Carbon–nitrogen interactions in European forests and semi-natural vegetation – Part 2: Untangling climatic, edaphic, management and nitrogen deposition effects on carbon sequestration potentials

[摘要] The effects of atmospheric nitrogen deposition ( N dep ) on carbon (C) sequestration in forests have often been assessed by relating differences in productivity to spatial variations of N dep 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 r ) deposition. We propose a methodology for untangling the effects of N dep from those of meteorological variables, soil water retention capacity and stand age, using a mechanistic forest growth model in combination with eddy covariance CO 2 exchange fluxes from a Europe-wide network of 22 forest flux towers. Total N r 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 ∕ d N dep ) 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 ∕ d N dep value. This model-enhanced analysis of the C and N dep flux observations at the scale of the European network suggests a mean overall dNEP ∕ d N dep response of forest lifetime C sequestration to N dep 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 N dep were non-linear, with no further growth responses at high N dep levels ( N dep   >  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 N dep levels implies that the forecast increased N r emissions and increased N dep levels in large areas of Asia may not positively impact the continent's forest CO 2 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.

[学科分类] 大气科学