Li, F., Agricultural College, Guangxi University, Nanning, Guangxi, 530005, China
Kang, S., Key Lab. Agric. Soil/Water Eng. A., Ministry of Education, Yangling, Shannxi, 712100, China
Zhang, J., Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong
Cohen, S., Inst. of Soil, Water/Environ. Sci., Volcani Center, Bet Dagan, 50250, Israel

Atmospheric CO2 levels are expected to exceed 700 μmol 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 CO 2 concentrations (350 and 700 μmol 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 CO 2. 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 (Nr, 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.