Supplying adequate nitrogen (N) to meet crop demand is critical for enhancing agricultural sustainability. Not only fertilizer N, but also N from other available sources should be considered in N supply capacity. We conducted a 10-year farming experiment using a split-plot design with two different main fertilizer management approaches and three N application strategies as add-on sub-treatments. Based on the experiment, we estimated the total N supply (TNsupply) for the summer maize cropping system, through considering environmental, soil, crop residue, and fertilizer N sources. An appropriate TNsupply was established by correlating TNsupply with the relative yield (RY), N input and output, and N use efficiency (NUE). The results revealed a wide variation in TNsupply (from 88 to 755 kg ha−1). The RY, N input, and N output fitted well to TNsupply using linear-plateau, linear, and linear-plateau models, respectively. The lower limits of TNsupply for achieving the maximum RY and N output were 361 and 358 kg ha−1, respectively. The relationship between N input and N output was described as linear-plateau. We determined the slope of the linear curve (55.4%) as the lower limit of NUE, beyond which the upper limit of TNsupply was determined to be less than 497 kg ha−1. Thus, appropriate TNsupply values ranged from 325 to 497 kg ha−1 for summer maize production, which could ensure enough N supply for higher yields and avoid excessive N input for higher NUE and lower environmental N loss. Our findings highlight that TNsupply can be an alternative indicator for evaluating N management.

Because of the strong competition for a limited resource of water and demand for food production, understanding yield and water productivity (WP) potentials and exploitable gaps in the current production of intensively rainfed maize (Zea mays L.) is essential on the regional scale in China. In this study, we conducted 411 site–year on-farm trials to assess the actual yield and WP of rainfed summer maize and its yield and WP potentials in Hebei Province, China. Each on-farm trial contained detailed information of three different treatments: no fertilizer application (CK), current farmers’ practices (FP, depending on local farmer field fertilization management), and optimum fertilizer application (OPT, depending on soil testing and balanced fertilization). Results revealed that the yield and WP of rainfed summer maize in Hebei Province were 7635 kg ha−1 and 20.7 kg ha−1 mm−1, respectively, and the yield and WP potentials were 12,148 kg ha−1 and 32.0 kg ha−1 mm−1, respectively. Thus, the farmers attained 62.8% of the yield potential and 64.7% of the WP potential. A wide variation was observed in terms of the yield and WP across various types of farming. Compared with high-yield and high-WP (HYHW) farming, in low-yield and low-WP (LYLW) farming, the yield decreased by 24.9% and WP decreased by 44.4%. Nitrogen fertilizer application rate and rain were the most significant factors for yield and WP gaps among farmers, respectively. Other factors, such as solar radiation (tSola), soil available phosphorus content (AP), potassium fertilizer application rate, and grass-referenced evapotranspiration from planting to maturity (ET0), contributed the most to the variations in the yield and WP. Scenario analysis indicated that the optimization of fertilization levels from current to optimal for each farming could increase the yield and WP by 9.7% and 14.8%, respectively; closing gaps between the farming groups and achievement of the standard of HYHW farming by all farmers could increase the yield and WP by 14.8% and 35.5%, respectively; and achieving the yield and WP potentials could increase the yield and WP by 59.1% and 54.8%, respectively. These findings provided farming-based evidence that optimal nutrient management, advanced and climate-adapted agronomy practices, and higher soil fertility are essential for future maize production.