{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,26]],"date-time":"2026-10-26T23:49:21Z","timestamp":1793058561531,"version":"4.5.0"},"reference-count":64,"publisher":"National Academy of Sciences","issue":"48","license":[{"start":{"date-parts":[[2025,11,24]],"date-time":"2025-11-24T00:00:00Z","timestamp":1763942400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by-nc-nd\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100000038","name":"Natural Sciences and Engineering Research Council of Canada","doi-asserted-by":"publisher","award":["RGPIN-2024-04188"],"award-info":[{"award-number":["RGPIN-2024-04188"]}],"id":[{"id":"10.13039\/501100000038","id-type":"DOI","asserted-by":"publisher"}]},{"name":"John Wesley Powell Center for Analysis and Synthesis","award":["G19AC00036"],"award-info":[{"award-number":["G19AC00036"]}]}],"content-domain":{"domain":["www.pnas.org"],"crossmark-restriction":true},"short-container-title":["Proc. Natl. Acad. Sci. U.S.A."],"published-print":{"date-parts":[[2025,12,2]]},"abstract":"<jats:p>\n                    CO\n                    <jats:sub>2<\/jats:sub>\n                    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\n                    <jats:sub>2<\/jats:sub>\n                    enrichment. Some ESMs with phenomenological representations of BNF predict a natural BNF increase in response to a doubling of CO\n                    <jats:sub>2<\/jats:sub>\n                    that aligns with a meta-analysis of CO\n                    <jats:sub>2<\/jats:sub>\n                    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\n                    <jats:sub>2<\/jats:sub>\n                    (135% increase). Overall, all current BNF representations in ESMs fall short of fully capturing its response to rising atmospheric CO\n                    <jats:sub>2<\/jats:sub>\n                    . Finally, we find a positive correlation between modeled present-day natural BNF and the CO\n                    <jats:sub>2<\/jats:sub>\n                    fertilization effect across ESMs, suggesting that overestimated natural BNF translates to an exaggerated CO\n                    <jats:sub>2<\/jats:sub>\n                    fertilization effect of approximately 11% in ESMs.\n                  <\/jats:p>","DOI":"10.1073\/pnas.2514628122","type":"journal-article","created":{"date-parts":[[2025,11,24]],"date-time":"2025-11-24T20:13:16Z","timestamp":1764015196000},"update-policy":"https:\/\/doi.org\/10.1073\/pnas.cm10313","source":"Crossref","is-referenced-by-count":8,"title":["Overestimated natural biological nitrogen fixation translates to an exaggerated CO\n                    <sub>2<\/sub>\n                    fertilization effect in Earth system models"],"prefix":"10.1073","volume":"122","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-4086-0561","authenticated-orcid":false,"given":"Sian","family":"Kou-Giesbrecht","sequence":"first","affiliation":[{"name":"School of Resource and Environmental Management, Simon Fraser University","place":["Burnaby, Canada"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9657-8071","authenticated-orcid":false,"given":"Carla R.","family":"Reis Ely","sequence":"additional","affiliation":[{"name":"Oak Ridge Institute for Science and Education","place":["Oak Ridge"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0703-9314","authenticated-orcid":false,"given":"Steven S.","family":"Perakis","sequence":"additional","affiliation":[{"name":"United States Geological Survey, Forest and Rangeland Ecosystem Science Center","place":["Corvallis"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Cory C.","family":"Cleveland","sequence":"additional","affiliation":[{"name":"Department of Ecosystem and Conservation Sciences, W.A. Franke College of Forestry and Conservation, University of Montana","place":["Missoula"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4736-9844","authenticated-orcid":false,"given":"Duncan N. L.","family":"Menge","sequence":"additional","affiliation":[{"name":"Department of Ecology, Evolution, and Environmental Biology, Columbia University","place":["New York"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8597-8619","authenticated-orcid":false,"given":"Sasha C.","family":"Reed","sequence":"additional","affiliation":[{"name":"United States Geological Survey, Southwest Biological Science Center","place":["Moab"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Benton N.","family":"Taylor","sequence":"additional","affiliation":[{"name":"Department of Organismic and Evolutionary Biology, Harvard University","place":["Roslindale"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sarah A.","family":"Batterman","sequence":"additional","affiliation":[{"name":"Cary Institute of Ecosystem Studies","place":["Millbrook"]},{"name":"School of Geography, University of Leeds","place":["Leeds, United Kingdom"]},{"name":"Smithsonian Tropical Research Institute","place":["Ancon, Panama"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Timothy E.","family":"Crews","sequence":"additional","affiliation":[{"name":"The Land Institute","place":["Salina"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5101-9666","authenticated-orcid":false,"given":"Katherine A.","family":"Dynarski","sequence":"additional","affiliation":[{"name":"Department of Ecosystem and Conservation Sciences, W.A. Franke College of Forestry and Conservation, University of Montana","place":["Missoula"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Maga","family":"Gei","sequence":"additional","affiliation":[{"name":"Association for Tropical Biology and Conservation","place":["Minneapolis"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2447-609X","authenticated-orcid":false,"given":"Michael J.","family":"Gundale","sequence":"additional","affiliation":[{"name":"Department of Forest Ecology and Management, Swedish University of Agricultural Sciences","place":["Umea, Sweden"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0423-2517","authenticated-orcid":false,"given":"David F.","family":"Herridge","sequence":"additional","affiliation":[{"name":"School of Environmental and Rural Science, University of New England","place":["Armidale, Australia"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sarah E.","family":"Jovan","sequence":"additional","affiliation":[{"name":"United States Department of Agriculture Forest Service Pacific Northwest Research Station","place":["Portland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6060-3290","authenticated-orcid":false,"given":"Mark B.","family":"Peoples","sequence":"additional","affiliation":[{"name":"Commonwealth Scientific and Industrial Research Organisation Agriculture and Food","place":["Canberra, Australia"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2925-3397","authenticated-orcid":false,"given":"Johannes","family":"Piipponen","sequence":"additional","affiliation":[{"name":"Department of Built Environment, Aalto University","place":["Espoo, Finland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5934-3214","authenticated-orcid":false,"given":"Emilio","family":"Rodr\u00edguez-Caballero","sequence":"additional","affiliation":[{"name":"Centro de Investigaci\u00f3n de Colecciones Cient\u00edficas de la Universidad de Almer\u00eda y Departamento de Agronom\u00eda, Universidad de Almer\u00eda","place":["Almer\u00eda, Spain"]},{"name":"Multiphase Chemistry Department, Max Planck Institute for Chemistry","place":["Mainz, Germany"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2188-551X","authenticated-orcid":false,"given":"Verity G.","family":"Salmon","sequence":"additional","affiliation":[{"name":"Environmental Science Division, Oak Ridge National Laboratory","place":["Oak Ridge"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9910-9377","authenticated-orcid":false,"given":"Fiona 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