The study investigates the impact of elevated atmospheric CO₂ concentration on net primary productivity (NPP) in a deciduous sweetgum forest over an 11-year period using a free-air CO₂ enrichment (FACE) experiment. Initially, NPP was significantly enhanced in plots exposed to 550 ppm CO₂ compared to ambient CO₂ levels, but this effect declined from 24% in 2001-2003 to 9% in 2008. The decline in NPP under elevated CO₂ was attributed to limited nitrogen (N) availability, which was supported by observations of declining foliar N concentration and reduced photosynthesis. Nitrogen fertilization experiments confirmed that N limitation constrained tree growth. The progressive nitrogen limitation (PNL) hypothesis, suggesting that increased N sequestration under elevated CO₂ reduces available N, was validated by the observed decline in δ15N in leaf litter and annual N uptake. The study highlights the importance of incorporating N limitation and feedback effects in ecosystem and global climate models to improve predictions of future climate change.The study investigates the impact of elevated atmospheric CO₂ concentration on net primary productivity (NPP) in a deciduous sweetgum forest over an 11-year period using a free-air CO₂ enrichment (FACE) experiment. Initially, NPP was significantly enhanced in plots exposed to 550 ppm CO₂ compared to ambient CO₂ levels, but this effect declined from 24% in 2001-2003 to 9% in 2008. The decline in NPP under elevated CO₂ was attributed to limited nitrogen (N) availability, which was supported by observations of declining foliar N concentration and reduced photosynthesis. Nitrogen fertilization experiments confirmed that N limitation constrained tree growth. The progressive nitrogen limitation (PNL) hypothesis, suggesting that increased N sequestration under elevated CO₂ reduces available N, was validated by the observed decline in δ15N in leaf litter and annual N uptake. The study highlights the importance of incorporating N limitation and feedback effects in ecosystem and global climate models to improve predictions of future climate change.