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.