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By contrast, nitrogen (N) concentrations in leaves generally tend to decline under elevated CO<jats:sub>2<\/jats:sub> (eCO<jats:sub>2<\/jats:sub>), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO<jats:sub>2<\/jats:sub>: (a) A \u201cdilution effect\u201d caused by increased concentration of leaf carbohydrates; and (b) inhibited nitrate assimilation caused by reduced supply of reductant from photorespiration under eCO<jats:sub>2<\/jats:sub>. This second hypothesis is fully tested in the field for the first time here, using tall trees of a mature <jats:italic>Eucalyptus <\/jats:italic>forest exposed to Free\u2010Air CO<jats:sub>2<\/jats:sub> Enrichment (EucFACE) for five years. Fully expanded young and mature leaves were both measured for net photosynthesis, photorespiration, total leaf N, nitrate (<jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/gcb14555-math-0001.png\" xlink:title=\"urn:x-wiley:13541013:media:gcb14555:gcb14555-math-0001\"\/>) concentrations, carbohydrates and <jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/gcb14555-math-0002.png\" xlink:title=\"urn:x-wiley:13541013:media:gcb14555:gcb14555-math-0002\"\/> reductase activity to test these hypotheses. Foliar N concentrations declined by 8% under eCO<jats:sub>2<\/jats:sub> in new leaves, while the <jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/gcb14555-math-0003.png\" xlink:title=\"urn:x-wiley:13541013:media:gcb14555:gcb14555-math-0003\"\/> fraction and total carbohydrate concentrations remained unchanged by CO<jats:sub>2<\/jats:sub> treatment for either new or mature leaves. Photorespiration decreased 31% under eCO<jats:sub>2<\/jats:sub> supplying less reductant, and in situ <jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/gcb14555-math-0004.png\" xlink:title=\"urn:x-wiley:13541013:media:gcb14555:gcb14555-math-0004\"\/> reductase activity was concurrently reduced (\u221234%) in eCO<jats:sub>2<\/jats:sub>, especially in new leaves during summer periods. Hence, <jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/gcb14555-math-0005.png\" xlink:title=\"urn:x-wiley:13541013:media:gcb14555:gcb14555-math-0005\"\/> assimilation was inhibited in leaves of <jats:italic>E.\u00a0tereticornis<\/jats:italic> and the evidence did not support a significant dilution effect as a contributor to the observed reductions in leaf N concentration. This finding suggests that the reduction of <jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/gcb14555-math-0006.png\" xlink:title=\"urn:x-wiley:13541013:media:gcb14555:gcb14555-math-0006\"\/> reductase activity due to lower photorespiration in eCO<jats:sub>2<\/jats:sub> can contribute to understanding how eCO<jats:sub>2<\/jats:sub>\u2010induced photosynthetic enhancement may be lower than previously expected. We suggest that large\u2010scale vegetation models simulating effects of eCO<jats:sub>2<\/jats:sub> on N biogeochemistry include both mechanisms, especially where <jats:inline-graphic xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"graphic\/gcb14555-math-0007.png\" xlink:title=\"urn:x-wiley:13541013:media:gcb14555:gcb14555-math-0007\"\/> is major N source to the dominant vegetation and where leaf flushing and emergence occur in temperatures that promote high photorespiration rates.