## 4 Discussion  
### Effects of Urea-FA Solution Combined Application on Soil Available Nutrients, Maize Yield, and NUE  
Organic wastes include crop straws and livestock manure, which can be used as raw materials for aerobic degradation to produce biochemical humic substances. The structure of humic substance shifted over time, including a wide range functional groups, such as carboxylic, phenolic and hydroxylic groups, during anaerobic process [42; 43]. Fulvic acid (FA) is one of the dominant components of humans [44], Originated from plant residues, FA is a type of organic polymer, containing carboxyl, quinone, phenolic hydroxyl, amide, and other functional groups, in this study (Figure S1). The phenolic hydroxyl and quinone groups in FA react with the sulfhydryli groups in urease, urease activity would be inhibited and ammonia volatilization from urea would be reduced [45]. In addition, the carboxyl and phenolic hydroxyl groups in FA interact with the amide groups in urea through coordination complexation, ion exchange, or hydrogen bond to form stable FA-urea complexes, delaying the decomposition of urea. The phenol hydroxyl and carboxyl groups in FA react with NH\({}_{4}\)\({}^{+}\) to generate stable FA-ammonium salts [46; 47; 48], improving the availability of NH\({}_{4}\)\({}^{+}\) to meet plant nitrogen demand at the critical growth stages. In this study, soil NH\({}_{4}\)\({}^{+}\) was significantly increased (Table 3), and maize grain yield and NUE were significantly improved by 12.61% and 20.70 percentage points, respectively, in the U+FA treatment compared with the U treatment (Table 2). Due to its loose, complex \"sponge-like\" structure and rich functional groups [49; 50], FA has a large specific surface area and a high adsorption capacity. Its carboxyl and phenolic hydroxyl groups can adsorb potassium ions to reduce potassium fixation by clay minerals [51; 52]. Compared to the U treatment, soil available potassium of the U+FA treatment was significantly higher (Table 3), contributing to the better maize growth in the treatment. Furthermore, the presence of FA alters the distribution of root exudate profile and it also tend to increase the release of oxalate and citrate from maize root [53; 54; 55]. Additionally, H is produced during FA decomposition [56], resulting in an available P increase in calcareous soils and the dissolution of insoluble P compounds. On the other hand, the adsorption of FA on the adsorption plane creates a repulsive negative electric potential and a steric hindrance on the mineral surface, which further inhibit P binding on the soil surface [57]. Furthermore, the carboxyl, hydroxyl, and alcoholic hydroxyl groups in FA can complex with such metal ions as Ca\({}^{2+}\) and Mg\({}^{2+}\), decreasing the fixation of phosphorus in soil and thereby increasing its mobility and availability [58; 59; 60]. Compared with the U treatment, the soil available phosphorus in the U+FA treatment was significantly higher at the seedling stage (Table 3), which would improve the vegetative growth and contribute to the later reproductive growth of maize in this treatment. In a word, the combined application of FA with urea in the U+FA treatment resulted in higher soil nutrient availability, which stimulated maize growth and in turn increased yield. Effects of Urea-FA Solution Combined Application on the Endogenous Hormones and \(N\) Metabolism-Related Enzyme Concentrations in Maize  
Plant endogenous hormones are important growth-regulating factors that respond to the changing environment. The hormone IAA triggers root cell signals, up-regulates H\({}^{+}\)-ATPase gene expression, and activates the enzymes and proteins related to cell wall relaxation and extension [61]. Compared with the U treatment, the U+FA treatment displayed significantly higher IAA concentrations in maize root at the V12 stage (Figure 1), indicating more active root development in this treatment. In addition, IAA often works synergistically with GA to influence cell expansion and tissue differentiation [62; 63].