Received April3, 1998 Summary The development of the Free-Air CO2 Enrichment

(FACE) facilities represents a substantial advance in experi-mental technology for studying ecosystem responses to ele-vated CO2. A challenge arising from the application of this technology is the utilization of short-term FACE results for predicting long-term ecosystem responses. This modeling study was designed to explore interactions of various processes on ecosystem productivity at elevated CO2 on the decadal scale. We used a forest model (FORDYN) to analyze CO2 re-sponses-particularly soil nitrogen dynamics, carbon produc-tion and storage-of a loblolly pine ecosystem in the Duke University Forest. When a 14-year-old stand was exposed to elevated CO2, simulated increases in annual net primary pro-ductivity (NPP) were 13, 10 and 7.5 % in Years 1, 2 and 10, respectively, compared with values at ambient CO2. Carbon storage increased by 4 % in trees and 9.2 % in soil in Year 10 in response to elevated CO2. When the ecosystem was exposed to elevated CO2 from the beginning of forest regrowth, annual NPP and carbon storage in trees and soil were increased by 32, 18 and 20%, respectively, compared with values at ambient CO2. In addition, simulation of a 20 % increase in mineraliza-tion rate led to a slight increase in biomass growth and carbon storage, but the simulated 20 % increase in fine root turnover rate considerably increased annual NPP and carbon storage in soil. The modeling results indicated that ( 1) stimulation ofNPP and carbon storage by elevated CO2 is transient and (2) effects of elevated CO2 on ecosystem processes--canopy develop-ment, soil nitrogen mineralization and root turnover-have great impacts on ecosystem C dynamics. A detailed under-standing of these processes will improve our ability to predict long-term ecosystem responses to CO2 enrichment