06 Feb 2023
 | 06 Feb 2023
Status: this preprint is open for discussion.

Evaluating Nitrogen Cycling in Terrestrial Biosphere Models: Implications for the Future Terrestrial Carbon Sink

Sian Kou-Giesbrecht, Vivek Arora, Christian Seiler, Almut Arneth, Stefanie Falk, Atul Jain, Fortunat Joos, Daniel Kennedy, Jürgen Knauer, Stephen Sitch, Michael O'Sullivan, Naiqing Pan, Qing Sun, Hanqin Tian, Nicolas Vuichard, and Sönke Zaehle

Abstract. Terrestrial carbon (C) sequestration is limited by nitrogen (N), a constraint that could intensify under CO2 fertilisation and future global change. The terrestrial C sink is estimated to currently sequester approximately a third of annual anthropogenic CO2 emissions based on an ensemble of terrestrial biosphere models, which have been evaluated in their ability to reproduce observations of the C, water, and energy cycles. However, their ability to reproduce observations of N cycling and thus the regulation of terrestrial C sequestration by N has been largely unexplored. Here, we evaluate an ensemble of terrestrial biosphere models with coupled C-N cycling and their performance at simulating N cycling, outlining a framework for evaluating N cycling that can be applied across terrestrial biosphere models. We find that models exhibit significant variability across N pools and fluxes, simulating different magnitudes and trends over the historical period, despite their ability to generally reproduce the historical terrestrial C sink. This suggests that the underlying N processes that regulate terrestrial C sequestration operate differently across models and may not be fully captured. Furthermore, models tended to overestimate tropical biological N fixation, vegetation C:N ratio, and soil C:N ratio but underestimate temperate biological N fixation relative to observations. However, there is significant uncertainty associated with measurements of N cycling processes given their scarcity (especially relative to those of C cycling processes) and their high spatiotemporal variability. Overall, our results suggest that terrestrial biosphere models that represent coupled C-N cycling (let alone those without a representation of N cycling) could be overestimating C storage per unit N, which could lead to biases in projections of the future terrestrial C sink under CO2 fertilisation and future global change. More extensive observations of N cycling processes are crucial to evaluate N cycling and its impact on C cycling as well as guide its development in terrestrial biosphere models.

Sian Kou-Giesbrecht et al.

Status: open (extended)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2023-167', Joshua Fisher, 24 Feb 2023 reply
  • RC1: 'Comment on egusphere-2023-167', Joshua Fisher, 27 Feb 2023 reply
    • AC1: 'Reply on RC1', Sian Kou-Giesbrecht, 16 Mar 2023 reply
      • RC2: 'Reply on AC1', Joshua Fisher, 16 Mar 2023 reply

Sian Kou-Giesbrecht et al.

Sian Kou-Giesbrecht et al.


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Short summary
Nitrogen (N) is an essential limiting nutrient to terrestrial carbon (C) sequestration. We evaluate N cycling in an ensemble of terrestrial biosphere models. We find that they simulate significant variability in N processes. Models tended to overestimate C storage per unit N in vegetation and soil, which could have consequences for projecting the future terrestrial C sink. However, N cycling measurements are highly uncertain and more are necessary to guide the development of N cycling in models.