Why are Nitrogen Concentrations in Plant Tissues Lower under Elevated CO₂? A Critical Examination of the Hypotheses

Plants grown under elevated atmospheric [CO₂] typically have decreased tissue concentrations of N compared with plants grown under current ambient [CO₂]. The physiological mechanisms responsible for this phenomenon have not been definitely established, although a considerable number of hypotheses have been advanced to account for it. In this review we discuss and critically evaluate these hypotheses. One contributing factor to the decreases in tissue N concentrations clearly is dilution of N by increased photosynthetic assimilation of C. In addition, studies on intact plants show strong evidence for a general decrease in the specific uptake rates (uptake per unit mass or length of root) of N by roots under elevated CO₂. This decreased root uptake appears likely to be the result both of decreased N demand by shoots and of decreased ability of the soil-root system to supply N. The best-supported mechanism for decreased N supply is a decrease in transpiration-driven mass flow of N in soils due to decreased stomatal conductance at elevated CO₂, although some evidence suggests that altered root system architecture may also play a role. There is also limited evidence suggesting that under elevated CO₂, plants may exhibit increased rates of N loss through volatilization and/or root exudation, further contributing to lowering tissue N concentrations.