## 2 Materials and Methods
### Sampling and Chemical Analyses  
The functional groups of FA were determined by Fourier Transform Infrared with waves number ranging from 400 cm\({}^{-1}\) to 4000 cm\({}^{-1}\) (Nicolet Nexus 410, thermo nicolet, \begin{table}
\begin{tabular}{c c c c c c c} \hline
**pH** & **Organic Matter Content** & **Total N** & **Nitrate N** & **Ammonium N** & **Available** & **Available** \\
**(I:2.5)** & & & & & **Phosphorus** & **Potassium** \\ \hline \multicolumn{6}{c}{(g kg\({}^{-1}\))} \\ \hline
7.85 & 12.01 & 0.65 & 72.35 & 9.44 & 13.22 & 92.22 \\ \hline \end{tabular}
\end{table} 
Table 1: The fundamental properties of soil in this experimental site. America). The spectrometer had a resolution of 2 cm\({}^{-1}\), signal-to-noise ratio of 50,000:1 and 32 scanning times (Supplementary Figure S1). Soil inorganic nitrogen (NH4\({}^{+}\)-N and NO3\({}^{-}\)-N) was extracted with 0.01 M CaCl\({}_{2}\) at soil to water ratio of 1:10 and quantified using a continuous flow injection analyzer (AA3-A001-02E, Bran-Luebbe, Germany). Soil available phosphorus was assessed by 0.5 M NaHCO\({}_{3}\) (pH 8.5) and colorimetrically quantified using the molybdenum antimony method [37]. Flame photometry was used to assess available potassium, which was extracted with 1.0 M of CH\({}_{3}\)COONH\({}_{4}\) at a pH of 7.0 [38]. At the seedling (July 9), jointing (July 20), 12-leaf (V12, August 13), and silking stages (September 11) of maize, plant height and stem diameter (between the second and third nodes from the bottom) were measured, and soil samples were gathered from the 0-20 cm layer in each pot for available nutrient analysis. At the jointing stage, the relative concentration of chlorophyll (i.e., SPAD value) was measured on the third completely grown leaf from the top using a chlorophyll meter (SPAD-502, Minolta, Japan). Net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr) were monitored on the ninth leaf from the top using a Li-6400 XT portable photosynthesis system (Licor Inc., Lincoln, NE, USA). In addition, the second fully expanded leaf from the top (i.e., the largest functional leaf) was collected from each plant, washed with deionized water, snap frozen in liquid nitrogen, and stored at \(-80\,^{\circ}\)C before being analyzed for endogenous hormones and carbon and nitrogen metabolism-related enzyme activities. At the V12 stage, leaves were collected again for analyses of carbon and nitrogen metabolism-related enzyme activities. In addition, root was sampled, washed with deionized water, snap frozen in liquid nitrogen, and stored at \(-80\,^{\circ}\)C before analyses for endogenous hormones and nitrogen metabolism-related enzyme activities. At September 29, maize was harvested. After ears per plant, rows per ear, grains per row, and 100-grain weight were recorded, plant samples were oven-dried first at \(105\,^{\circ}\)C for 15 min and then at \(65\,^{\circ}\)C to constant weight. The concentrations of endogenous hormones, including auxin (IAA), gibberellin (GA), abscisic acid (ABA), and zeatin nucleoside (ZR), and the activities of carbon and nitrogen metabolism-related enzymes, including glutamine synthetase (GS), glutathione reductase (GSR), nitrate reductase (NR), glutamate dehydrogenase (GLDH), nitrite reductase (NiR), glutamate synthase (Fd-GOGAT), pyruvate phosphate dikinase (PPDK), and phosphoenolpyruvate carboxylase (PEPC), in root and leave were estimated using the ELISA kit (Shanghai HengYuan Biological Technology Co. Ltd., Shanghai, China). An ELISA kit uses a double-antibody sandwich method to determine the concentrations of various enzymes in samples. The purified enzyme capture antibody was placed on a microplate to make a solid phase antibody. To form the antibody-antigen enzyme-labeled antibody complex, the enzyme-was applied to the coated micropores one at a time and then combined with the detecting antibody-labeled using the enzyme labeled reagent horseradish peroxidase (HRP) to form antibody antigen enzyme labeled antibody complex.