Abstract
terrestrial carbon (C) sequestration has increased over the last few
decades. The drivers of carbon sequestration, the geographical spread
and magnitude of this sink are however hotly debated. Photosynthesis
determines the total C uptake of terrestrial ecosystems and is a major
flux of the global C balance. We contribute to the discussion on
enhanced C sequestration by analyzing the influence of nitrogen (N)
deposition on photosynthetic capacity (Amax) of forest
canopies. Eddy covariance measurements of net exchange of carbon provide
estimates of gross primary production, from which Amax is
derived with a novel approach. Canopy Amax is combined with
modeled N deposition, environmental variables and stand characteristics
to study the relative effects on Amax for a unique global
data set of 80 forest FLUXNET sites. Canopy Amax relates
positively to N deposition for evergreen needleleaf forests below an
observed critical load of ~ 8 kg N ha-1 yr-1, with
a slope of 2.0 ± 0.4 (S.E.) µmol CO2
m-2 s-1 per 1 kg N ha-1
yr-1. Above this threshold canopy Amax levels off,
exhibiting a saturating response in line with the N saturation
hypothesis. Climate effects on canopy Amax cannot be
separated from the effect of N deposition due to considerable
covariation. For deciduous broadleaf forests and forests in the
temperate (-continental) climate zones, the analysis shows the N
deposition effect to be either small or absent. Leaf area index and
foliar N concentration are positively but weakly related to
Amax. We conclude that flux tower measurements of C fluxes
provide valuable data to study physiological processes at the canopy
scale. Future efforts need to be directed toward standardizing measures
N cycling and pools within C monitoring networks to gain a better
understanding of C and N interactions, and to disentangle the role of
climate and N deposition in forest ecosystems.
Original language | English |
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Pages (from-to) | 187-199 |
Number of pages | 13 |
Journal | Global Biogeochemical Cycles |
Volume | 27 |
Issue number | 1 |
DOIs | |
Publication status | Published - Mar 2013 |
Keywords
- canopy photosynthesis
- photosynthetic capacity
- nitrogen deposition
- carbon sequestration
- nitrogen saturation
- FLUXNET