Nitrogen is consistently present in rain, a natural part of the global nitrogen cycle that has been significantly amplified by human activity. Nitrogen enters precipitation through “wet deposition,” where various nitrogen compounds dissolve directly into raindrops, snow, or fog as they fall through the atmosphere. While nitrogen gas makes up about 78% of the Earth’s atmosphere, it is in an inert state and does not dissolve into water. Therefore, the nitrogen found in rain is always a reactive compound.
The Chemical Forms of Nitrogen Found in Rainwater
The nitrogen found dissolved in precipitation exists primarily as two types of reactive compounds: oxidized and reduced forms. The oxidized form is Nitrate (NO3-), a negatively charged ion that is highly soluble in water. Nitrate is formed when nitrogen oxides (NOx) react with water vapor and oxygen, often creating nitric acid (HNO3).
The reduced form is Ammonium (NH4+), a positively charged ion that is also readily dissolved in water. Ammonium is generated when gaseous Ammonia (NH3) absorbs a hydrogen ion from the surrounding air or water. The balance between these two forms greatly influences the rain’s chemistry; rain rich in nitric acid will be more acidic, while rain containing high levels of ammonia can help neutralize acidity.
Both nitrate and ammonium are considered bioavailable, meaning they can be directly utilized by plants and other organisms. Because these ions are so water-soluble, they are easily washed out of the atmosphere during precipitation events. This cleansing process delivers a constant supply of nitrogen to the Earth’s surface, acting as an amplified fertilization mechanism.
Natural and Anthropogenic Sources of Atmospheric Nitrogen
Reactive nitrogen compounds enter the atmosphere through both natural and human-caused processes. One significant natural source is the immense energy from lightning, which causes atmospheric nitrogen gas (N2) and oxygen to combine, forming nitrogen oxides (NOx). Microbial activity in soils, such as the natural decay of organic matter, also releases nitrogen compounds, including ammonia, which can then volatilize into the atmosphere.
However, human activities, or anthropogenic sources, now account for a vast majority of the atmospheric nitrogen deposition globally, estimated to be around 60% of the total atmospheric nitrogen source. The burning of fossil fuels in vehicles, power plants, and industrial activities is the primary source of oxidized nitrogen, releasing large quantities of nitrogen oxides (NOx).
Agricultural practices contribute the largest share of reduced nitrogen to the atmosphere, primarily in the form of ammonia (NH3). This ammonia is released through the volatilization of synthetic nitrogen fertilizers, as well as from the decomposition of animal manure and livestock waste. Human activity has caused a nearly three-fold increase in soluble nitrogen deposition over land since 1850, drastically altering the natural flow of this element.
Ecological Impact of Nitrogen Deposition
Once deposited, atmospheric nitrogen acts as a fertilizer, which can initially stimulate plant growth in nitrogen-limited ecosystems, such as forests. This fertilization can increase the amount of carbon stored in these ecosystems, a perceived temporary benefit. However, the excessive input of this reactive nitrogen load has negative consequences that outweigh initial positive effects.
The most visible negative impact is Eutrophication, which occurs when excessive nutrient loading enters bodies of water. This influx of nitrogen fuels the rapid overgrowth of algae and cyanobacteria, creating dense algal blooms. When these blooms die, their decomposition by bacteria consumes vast amounts of dissolved oxygen in the water. This creates “dead zones” that lead to fish kills and the loss of aquatic life.
On land, the constant addition of nitrogen can lead to soil acidification, particularly when ammonium is converted to nitrate by soil microbes. This process releases hydrogen ions, which lowers the soil’s pH and leaches away other beneficial nutrients like calcium and magnesium. Furthermore, the over-fertilization leads to a loss of biodiversity. Fast-growing, nitrogen-loving plant species outcompete native plants that are adapted to low-nutrient conditions. This changes the structure of the ecosystem, leading to a homogenization of plant communities and a decline in species richness.