We studied the effects of doubled atmospheric CO2 concentration on the density and diversity of microarthropods of the Shortgrass Steppe of the Pawnee National Grasslands in Colorado. Samples were extracted from soil cores taken from three ambient CO2 chambers, three doubled CO2 chambers and three control plots. The treatments were established in the spring of 1997 and the first year's data was collected after the first full growing season, in the fall of 1998. In all, we collected 18 soil cores per year or two cores per chamber or plot, each of which was separated into five depths. The total microarthropods for combined depths, found in the first year, equaled 744 in the ambient CO2 chambers, 523 in the control plots and 670 in the doubled CO2 chambers. While the total microarthropod densities were similar across treatments, the numbers of cryptostimatid, astigmatid, mesostigmatid and prostigmatid mites began to show differences by functional groupings. The cryptostigmatid mites had the highest densities in the ambient chambers and control plots. The doubled CO2 chambers had higher densities of predatory prostigmatid mites. The populations did not demonstrate significant treatment affects but did have significant treatment by depth interactions. The aggregation of mite populations to lower soil horizons was apparent.

Key words: CO2, microarthropods, mites