Abstract: Aims This study examines the growth responses of major afforestation tree species in Northeast China, specifically Fraxinus mandshurica (a broadleaf species associated with arbuscular mycorrhizae) and Larix gmelinii (a coniferous species associated with ectomycorrhizae), to varying levels of dry and wet reduced nitrogen deposition. By investigating the impact of increased reduced nitrogen on these seedlings, this research aims to enhance our understanding of the relationship between different seedling growth patterns and atmospheric nitrogen deposition forms, thereby providing a theoretical foundation for precise seedling cultivation. Methods The research utilized Fraxinus mandshurica with arbuscular mycorrhizal (AM) seedlings and Larix gmelinii with ectomycorrhizal (ECM) seedlings as test subjects to simulate varying concentrations of dry and wet reduced nitrogen deposition: 0 (CK), 35 (ND-35), 70 (ND-70), 35 (NW-35), and 70 kg N ha-1 yr-1 (NW-70). We assessed changes in seedling growth, photosynthetic capacity, root development, and mycorrhizal infection rates of two types of seedlings. Important findings Under conditions of atmospheric dry and wet nitrogen deposition, seedlings of both mycorrhizal types gradually reduce their dependence on mycorrhizal fungi. The primary response forms are the enhancement of their own photosynthetic performance and an increase in root absorption capacity. (1) During dry sedimentation of Fraxinus mandshurica with arbuscular mycorrhizal, a significant enhancement in leaf accumulation and total biomass was observed, attributed to changes in photosynthetic capacity. Under the ND-70 treatment, the net photosynthetic rate, total chlorophyll, and leaf biomass increased by 49.61% compared to the control, with additional increases of 76.29% and 53.84%, respectively. In contrast, during wet deposition, nitrogen use efficiency improved primarily due to an increased contact area between absorbing roots and soil. Under the NW-35 and NW-70 treatments, the surface area of absorbing roots increased by 14.96% and 16.17%, respectively, compared to the control. (2) Larix gmelinii, characterized by ectomycorrhizal roots, exhibited a more pronounced response to wet sedimentation, primarily enhancing the absorptive capacity of the root system through the elongation and thinning of the absorptive roots. Under the NW-70 treatment, the absorptive root length increased by 20.70% compared to the control, while the average absorptive root diameter and cortical thickness decreased by 10.14% and 27.25%, respectively. This research provides an in-depth analysis of the relationship between mycorrhizal types and atmospheric reduced deposition, serving as a reference for the precise nutrient management of seedlings with varying mycorrhizal types.

Key words: Fraxinus mandshurica, Larix gmelinii, nitrogen deposition, seedling growth, photosynthetic capacity, absorbing roots, mycorrhizal development