The drivers of terrestrial carbon sequestration, as well as the geographical spread and magnitude of this land sink
are currently debated. Nitrogen deposition is one potential driver for enhancements in carbon sequestration. We
studied the influence of nitrogen (N) deposition on photosynthetic capacity using eddy covariance measurements
of net exchange of carbon. From the provided estimates of gross primary production we derived the photosynthetic
capacity (Amax) of forests. We used Amax to study the impact of N deposition, while accounting for climate and
stand characteristics of forest canopies for a global dataset of 80 forest FLUXNET sites. Canopy Amax related
positively to N deposition below an observed critical load for evergreen needleleaf forests and leveled off for
higher N deposition rates. The determination of a pure N deposition effect was prevented by correlations between
climate and N deposition. Without considering climate effects we identified an upper limit of the Amax – N
deposition relationship for evergreen needleleaf forests; 2.0 ± 0.4 (S.E.) µmol CO2 m-2 s-1 per 1 kg N ha-1 yr-1.
For deciduous broadleaf forests, as well as for temperate evergreen needleleaf forests, Amax appeared insensitive to
N deposition. However, in N limited evergreen needleleaf forests, we conclude that N deposition plays an essential
role in determining canopy physiology and carbon cycling by increasing Amax
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