The current rise in atmospheric CO₂, a consequence of human activities such as fossil fuel burning and deforestation, is thought to stimulate plant growth in many ecosystems. If increased C assimilation by plants is translated into increased soil organic carbon (SOC), terrestrial ecosystems might help mitigate rising CO₂ emissions. However, higher plant growth rates in a CO₂-rich world can only be sustained if the soil supplies plants with additional nutrients. Therefore, the effect of elevated CO₂ on nutrient cycling is of key importance when predicting the potential for C storage in plant biomass and soils. The impact of higher CO₂ levels on the flow of C and nutrient through ecosystems depends on a set of complex interactions between soil and plants. We therefore need field experiments under realistic conditions to make accurate predictions on the effects of rising atmospheric CO₂. Over the last two decades, many Open Top Chamber (OTC) and Free Air Carbon dioxide Enrichment (FACE) experiments have been conducted, covering a wide range of terrestrial ecosystems. Yet, because of spatial variability, the sensitivity of these experiments to detect CO₂ induced changes is low. A quantitative integration of results across multiple studies might overcome this problem. Meta-analytic methods enable placing confidence limits around effect sizes; therefore they provide a robust statistical test for overall CO₂ effects across multiple studies. Using meta-analysis, we summarized the results of 116 studies on plant biomass production, SOM dynamics and biological N₂ fixation in FACE and OTC experiments. Averaged over all studies, elevated CO₂ stimulated above- and belowground plant biomass by 20% and 30%, respectively. Despite the stimulation of microbial respiration by 7.5%, elevated CO₂ still caused soil C contents to increase by 0.9% per year. These results suggest that the effect of elevated CO₂ on soil C input outweighs its effect on microbial decomposition. However, elevated CO₂ had a relative strong effect on potential mineralizable and soluble C, suggesting that C sequestration mostly occurred in labile pools with limited storage potential. Stimulation of gross N immobilization at elevated CO₂ was significant (+22%), but gross and net N mineralization rates remained unaffected. Combined with a significant increase in microbial N contents (+6%), this suggests that higher CO₂ levels enhanced microbial N demand. An increase in competition for available N between soil microbes and plants could potentially limit plant growth and thus soil C input under elevated CO₂. Indeed, elevated CO₂ only increased soil C contents when N was added at rates exceeding typical N deposition. These results are in line with the Progressive Nitrogen Limitation (PNL) theory, which states that when elevated CO₂ stimulates biomass production in unfertilized ecosystems, the resulting increase in soil C inputs will gradually reduce N availability. Some believe that an increase in atmospheric CO₂ will stimulate N₂ fixation, thereby providing N needed for C sequestration. However, averaged over all experiments, N₂ fixation was unresponsive to elevated CO₂ unless other essential nutrients were added. Thus, our meta-analysis corroborates the untested hypotheses that increased soil C input and soil C sequestration under elevated CO₂ are limited directly by N availability, and indirectly by nutrients needed to support N₂ fixation. Together, these findings suggest that the potential for rapid soil C storage under elevated CO₂ will be small.