<p>Lichens are known for being highly sensitive to air pollution, including nitrogen-related compounds, both reduced and oxidized. The future scenarios forecast a decline in nitrate emissions in Poland, with a subsequent increase in ammonia deposition. This study aimed to evaluate the effect of nitrate and ammonium on selected physiological parameters of two lichen species differing in N-sensitivity: <i>Hypogymnia physodes</i> and <i>Xanthoria parietina</i>. Reduced and oxidized nitrogen doses corresponding to wet nitrogen deposition scenarios in 2050 were used to verify whether future changes in nitrogen pollution could potentially affect the physiology of these lichen species. Selected lichen species were kept in a climatic chamber for three weeks and sprayed with different ammonium and nitrate solutions, and the content of photosynthetic pigments, TBARS concentration, and dehydrogenase activity were checked. We found that nitrate and ammonium treatment did not cause clear signs of oxidative stress in both species. Ammonium treatment caused a significant decline in mycobiont’s vitality of <i>Hypogymnia physodes</i>, while <i>Xanthoria parietina</i> handled the nitrogen treatment well. No clear pattern in photosynthetic pigment concentration for both species was found. We concluded that <i>Xanthoria parietina</i> is well suited to nitrogen stress, in both reduced and oxidized forms, and that the forecast decline in nitrate emissions may still impact the vitality of N-sensitive lichen species. We also propose the assessment of dehydrogenase activity in N-sensitive lichen species as a potential early indicator of elevated nitrogen concentration in the environment.</p>