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However, recent datasets indicate that some ecosystems may be experiencing widespread oligotrophication\u2014declining N availability\u2014which is suggested to be a response to elevated atmospheric carbon dioxide (CO\n                    <jats:sub>2<\/jats:sub>\n                    )\n                    <jats:sup>2<\/jats:sup>\n                    . Plant N isotope (\u03b4\n                    <jats:sup>15<\/jats:sup>\n                    N) chronologies have served as primary evidence for oligotrophication, but there is wide disagreement whether rising CO\n                    <jats:sub>2<\/jats:sub>\n                    or temporal changes in N deposition explain these patterns\n                    <jats:sup>3\u20136<\/jats:sup>\n                    . Here we construct \u03b4\n                    <jats:sup>15<\/jats:sup>\n                    N tree-ring chronologies using archived samples from Sweden\u2019s 23.5-million-hectare forest area from 1961 to 2018. The study area spans a 1,500-km latitudinal distance where N deposition varies fourfold, but where rising CO\n                    <jats:sub>2<\/jats:sub>\n                    is spatially uniform. Our data show declining \u03b4\n                    <jats:sup>15<\/jats:sup>\n                    N chronologies throughout Sweden, including forests in the far north where atmospheric N deposition rates are very low. Linear mixed-effects models showed that rising CO\n                    <jats:sub>2<\/jats:sub>\n                    is the strongest predictor of \u03b4\n                    <jats:sup>15<\/jats:sup>\n                    N values, whereas N deposition variables, temperature and forest basal area had lower explanatory power. Our findings suggest that elevated atmospheric CO\n                    <jats:sub>2<\/jats:sub>\n                    is causing oligotrophication in boreal forests, which has implications for predicting their future role as sinks in the global carbon cycle\n                    <jats:sup>7\u20139<\/jats:sup>\n                    .\n                  <\/jats:p>","DOI":"10.1038\/s41586-025-10039-5","type":"journal-article","created":{"date-parts":[[2026,2,18]],"date-time":"2026-02-18T16:02:14Z","timestamp":1771430534000},"page":"629-635","update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":15,"title":["Rising atmospheric CO2 reduces nitrogen availability in boreal forests"],"prefix":"10.1038","volume":"650","author":[{"ORCID":"https:\/\/orcid.org\/0009-0003-3151-0740","authenticated-orcid":false,"given":"Kelley R.","family":"Bassett","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9498-5183","authenticated-orcid":false,"given":"Stefan F.","family":"Hupperts","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5222-7878","authenticated-orcid":false,"given":"Sandra","family":"J\u00e4mtg\u00e5rd","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lars","family":"\u00d6stlund","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8295-665X","authenticated-orcid":false,"given":"Jonas","family":"Fridman","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0703-9314","authenticated-orcid":false,"given":"Steven S.","family":"Perakis","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2447-609X","authenticated-orcid":false,"given":"Michael J.","family":"Gundale","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2026,2,18]]},"reference":[{"key":"10039_CR1","doi-asserted-by":"publisher","first-page":"889","DOI":"10.1126\/science.1136674","volume":"320","author":"JN Galloway","year":"2008","unstructured":"Galloway, J. 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