| Summary: | Both fulvic acid (FA) and nitrogen (N) play important roles in agricultural production in China. Plants typically show a higher nitrogen utilization efficiency (NUE) under FA application. However, the role of FA application in apple growth and NUE remains unclear. A hydroponic culture experiment was performed, and M9T337 seedlings (a dwarf apple rootstock) were used as the experimental subjects. The biomass, photosynthesis, accumulation, and distribution of photosynthates, N absorption and assimilation, and relative gene expression in the seedlings were examined after treatment with five different concentrations of FA (0, 60, 120, 180, and 240 mg·L<sup>−1</sup>, represented by CK, FA1, FA2, FA3, FA4, respectively). The results showed that the seedling dry weight and <sup>15</sup>NUE were enhanced by FA, and both were highest under the FA2 (the concentration of fulvic acid is 120 mg·L<sup>−1</sup>) treatment. Further analysis revealed that under the FA2 treatment, the root morphology was optimized, and the root activity was relatively high. Compared with CK (control, the concentration of fulvic acid is 0 mg·L<sup>−1</sup>), the FA2 treatment strengthened photosynthesis, elevated the key enzyme activities related to C metabolism, upregulated the gene expression of sugar transport proteins, and increased the root sorbitol and sucrose contents, which suggested that the FA2 treatment optimally affected the root growth and N absorption because it enhanced photosynthate synthesis and the leaf-to-root translocation of photoassimilates. The seedlings in the FA2 treatment group also showed a significantly higher NO<sub>3</sub><sup>−</sup> influx rate and <i>NRT</i> (nitrate transporter) gene expression in the roots. Moreover, relatively high N metabolism-related enzyme activities in the leaves and roots were also observed under the FA2 treatment. The isotope labeling results showed that the optimal FA2 supply not only promoted seedling <sup>15</sup>N absorption but also optimized the distribution of C and N in the seedlings. These results suggested that an optimal FA supply (120 mg·L<sup>−1</sup>) enhanced seedling NUE by strengthening photoassimilate synthesis and transport from leaves to roots, regulating N absorption, assimilation, and distribution.
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