CO ₂ enhancement of forest productivity constrained by limited nitrogen availability

TLDR

Rising atmospheric CO₂ is expected to stimulate terrestrial plant production, but climate–carbon cycle models predict that nitrogen‑cycle feedbacks may limit this fertilization effect, creating uncertainty in projections. The study evaluated whether nitrogen limitation reduced the CO₂ fertilization of net primary productivity over 11 years in a free‑air CO₂ enrichment experiment on a sweetgum forest stand in Tennessee. The experiment showed an initial 24 % NPP increase under 550 ppm CO₂ that declined to 9 % after eight years, indicating that nitrogen availability limited and weakened the fertilization effect, and that sustained CO₂ fertilization cannot be assumed in global models.

Abstract

Stimulation of terrestrial plant production by rising CO 2 concentration is projected to reduce the airborne fraction of anthropogenic CO 2 emissions. Coupled climate–carbon cycle models are sensitive to this negative feedback on atmospheric CO 2 , but model projections are uncertain because of the expectation that feedbacks through the nitrogen (N) cycle will reduce this so-called CO 2 fertilization effect. We assessed whether N limitation caused a reduced stimulation of net primary productivity (NPP) by elevated atmospheric CO 2 concentration over 11 y in a free-air CO 2 enrichment (FACE) experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee. During the first 6 y of the experiment, NPP was significantly enhanced in forest plots exposed to 550 ppm CO 2 compared with NPP in plots in current ambient CO 2 , and this was a consistent and sustained response. However, the enhancement of NPP under elevated CO 2 declined from 24% in 2001–2003 to 9% in 2008. Global analyses that assume a sustained CO 2 fertilization effect are no longer supported by this FACE experiment. N budget analysis supports the premise that N availability was limiting to tree growth and declining over time —an expected consequence of stand development, which was exacerbated by elevated CO 2 . Leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to elevated CO 2 ; these observations are consistent with stand-level model projections. This FACE experiment provides strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.