Abstract. The effects of atmospheric nitrogen deposition
(Ndep) on carbon (C) sequestration in forests have often been assessed
by relating differences in productivity to spatial variations of Ndep
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 (Nr) deposition. We propose a methodology for
untangling the effects of Ndep from those of meteorological variables,
soil water retention capacity and stand age, using a mechanistic forest
growth model in combination with eddy covariance CO2 exchange fluxes
from a Europe-wide network of 22 forest flux towers. Total Nr
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 response of forest net ecosystem productivity to nitrogen deposition
(dNEP ∕ dNdep) was estimated after accounting for the effects on gross
primary productivity (GPP) 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 dGPP ∕ dNdep value. This
model-enhanced analysis of the C and Ndep flux observations at the
scale of the European network suggests a mean overall dNEP ∕ dNdep
response of forest lifetime C sequestration to Ndep of the order of
40–50 g C per g 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
Ndep were non-linear, with no further growth responses at high
Ndep levels (Ndep > 2.5–3 g N m−2 yr−1)
but accompanied by increasingly large ecosystem N losses by leaching and
gaseous emissions. The reduced increase in productivity per unit N deposited
at high Ndep levels implies that the forecast increased Nr
emissions and increased Ndep levels in large areas of Asia may not
positively impact the continent's forest CO2 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.
Carbon–nitrogen interactions in European forests and semi-natural vegetation – Part 2: Untangling climatic, edaphic, management and nitrogen deposition effects on carbon sequestration potentials