<p>The effects of atmospheric nitrogen deposition (N<sub>dep</sub>) on carbon (C) sequestration in forests have often been assessed by relating differences in productivity to spatial variations of N<sub>dep</sub> across a large geographic domain. These correlations generally suffer from covariation of other confounding variables related to climate and other growth-limiting factors, as well as large uncertainties in total (dry + wet) reactive nitrogen (N<sub>r</sub>) deposition. We propose a methodology for untangling the effects of N<sub>dep</sub> from those of meteorological variables, soil water retention capacity and stand age, using a mechanistic forest growth model in combination with eddy covariance CO<sub>2</sub> exchange fluxes from a Europe-wide network of forest flux towers. Total N<sub>r</sub> deposition rates were estimated from local measurements as far as possible. The forest data were compared with data from natural or semi-natural, non-woody vegetation sites. The carbon sequestration response of forests to nitrogen deposition (dC / dN) was estimated after accounting for the effects of the co-correlates by means of a meta-modelling standardization procedure, which resulted in a reduction by a factor of about 2 of the uncorrected, apparent dC / dN value. This model-enhanced analysis of the C and N<sub>dep</sub> flux observations at the scale of the European network suggests a mean overall dC / dN response of forest lifetime C sequestration to Ndep of the order of 40–50 g (C) g<sup>−1</sup> (N), which is slightly larger but not significantly different from the range of estimates published in the most recent reviews. Importantly, patterns of gross primary and net ecosystem productivity versus N<sub>dep</sub> were non-linear, with no further responses at high N<sub>dep</sub> levels (N<sub>dep</sub> > 2.5–3 g (N) m<sup>−2</sup> yr<sup>−1</sup>) partly due to large ecosystem N losses by leaching and gaseous emissions. The reduced increase in productivity per unit N deposited at high N<sub>dep</sub> levels implies that the forecast increased N<sub>r</sub> emissions and increased N<sub>dep</sub> levels in large areas of Asia may not positively impact the continent's forest CO<sub>2</sub> sink. The large level of unexplained variability in observed carbon sequestration efficiency (CSE) across sites further adds to the uncertainty in the dC / dN response.</p>