Abstract
Stimulation of terrestrial productivity by rising CO2 concentration is projected to reduce the airborne fraction of anthropogenic CO2 emissions; coupled climate-carbon (C) cycle models, including those used in the IPCC Fourth Assessment Report (AR4), are sensitive to this negative feedback on atmospheric CO2 1. The representation of the so-called CO2 fertilization effect in the 11 models used in AR4 and subsequent models2,3 was broadly consistent with experimental evidence from four free-air CO2 enrichment (FACE) experiments, which indicated that net primary productivity (NPP) of forests was increased by 23 ± 2% in response to atmospheric CO2 enrichment to 550 ppm4. Substantial uncertainty remains, however, because of the expectation that feedbacks through the nitrogen (N) cycle will reduce the CO2 stimulation of NPP5,6; these feedbacks were not included in the AR4 models and heretofore have not been confirmed by experiments in forests7. Here, we provide new evidence from a FACE experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee, USA, that N limitation has significantly reduced the stimulation of NPP by elevated atmospheric CO2 concentration (eCO2). Isotopic evidence and N budget analysis support the premise that N availability in this forest ecosystem has been declining over time, and declining faster in eCO2. Model analyses and evidence from leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to eCO2. These results provide a strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.
