The factors driving plant species diversity in different forest types and layers are still insufficiently understood. Therefore, we investigated the limiting factors of nutrient and water use efficiency in three forest layers (arbor, shrub and herb layers) and two forest types (plantation forest and natural forest), as well as their potential relationship with plant species diversity. The study area is located in mid-latitude evergreen broad-leaved forest ecosystems in southern Anhui province. The nitrogen content in the soil (2.90 g kg⁻¹) exceeded the national average for forest ecosystems (1.06 g kg⁻¹), whereas the phosphorus content (0.43 g kg⁻¹) was below the national average for such ecosystems (0.65 g kg⁻¹). The plant species diversity in the arbor layer was comparatively low, and nitrogen exerted a more substantial influence on it. In contrast, phosphorus had the most significant impact on the shrub and herb layers. In conclusion, nitrogen has little effect on plant growth patterns in this area, while there is a potential role for phosphorus in regulating plant succession rates. There are phenomenon of excessive nitrogen deposition and phosphorus loss in this forest ecosystem. Despite the fact that the plant species diversity indices (the Pielou Evenness Index, Gleason Richness Index, Shannon–Wiener Diversity Index and Simpson Dominance Index) in natural forests were higher than that in plantation forests, the disparity between them was not significant (p > 0.05). This may be due to similar water use efficiency and climate. The findings of this research hold substantial significance for forecasting the alterations in plant species diversity, functionality, and their influencing factors within mid-latitude evergreen broad-leaved forests.

Accelerated urbanization has intensified nitrogen deposition and fine particulate matter (PM_2.5) pollution. While urban landscape plants play a vital role in atmospheric remediation, systematic exploration of their adaptation strategies to these dual stressors remains limited. This study investigated the dynamic responses of antioxidant defense systems and nitrogen/weight ratios of Iris germanica L. and Portulaca grandiflora Hook. under four nitrogen deposition scenarios (N0, N1, N2, and N4 with nitrogen concentrations of 0, 15, 30, and 60 kg N·hm⁻²·a⁻¹, respectively) combined with constant PM_2.5 exposure (50 μg/m³). Through fumigation experiments, we demonstrated that Iris germanica L. showed higher sensitivity to inorganic nitrogen-laden PM_2.5 stress than Portulaca grandiflora Hook. Both species exhibited stronger antioxidant enzyme (SOD, CAT, POD) activities in the high-growth season compared to the low-growth season. Nitrogen allocation analysis revealed that Portulaca grandiflora Hook. maintained stable nitrogen content across treatments, while Iris germanica L. showed progressive nitrogen loss under high nitrogen-laden PM_2.5 exposure. These findings establish Portulaca grandiflora Hook.’s superior resilience through two synergistic mechanisms: modulated antioxidant systems and efficient nitrogen remobilization. This comparative study provides actionable insights for selecting pollution-tolerant species in urban green infrastructure planning.