## 1 Introduction  
Maize (_Zaa mays_ L.) is a nicotinamide adenine dinucleotide phosphate malic enzyme (NADP-ME) type C\({}_{4}\) plant [1]. As one of the most important food crops, maize has a larger production area than rice in China. With the development of intensification agricultural systems, maize production is becoming increasingly important. The North China Plain (NCP) is a major grain-producing region in China. It contributes 31% of the total maize yield, where it accounts for 32% of the total maize planting area in China [2]. However, soil nutrient supply generally cannot meet the demand of maize during its growth. The element nitrogen (N) is essential for the syntheses of genetic materials, metabolic enzymes, hormones, and proteins in plants. It has a strong influence on the growth, development, yield, and quality of crops [3]. In order to achieve high maize yield and quality, chemical fertilizers, especially N fertilizers, are commonly applied to soils. During the summer maize growing season, many observed N fertilizers have been observed, which can be as high 300-360 kg N ha\({}^{-1}\) yr\({}^{-1}\)[4; 5; 6], which surpasses the crop requirements of approximately 200 kg N ha\({}^{-1}\) yr\({}^{-1}\)[7; 8; 9]. The disproportionate use of N fertilizer results in substantial waste of resources, increased costs and decreased economic benefits, all of which seriously hinder the region's agricultural development. As a result, the new fertilization methods and nitrogen use efficiency should be studied with great efforts during the summer maize growth period. Urea (46% N) has the highest N content of all inorganic N fertilizers and contributes to approximately 40% of total N fertilizer capacity in China. At 20-25 \({}^{\circ}\)C, urea applied to the soil would be hydrolyzed by microorganisms and urease secreted by plant root into ammonium bicarbonate within 5-7 days, and the produced NH\({}_{4}\)\({}^{+}\) ions would be absorbed by plants for growth [10; 11]. Those NH\({}_{4}\)\({}^{+}\) ions that are not absorbed by plants would be adsorbed by the negatively charged soil colloids or oxidized to NO\({}_{2}\)\({}^{-}\) by ammonia-oxidizing bacteria and then to NO\({}_{3}\)\({}^{-}\) by nitrifying bacteria [12]. They are readily carried away by runoff into the surface waters or leached into the subsurface soil layers and then the underground water, causing eutrophication of waters. Under anaerobic conditions, NO\({}_{3}\)\({}^{-}\) would be reduced to NO, N\({}_{2}\)O, and N\({}_{2}\) by bacteria and fungi, causing emissions of greenhouse gases [13]. Therefore, excessive urea application would not only bring environmental concerns, but also lead to low nitrogen use efficiency (NUE). It is reported that the average NUE of summer maize over is only 25-30% in the NCP, which is far lower than that in developed countries [9]. Hence, improving NUE, reducing environmental pollution and greenhouse gas emissions caused by excessive fertilization, and developing efficient and sustainable and intensive agricultural have become the focus of food production. Integrating inorganic fertilizer with organic amendments is an effective strategy for ensuring the sustainability of agricultural system [14; 15]. Fulvic acid (FA) is a widely used amendment in soils environment that has been proved to be beneficial in promoting plant growth and increasing crop yields [16; 17]. According to the extraction source of fulvic acid, it can be divided into natural and biochemical fulvic acid. Natural fulvic acid is a kind of macromolecule organic weak acid mixture formed by a series of decomposition and transformation of animal and plant remains under microbial, geophysical and chemical action, which can be extracted from soil, water and coal [18; 19; 20]. Biochemical fulvic acid is a type of weakly acidic organic matter extracted from agricultural waste by microbial fermentation technology or straw-based ammonium sulfite pulping process [21]. Its chemical properties are similar to those of natural fulvic acid [22; 23].