A global synthesis of the effect of water and nitrogen input on maize (Zea mays) yield, water productivity and nitrogen use efficiency

Global maize (Zea mays) yields are limited by water and nutrient availability. Although water and nitrogen (N) have long been recognized for their beneficial effects (alone or interactively) on maize production, synthetic analysis on the effects of water and N inputs on maize yield, water productivity (WP, yield per unit water usage) and N use efficiency (NUE, yield per unit N applied) at a global scale is lacking. The objectives of this study were to quantify the effects of water and/or N inputs on both yield, WP and NUE, and water and N saving potential. Based on yield (n = 1406), WP (n = 1406), and NUE (n = 1131) observations from 83 studies conducted in 15 countries, the meta-analysis found that the optimal maize yield was achieved when maize planting density (number of plants per unit area) was 8.7 plants m−2. Generally speaking, maize yield, WP, and NUE were inversely correlated to mean annual temperature. The positive effect of inputting either water or N on all variables faded away when the optimal input levels were exceeded. The achieved yield, WP, and NUE were significantly lower than the attainable values by 27.9%, 27.9%, and 20.5%, respectively. The overall global mean water and N saving potential was 10 and 18%, respectively. Meanwhile, optimizing water or N input separately did not consistently enhance yield, WP, or NUE, while above-optimal water input increased yield and WP more than below-optimal water input by 3.4% and 3.3%, respectively. These results showed that current water and N management practices have created large gaps between the actual and attainable productivity and efficiency indices (e.g. yield, WUE, and NUE), and that simultaneous optimization of water and fertilizer N inputs is crucial for achieving maximum maize yield