Weathering release occurs of Ca and other ions into surface waters. Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide, SO, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to the rocks on which it
Lichens and nitrogen cycling and decrease the growth of the lichen. If the lichen cannot increase its photobiont growth, high nitrogen uptake may result in a higher concentration than it physiologically requires which will negatively affect the lichen and it's host plant as the other nutrients are too limiting. Lichen’s response to nutrient enrichment is both species-specific and dependent its environmental factors e.g. nutrient concentration, light availability and water supply. Lichen is nitrogen sensitive and change in nitrogen availability can affect its health greatly. Two main nitrogen stress factors for lichens are nitrogen deficiency and high nitrogen deposition. Both types of nitrogen stress result