Nitrogen fertilisation influences low CO₂ effects on plant performance

Low atmospheric CO₂ conditions prevailed for most of the recent evolutionary history of plants. Such concentrations reduce plant growth compared with modern levels, but low-CO₂ effects on plant performance may also be affected by nitrogen availability, since low leaf nitrogen decreases photosynthesis, and CO₂ concentrations influence nitrogen assimilation. To investigate the influence of N availability on plant performance at low CO₂, we grew Elymus canadensis at ambient (∼400 μmol mol^-1) and subambient (∼180 μmol mol^-1) CO₂ levels, under four N-treatments: Nitrate only ammonium only a full and a half mix of nitrate and ammonium. Growth at low CO₂ decreased biomass in the full and nitrate treatments, but not in ammonium and half plants. Low CO₂ effects on photosynthetic and maximum electron transport rates were influenced by fertilisation, with photosynthesis being most strongly impacted by low CO₂ in full plants. Low CO₂ reduced stomatal index in half plants, suggesting that the use of this indicator in paleo-inferences can be influenced by N availability. Under low CO₂ concentrations, nitrate plants discriminated more against ¹⁵N whereas half plants discriminated less against ¹⁵N compared with the full treatment, suggesting that N availability should be considered when using N isotopes as paleo-indicators.