Responses of soil respiration to atmospheric and climatic change will have profound impacts on ecosystem and global carbon (C) cycling in the future. This study was conducted to examine the effects on soil respiration of the concurrent driving factors of elevated atmospheric C O 2 concentration, air warming, and changing precipitation in a constructed old-field grassland. During the 19-month experimental period, from June 2003 to December 2004, higher C O 2 concentration and soil water availability significantly increased mean soil respiration by 35.8% and 15.7%, respectively. The effects of air warming on soil respiration varied seasonally from small reductions to significant increases to no response, with no significant main effect. In the wet side of elevated C O 2 chambers, air warming consistently caused increases in soil respiration, whereas with the other three combinations of C O 2 and water treatments, warming tended to decrease soil respiration over the growing season but increase it over the winter. There were no interactive effects on soil respiration among any two or three treatment factors, regardless of time period. Treatment-induced changes in soil temperature and moisture together explained 49%, 44%, and 56% of the seasonal variations of soil respiration responses to elevated C O 2 , air warming, and changing precipitation, respectively. Additional indirect effects of seasonal dynamics and responses of plant growth on C substrate supply were indicated. By identifying the direct and indirect effects of the global change forcing factors on soil respiration, the findings are critical for model projection of ecosystem and global C cycling ( Global Change Biology, 2007).