Overestimated natural biological nitrogen fixation translates to an exaggerated CO <sub>2</sub> fertilization effect in Earth system models

CO<sub>2</sub> fertilization of the terrestrial biosphere is limited by nitrogen. Biological nitrogen fixation (BNF) is the dominant natural nitrogen source to the terrestrial biosphere and can alleviate nitrogen limitation but is poorly constrained in Earth system models (ESMs). Here, we compare terrestrial BNF from an ensemble of ESMs of the 6th Coupled Model Intercomparison Project to a new global synthesis of observations across natural and agricultural biomes. We find that compared to observations, ESMs underestimate agricultural BNF but overestimate natural BNF in the present day by over 50%. Natural BNF is overestimated in the most productive ecosystems that contribute most to the terrestrial carbon sink (forests and grasslands). ESMs with different BNF representations yield a range of BNF responses to CO<sub>2</sub> enrichment. Some ESMs with phenomenological representations of BNF predict a natural BNF increase in response to a doubling of CO<sub>2</sub> that aligns with a meta-analysis of CO<sub>2</sub> enrichment experiments (31% increase) but fail to account for the substantial carbon cost of BNF. In contrast, ESMs with mechanistic representations of BNF account for its carbon cost as well as its regulation by nitrogen limitation but overestimate the BNF response to a doubling of CO<sub>2</sub> (135% increase). Overall, all current BNF representations in ESMs fall short of fully capturing its response to rising atmospheric CO<sub>2</sub>. Finally, we find a positive correlation between modeled present-day natural BNF and the CO<sub>2</sub> fertilization effect across ESMs, suggesting that overestimated natural BNF translates to an exaggerated CO<sub>2</sub> fertilization effect of approximately 11% in ESMs.

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Publication Details

Journal
Proceedings of the National Academy of Sciences
Published
2025-11-24
DOI
https://doi.org/10.1073/pnas.2514628122
Citations
1
Primary Topic
Plant Water Relations and Carbon Dynamics
Type
article
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1.11
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article

Overestimated natural biological nitrogen fixation translates to an exaggerated CO 2 fertilization effect in Earth system models

Johannes Piipponen, Duncan N. L. Menge, Cory C. Cleveland, Verity Salmon et al.
1 citations
Proceedings of the National Academy of Sciences
Plant Water Relations and Carbon Dynamics
1.11
article

Overestimated natural biological nitrogen fixation translates to an exaggerated CO 2 fertilization effect in Earth system models

Johannes Piipponen, Duncan N. L. Menge, Cory C. Cleveland, Verity Salmon, C. A. Williams, Steven S. Perakis, Maga Gei, Michael J. Gundale, Mark B. Peoples, Bettina Weber, Sasha C. Reed, Sarah Jovan, Katherine A. Dynarski, Benton N. Taylor, Nina Wurzburger, Anika P. Staccone, Sarah A. Batterman, Carla Roberta Gonçalves Reis, Sian Kou‐Giesbrecht, Fiona M. Soper, Emilio Rodríguez‐Caballero, Timothy E. Crews, D. F. Herridge
article en
1 citations

Abstract

CO<sub>2</sub> fertilization of the terrestrial biosphere is limited by nitrogen. Biological nitrogen fixation (BNF) is the dominant natural nitrogen source to the terrestrial biosphere and can alleviate nitrogen limitation but is poorly constrained in Earth system models (ESMs). Here, we compare terrestrial BNF from an ensemble of ESMs of the 6th Coupled Model Intercomparison Project to a new global synthesis of observations across natural and agricultural biomes. We find that compared to observations, ESMs underestimate agricultural BNF but overestimate natural BNF in the present day by over 50%. Natural BNF is overestimated in the most productive ecosystems that contribute most to the terrestrial carbon sink (forests and grasslands). ESMs with different BNF representations yield a range of BNF responses to CO<sub>2</sub> enrichment. Some ESMs with phenomenological representations of BNF predict a natural BNF increase in response to a doubling of CO<sub>2</sub> that aligns with a meta-analysis of CO<sub>2</sub> enrichment experiments (31% increase) but fail to account for the substantial carbon cost of BNF. In contrast, ESMs with mechanistic representations of BNF account for its carbon cost as well as its regulation by nitrogen limitation but overestimate the BNF response to a doubling of CO<sub>2</sub> (135% increase). Overall, all current BNF representations in ESMs fall short of fully capturing its response to rising atmospheric CO<sub>2</sub>. Finally, we find a positive correlation between modeled present-day natural BNF and the CO<sub>2</sub> fertilization effect across ESMs, suggesting that overestimated natural BNF translates to an exaggerated CO<sub>2</sub> fertilization effect of approximately 11% in ESMs.

Proceedings of the National Academy of SciencesVol. 122(48)
Oak Ridge Associated Universities (US), Smithsonian Tropical Research Institute (PA), United States Geological Survey (US), Oak Ridge National Laboratory (US), Clark University (US), University of Leeds (GB), Harvard University (US), University of Georgia (US), Simon Fraser University (CA), University of New England (US), Minnesota Medical Association (US), Swedish University of Agricultural Sciences (SE), Cary Institute of Ecosystem Studies (US), Graz University of Technology (AT), The Land Institute (US), Health Sciences and Nutrition (AU), Evolutionary Genomics (United States) (US), Max Planck Institute for Chemistry (DE), Mill Valley Public Library (US), Pacific Northwest Research Station (US), Institute of Plant Biology (HU), Agriculture and Food (AU), Hospital for Tropical Diseases (VN), Animal, Food and Health Sciences (AU), Oak Ridge Institute for Science and Education, Southwest Biological Science Center, McGill University (CA), University of Almería (ES), University of Montana (US), Columbia University (US), University of New England (AU), Aalto University (FI)
Openalex Percentile: Top 17%
Plant Water Relations and Carbon Dynamics
1.11
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