The response of ecosystems to elevated atmospheric CO₂ is affected by nitrogen (N) availability. It has been hypothesized that N limitation becomes progressively stronger (progressive N limitation, PNL). Most long-term free air CO₂ enrichment studies did not see a PNL. This paper shows that enhanced biological N₂ fixation only prevents PNL in plant communities with symbiotic N₂ fixation. In most ecosystems a stimulation of gross N mineralization prevents the development of a PNL.