Carbon dioxide enrichment decreases critical nitrate and nitrogen concentrations in wheat

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Abstract
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.