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 mumol 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 aestivum L.) grown under two atmospheric CO2 concentrations (350 and 700 mumol 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 Nr, 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.