Determining effects of water and nitrogen inputs on wheat yield and water productivity and nitrogen use efficiency in China: A quantitative synthesis

A major challenge in wheat (Triticum aestivum L.) production is the optimization of yields and resource use efficiency. Using a data assimilation approach based on 126 studies performed in China between 1980 and 2018, this study quantified how water and nitrogen (N) inputs affect wheat yields (1332 observations), water productivity (WPc, 648), and fertiliser N use efficiency (NUEf, 299). The interactive effects of soil nutrients (e.g., initial concentrations of N, phosphorus, and potassium), climate conditions (temperature and precipitation), and management practices (water and N input, and irrigation method) were also examined. The mean wheat yield, WPc, and NUEf were 6.5 ± 1.9 t ha−1, 1.8 ± 0.5 kg m-3, and 30.5 ± 12.6 kg kg−1, respectively. The input of water increased wheat yield, WPc, and NUEf only when the input amounts were less than 295, 104, and 378 mm, respectively; input of N increased wheat yields and WPc until N inputs were greater than 226 and 342 kg ha-1, respectively. Additionally, variable partitioning analysis suggested that management practices, climate parameters, and soil properties alone explained 24 %, 26 %, and 26 % of the variation of the response ratio (effect size between the treatment and control) for yield (RRY), WPc (RRWPc), and NUEf (RRNUEf), respectively. Specifically, the initial soil N and potassium concentrations critically affected RRY, RRWPc, and RRNUEf. RRY and RRNUEf were negatively correlated with mean annual temperature. Therefore, this study will be helpful for large-scale modelling or design of water and/or N management practices to improve wheat yield, WPc, and NUEf.