Abstract
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.
Highlights
Atmospheric reactive nitrogen (Nr) deposition (Ndep) has often been suggested to be a major driver of the large forest carbon (C) sink observed in the Northern Hemisphere (Reay et al, 2008; Ciais et al, 2013), but this view has been challenged, both in temperate (Nadelhoffer et al, 1999; Lovett et al, 2013) and in boreal regions (Gundale et al, 2014)
Regardless of its age and establishment date, an initial phase of around 20–25 years occurs, during which gross primary productivity (GPP) increases sharply from zero to a potential value attained upon canopy closure (Fig. 1b), while net ecosystem productivity (NEP) switches from a net C source to a net C sink after about 10 years (Fig. 1d)
In parallel, modelled total N losses start to decrease after the 1980s, even for sites long past canopy closure (Fig. 1e–f), but this mostly applies to stands subject to the largest nitrogen deposition (Ndep) levels, i.e. where the historical high Ndep values of the 1980s, added to the internal N supply, were well in excess of growth requirements in the model
Summary
Atmospheric reactive nitrogen (Nr) deposition (Ndep) has often been suggested to be a major driver of the large forest carbon (C) sink observed in the Northern Hemisphere (Reay et al, 2008; Ciais et al, 2013), but this view has been challenged, both in temperate (Nadelhoffer et al, 1999; Lovett et al, 2013) and in boreal regions (Gundale et al, 2014). The measure of carbon sequestration is not the NPP, but the long-term net ecosystem carbon balance (NECB; Chapin et al, 2006) or the net biome productivity at a large spatial scale (NBP; Schulze et al, 2010), whereby heterotrophic respiration (Rhet) and all other C losses, including exported wood products and other disturbances over a forest lifetime, reduce the fraction of photosynthesized C (gross primary production, GPP) that is sequestered in the ecosystem. There is considerable debate as to the magnitude of the fertilization role that atmospheric Nr deposition may play on forest carbon balance, as illustrated by the controversy over the study by Magnani et al (2007) and subsequent comments by Högberg (2007), De Schrijver et al (2008), Sutton et al (2008) and others. Recent reviews have suggested mean dC/dN responses generally well below 100 g C per g N, ranging from 61–98 for aboveground biomass increment in US forests (Thomas et al, 2010) to 35–65 for above-ground biomass and soil organic matter (Erisman et al, 2011; Butterbach-Bahl and Gundersen, 2011), 16–33 for the whole ecosystem (Liu and Greaver, 2009), 5–75 (mid-range 20–40) for the whole ecosystem in European forests and heathlands (de Vries et al, 2009), and down to 13–14 for above-ground woody biomass in temperate and boreal forests (Schulte-Uebbing and de Vries, 2018) and 10–70 for the whole ecosystem for forests globally, increasing from tropical to temperate to boreal forests (de Vries et al, 2014a; Du and de Vries, 2018)
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