A synthetic analysis of the effect of water and nitrogen inputs on wheat yield and water- and nitrogen-use efficiencies in China

Water and nitrogen (N) inputs are essential for increasing crop yields, but overuse inevitably causes substantial nitrate leaching and the subsequent contamination of groundwater. Water and N use in wheat production in China is relatively inefficient, so a synthesis of data for the effects of water and N inputs on wheat yield, water-use efficiency (WUE) and nitrogen-use efficiency (NUE) is necessary. We conducted a meta-analysis based on 4162 yield, 4158 WUE and 3694 NUE observations from 332 peer-reviewed publications to quantify the effects of water and N inputs on wheat yield, WUE and NUE and to clarify the potentials for saving water and N for wheat production in China. The positive effects of water or N input on all variables decreased when the optimum levels of input were exceeded. Actual yield, WUE and NUE were 29.8, 47.2 and 41.8% significantly lower, respectively, than those potentially attainable. Results differed greatly among regions due to differences in management and climate. The differences between actual and attainable yield, WUE and NUE were highest in the northwestern region and lowest in the north-central region. The difference in WUE was largest in the lower and middle reaches of the Yangtze River. The overall water- and N-saving potentials for wheat production across China were 22.4 and 26.0%, respectively. Yield was highest at a plant density of 180 plants m(-2). The effects of the water and N inputs were more pronounced in areas with silty soil and high levels of soil organic carbon. Yield, WUE and NUE were negatively correlated with mean seasonal temperature, so suitable measures should be taken to control temperatures <15 degrees C. This analysis provides scientific guidance for improving the use efficiencies of resources, reducing environmental pollution, advancing China's food security and improving yield stability.