| Summary: | <i>Research Highlights:</i> This study identifies the nitrogen (N) deposition effect on understory plants by altering directly soil nutrients or indirectly altering environmental factors in subtropical plantation. <i>Background and Objectives:</i> N deposition is a major environmental issue and has altered forest ecosystem components and their functions. The response of understory vegetation to N deposition is often neglected due to a small proportion of stand productivity. However, compared to overstory trees, understory species usually have a higher nutrient cycle rate and are more sensitive to environmental change, so should be of greater concern. <i>Materials and Methods:</i> The changes in plant biomass, N, phosphorus (P), and nonstructural carbohydrates (NSCs) of three dominant understory species, namely <i>Dicranopteris dichotoma</i>, <i>Lophatherum gracile</i>, and <i>Melastoma dodecandrum</i>, were determined following four years of experimental N addition (100 kg hm<sup>−2</sup> year<sup>−1</sup> of N) in a Chinese fir plantation. <i>Results:</i> N addition increased the tissue N concentrations of all the understory plants by increasing soil mineral N, while N addition decreased the aboveground biomass of <i>D. dichotoma</i> and <i>L. gracile</i> significantly—by 82.1% and 67.2%, respectively. The biomass of <i>M. dodecandrum</i> did not respond to N addition. In contrast, N addition significantly increased the average girth growth rates and litterfall productivity of overstory trees—by 18.28% and 36.71%, respectively. NSCs, especially soluble sugar, representing immediate products of photosynthesis and main energy sources for plant growth, decreased after N addition in two of the three species. The plant NSC/N and NSC/P ratios showed decreasing tendencies, but the N/P ratio in aboveground tissue did not change with N addition. <i>Conclusions:</i> N addition might inhibit the growth of understory plants by decreasing the nonstructural carbohydrates and light availability indirectly rather than by changing nutrients and N/P stoichiometry directly, although species-specific responses to N deposition occurred in the Chinese fir plantation.
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