO2 enrichment was not performed.  After six growing seasons under this protocol, they assessed the impact of the extra CO2 on (1) N-mineralisation in the grassland soil and (2) the activities of the soil microbial enzymes invertase, xylanase, urease, protease, arylsulfatase and alkaline phosphatase in both the spring and summer.

What was learned
N-mineralisation was increased significantly (P = 0.02, a priori linear contrast) by 30% in the spring and insignificantly (P = 0.6) by 3% in the summer by the extra CO2.  In addition, at both sampling times all measured enzyme activities were higher in the CO2-enriched treatment, with the single exception of xylanase in summer.  The strongest responder in the spring was alkaline phosphatase (up 32%, P = 0.02), followed by urease (up 21%, P = 0.13).  In the summer, the best responder was urease (up 21%, P = 0.2), followed by protease (up 17%, P = 0.09) and invertase (up 14%, P = 0.07).  The authors attribute the increased N-mineralisation and enzyme activity of the soil biota to the higher moisture content of the soil in the CO2-enriched plots (due to a CO2-induced decrease in stand transpiration) and/or the CO2-induced increase in root biomass (up 24%, P = 0.02, in June 1999).

What it means
Soil microorganisms hold a key position in terrestrial ecosystems, as they mineralize organic matter and make its nitrogen available for use by plants.  Hence, it is encouraging to note, in the words of the authors, "that elevated CO2 will enhance below-ground C- and N-cycling in grasslands," even nutrient-poor grasslands, such as the one they studied.

Reference
Leadley, P.W., Niklaus, P., Stocker, R. and Korner, C.  1997.  Screen-aided CO2 control (SACC): a middle-ground between FACE and open-top chamber.  Acta Oecologia 18: 207-219.