Effects of atmospheric CO2 enrichment, water status and applied nitrogen on water- and nitrogen-use efficiencies of wheat

Atmospheric CO₂ levels are expected to exceed 700 mol mol^–1 by the end of the 21st century. The influence of increased CO₂ concentration on crop plants is of major concern. This study investigated water- and nitrogen-use efficiency (WUE and NUE, respectively, were defined by the amount of biomass accumulated per unit water or N uptake) of spring wheat (Triticum aestivumL.) grown under two atmospheric CO₂ concentrations (350 and 700 mol mol^–1), two soil moisture treatments (well-watered and drought) and five nitrogen amendment treatments. Results showed that enriched CO₂ concentration increased canopy WUE, and more N supply led to higher WUE under the increased CO₂. Canopy WUE was significantly lower in well-watered treatments than in drought treatment, but increased with the increased N supply. Elevated CO₂ reduced the apparent recovery fraction of applied N by the plant root system (N_r, defined as the ratio of the increased N uptake to N applied), but increased the NUE and agronomic N efficiency (NAE, defined as the ratio of the increased biomass to N applied). Water limitation and high N application reduced the N_r, NUE and NAE, indicating a poor N efficiency. In addition, there was a close relationship between the root mass ratio and NUE. Canopy WUE was negatively related to the root mass ratio and NUE. Our results indicated that CO₂ enrichment enhanced WUE more at high N application, but increased NUE more when N application was less.