Nitrate for Me, Ammonium for You: The interdependence of plant nitrogen uptake and plant responses to carbon dioxide is well established, but the influence of inorganic nitrogen form—i.e., whether nitrate or ammonium—has been largely ignored. Bloom et al. (p. 899 ) present evidence from five independent methods in both a monocot and dicot species that carbon dioxide inhibition of nitrate assimilation is a major determinant of plant responses to rising atmospheric concentrations of carbon dioxide. This finding explains several phenomena, including carbon dioxide acclimation and decline in food quality. The large variation in these phenomena among species, locations, or years derives from the large variation in the relative dependence of plants on nitrate and ammonium as nitrogen sources among species, locations, or years. The relative importance of ammonium and nitrate for plant N nutrition in future cropping systems will be critical for quantity and quality of food.

The CO₂ concentration in Earth's atmosphere may double during this century. Plant responses to such an increase depend strongly on their nitrogen status, but the reasons have been uncertain. Here, we assessed shoot nitrate assimilation into amino acids via the shift in shoot CO₂ and O₂ fluxes when plants received nitrate instead of ammonium as a nitrogen source (ΔAQ). Shoot nitrate assimilation became negligible with increasing CO₂ in a taxonomically diverse group of eight C₃ plant species, was relatively insensitive to CO₂ in three C₄ species, and showed an intermediate sensitivity in two C₃‐C₄ intermediate species. We then examined the influence of CO₂ level and ammonium vs. nitrate nutrition on growth, assessed in terms of changes in fresh mass, of several C₃ species and a Crassulacean acid metabolism (CAM) species. Elevated CO₂ (720 μmol CO₂/mol of all gases present) stimulated growth or had no effect in the five C₃ species tested when they received ammonium as a nitrogen source but inhibited growth or had no effect if they received nitrate. Under nitrate, two C₃ species grew faster at sub‐ambient (∼310 μmol/mol) than elevated CO₂. A CAM species grew faster at ambient than elevated or sub‐ambient CO₂ under either ammonium or nitrate nutrition. This study establishes that CO₂ enrichment inhibits shoot nitrate assimilation in a wide variety of C₃ plants and that this phenomenon can have a profound effect on their growth. This indicates that shoot nitrate assimilation provides an important contribution to the nitrate assimilation of an entire C₃ plant. Thus, rising CO₂ and its effects on shoot nitrate assimilation may influence the distribution of C₃ plant species.

A major contributor to the global carbon cycle is plant respiration. Elevated atmospheric CO₂ concentrations may either accelerate or decelerate plant respiration for reasons that have been uncertain. We recently established that elevated CO₂ during the daytime decreases plant mitochondrial respiration in the light and protein concentration because CO₂ slows the daytime conversion of nitrate (NO₃⁻) into protein. This derives in part from the inhibitory effect of CO₂ on photorespiration and the dependence of shoot NO₃⁻ assimilation on photorespiration. Elevated CO₂ also inhibits the translocation of nitrite into the chloroplast, a response that influences shoot NO₃⁻ assimilation during both day and night. Here, we exposed Arabidopsis (Arabidopsis thaliana) and wheat (Triticum aestivum) plants to daytime or nighttime elevated CO₂ and supplied them with NO₃⁻ or ammonium as a sole nitrogen (N) source. Six independent measures (plant biomass, shoot NO₃⁻, shoot organic N, ¹⁵N isotope fractionation, ¹⁵NO₃⁻ assimilation, and the ratio of shoot CO₂ evolution to O₂ consumption) indicated that elevated CO₂ at night slowed NO₃⁻ assimilation and thus decreased dark respiration in the plants reliant on NO₃⁻. These results provide a straightforward explanation for the diverse responses of plants to elevated CO₂ at night and suggest that soil N source will have an increasing influence on the capacity of plants to mitigate human greenhouse gas emissions.