<\/jats:p>","DOI":"10.1111\/gcb.14555","type":"journal-article","created":{"date-parts":[[2019,2,21]],"date-time":"2019-02-21T02:01:36Z","timestamp":1550714496000},"page":"1282-1295","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":70,"title":["Lower photorespiration in elevated CO<sub>2<\/sub> reduces leaf N concentrations in mature <i>Eucalyptus <\/i>trees in the field"],"prefix":"10.1111","volume":"25","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-0315-8981","authenticated-orcid":false,"given":"Agnieszka","family":"Wujeska\u2010Klause","sequence":"first","affiliation":[{"name":"Hawkesbury Institute for the Environment, Western Sydney University Penrith New South Wales Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9478-7593","authenticated-orcid":false,"given":"Kristine Y.","family":"Crous","sequence":"additional","affiliation":[{"name":"Hawkesbury Institute for the Environment, Western Sydney University Penrith New South Wales Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1341-0741","authenticated-orcid":false,"given":"Oula","family":"Ghannoum","sequence":"additional","affiliation":[{"name":"Hawkesbury Institute for the Environment, Western Sydney University Penrith New South Wales Australia"},{"name":"Translational Photosynthesis Centre of Excellence, Western Sydney University Penrith New South Wales Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9699-2272","authenticated-orcid":false,"given":"David S.","family":"Ellsworth","sequence":"additional","affiliation":[{"name":"Hawkesbury Institute for the Environment, Western Sydney University Penrith New South Wales Australia"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2019,2,20]]},"reference":[{"key":"e_1_2_7_2_1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1469-8137.2004.01224.x"},{"key":"e_1_2_7_3_1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1365-3040.2007.01641.x"},{"key":"e_1_2_7_4_1","doi-asserted-by":"publisher","DOI":"10.1111\/aab.12045"},{"key":"e_1_2_7_5_1","doi-asserted-by":"publisher","DOI":"10.1023\/A:1004206624311"},{"key":"e_1_2_7_6_1","doi-asserted-by":"publisher","DOI":"10.18637\/jss.v067.i01"},{"key":"e_1_2_7_7_1","doi-asserted-by":"publisher","DOI":"10.1093\/treephys\/21.2-3.137"},{"key":"e_1_2_7_8_1","doi-asserted-by":"publisher","DOI":"10.1007\/s11120-014-0056-y"},{"key":"e_1_2_7_9_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.pbi.2015.03.002"},{"key":"e_1_2_7_10_1","doi-asserted-by":"publisher","DOI":"10.1890\/11-0485.1"},{"key":"e_1_2_7_11_1","doi-asserted-by":"publisher","DOI":"10.1126\/science.1186440"},{"key":"e_1_2_7_12_1","doi-asserted-by":"publisher","DOI":"10.1038\/nclimate2183"},{"key":"e_1_2_7_13_1","doi-asserted-by":"publisher","DOI":"10.1080\/00103627509366547"},{"key":"e_1_2_7_14_1","doi-asserted-by":"publisher","DOI":"10.1093\/treephys\/24.9.961"},{"key":"e_1_2_7_15_1","doi-asserted-by":"publisher","DOI":"10.1007\/s11104-015-2426-4"},{"key":"e_1_2_7_16_1","doi-asserted-by":"publisher","DOI":"10.1111\/gcb.12314"},{"key":"e_1_2_7_17_1","doi-asserted-by":"publisher","DOI":"10.1111\/gcb.13819"},{"key":"e_1_2_7_18_1","doi-asserted-by":"publisher","DOI":"10.1007\/s40415-014-0083-0"},{"key":"e_1_2_7_19_1","doi-asserted-by":"publisher","DOI":"10.1146\/annurev.arplant.48.1.609"},{"key":"e_1_2_7_20_1","doi-asserted-by":"publisher","DOI":"10.1111\/gcb.13151"},{"key":"e_1_2_7_21_1","doi-asserted-by":"publisher","DOI":"10.1016\/S0031-9422(00)85818-5"},{"key":"e_1_2_7_22_1","doi-asserted-by":"publisher","DOI":"10.1111\/pce.12468"},{"key":"e_1_2_7_23_1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1365-2486.2004.00867.x"},{"key":"e_1_2_7_24_1","doi-asserted-by":"publisher","DOI":"10.1038\/nclimate3235"},{"key":"e_1_2_7_25_1","doi-asserted-by":"publisher","DOI":"10.1007\/BF00377192"},{"key":"e_1_2_7_26_1","volume-title":"The allocation of protein nitrogen in the photosynthetic apparatus: Costs, consequences, and control","author":"Evans J. 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