Variability of projected terrestrial biosphere responses to elevated levels of atmospheric CO<sub>2</sub> due to uncertainty in biological nitrogen fixation
Including a terrestrial nitrogen (N) cycle in Earth system models has led to substantial attenuation of predicted biosphere–climate feedbacks. However, the magnitude of this attenuation remains uncertain. A particularly important but highly uncertain process is biological nitrogen fixation (BNF), wh...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-03-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/13/1491/2016/bg-13-1491-2016.pdf |
| Summary: | Including a terrestrial nitrogen (N) cycle in Earth system models has led to
substantial attenuation of predicted biosphere–climate feedbacks. However,
the magnitude of this attenuation remains uncertain. A particularly important
but highly uncertain process is biological nitrogen fixation (BNF), which is
the largest natural input of N to land ecosystems globally. In order to
quantify this uncertainty and estimate likely effects on terrestrial
biosphere dynamics, we applied six alternative formulations of BNF spanning
the range of process formulations in current state-of-the-art biosphere
models within a common framework, the O-CN model: a global map of static BNF
rates, two empirical relationships between BNF and other ecosystem variables
(net primary productivity and evapotranspiration), two process-oriented
formulations based on plant N status, and an optimality-based approach. We
examined the resulting differences in model predictions under ambient and
elevated atmospheric [CO<sub>2</sub>] and found that the predicted global BNF rates
and their spatial distribution for contemporary conditions were broadly
comparable, ranging from 108 to 148 Tg N yr<sup>−1</sup> (median:
128 Tg N yr<sup>−1</sup>), despite distinct regional patterns associated with
the assumptions of each approach. Notwithstanding, model responses in BNF
rates to elevated levels of atmospheric [CO<sub>2</sub>] (+200 ppm)
ranged between −4 Tg N yr<sup>−1</sup> (−3 %) and 56 Tg N yr<sup>−1</sup>
(+42 %) (median: 7 Tg N yr<sup>−1</sup> (+8 %)). As a consequence, future
projections of global ecosystem carbon (C) storage (+281 to +353 Pg C,
or +13 to +16 %) as well as N<sub>2</sub>O emission (−1.6 to +0.5 Tg N yr<sup>−1</sup>, or −19 to +7 %) differed significantly across the different
model formulations. Our results emphasize the importance of better
understanding the nature and magnitude of BNF responses to change-induced
perturbations, particularly through new empirical perturbation experiments
and improved model representation. |
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| ISSN: | 1726-4170 1726-4189 |