Carbon dioxide enrichment decreases critical nitrate and nitrogen concentrations in wheat

Atmospheric carbon dioxide (CO₂) levels are increasing. In a glasshouse experiment with wheat grown at 5 levels of nitrate (NO₃) supply, CO₂ enrichment (1500 cm³/m³) substantially decreased critical concentrations of N03‐N and total‐N in stem bases and leaves. For example, critical NO₃‐N concentrations in stem bases at Feekes Stages 1.5, 5, and 10.3, were 4.5, 2.0, and 2.0 mg/g dry wt, respectively, for CO₂‐enriched plants, compared with 7.5, 6.2 and 6.4 mg/g dry wt, respectively, for control plants grown at the ambient level of CO₂. However, concentrations of NO₃‐N in the rooting medium required to produce maximum dry matter accumulation by CO₂‐enriched plants were similar to those of control plants at the three growth stages. Critical concentrations of NO₃‐N and total‐N declined with time in stem bases and leaves of plants grown at both ambient and elevated CO₂ levels, but the decline was greater for CO₂‐enriched plants. It was concluded that diagnostic criteria based on current critical N concentrations may become invalid as the atmospheric level of CO₂ increases.