Physiological and Biochemical Adaptation of Common Garden Plants to Inorganic Nitrogen-Laden Fine Particulate Matter Stress

Accelerated urbanization has intensified nitrogen deposition and fine particulate matter (PM2.5) 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 PM2.5 exposure (50 μg/m3). Through fumigation experiments, we demonstrated that Iris germanica L. showed higher sensitivity to inorganic nitrogen-laden PM2.5 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 PM2.5 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.