Effects of CO2 on plants are often confounded with effects of nutrition. Coleman et al. (1993) showed that nitrogen concentrations in plants grown under different CO2 levels did not differ if plants of equal sizes were compared but varied at equal times and equal sizes for different levels of nitrogen additions. I will suggest why this must occur under the conditions of the experiment. I will also suggest why nitrogen productivity should be used to interpret experimental results rather than the dubious nitrogen use efficiency.
Keywords: Nitrogen concentration; Nitrogen productivity; Nitrogen use efficiency.