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

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= dN${}_{dep}$) 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/dN${}_{dep}$ value. ... mehrThis model-enhanced analysis of the C and N${}_{dep}$ flux observations at the scale of the European network suggests a mean overall dNEP/dN${}_{dep}$ 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 gNm${}^{-2}$ 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 Ndep 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.