Snow contains nitrogen in various forms, acting as a natural delivery system for this element to the Earth’s surface. This process is known as atmospheric deposition, which involves the transfer of gases and particles from the air to land and water. Nitrogen is abundant in the atmosphere, but most of it is inert nitrogen gas (\(\text{N}_2\)). A smaller fraction exists as reactive nitrogen compounds that are readily incorporated into precipitation. When snow falls, it acts as a filter, capturing these nitrogen compounds and depositing them on the landscape. This deposition can profoundly influence ecosystems, especially in regions where snow is present for long periods.
The Chemical Forms of Nitrogen Captured by Snow
The nitrogen found in snow is primarily in two inorganic forms that plants can readily use: nitrate (\(\text{NO}_3^-\)) and ammonium (\(\text{NH}_4^+\)). These chemical species represent the oxidized and reduced states of reactive nitrogen, respectively.
Nitrate is a negatively charged ion formed when nitrogen oxides react with other atmospheric components. This form of nitrogen is generally highly mobile in soil and water once the snow melts.
Ammonium is a positively charged ion derived from airborne ammonia (\(\text{NH}_3\)). This form is often attracted to and held by negatively charged soil particles, making it less mobile than nitrate. Research shows that snow tends to be relatively enriched in nitrate compared to rain, which often has a higher proportion of ammonium. The ratio of these two forms can vary significantly depending on local sources of atmospheric pollution.
How Nitrogen Enters the Atmosphere and Snow
Reactive nitrogen compounds enter the atmosphere through both natural processes and human activities. Natural sources include electrical discharges during thunderstorms, where the immense energy from lightning converts inert atmospheric nitrogen gas into nitrogen oxides (\(\text{NO}_x\)). Biological processes also contribute, such as the decomposition of organic matter and the volatilization of nitrogen from soils.
Human activities are now the dominant source of reactive nitrogen in many regions. The combustion of fossil fuels in vehicles and power plants releases large amounts of nitrogen oxides into the air. Agricultural practices are another major source, as the use of nitrogen-based fertilizers and animal waste management release ammonia gas (\(\text{NH}_3\)) into the atmosphere. These gases and fine particulate matter are then incorporated into the hydrological cycle.
The physical process of transfer to the snow is called wet deposition, where compounds are dissolved into cloud droplets that form snowflakes. Snow is particularly efficient at “scavenging,” where falling ice crystals physically sweep up and absorb gases and microscopic particles as they descend through the atmosphere. This action means that fresh snowfall can effectively remove significant amounts of airborne pollutants, which are then held within the snowpack until the spring thaw.
Ecological Significance of Nitrogen Deposition
The nitrogen stored in the snowpack is released in a concentrated pulse during the spring melt, affecting terrestrial and aquatic ecosystems. This sudden influx of nutrients acts as a fertilizer, stimulating the growth of plants and microorganisms in nutrient-limited environments, such as high-altitude forests and alpine regions. The timing of this “snowmelt nitrogen pulse” often coincides perfectly with the start of the growing season.
When atmospheric nitrogen deposition is excessive, it can lead to nitrogen saturation. This occurs when the amount of nitrogen entering an ecosystem surpasses what plants and microbes can assimilate. The overload causes significant ecological changes, including soil acidification and the leaching of excess nitrate into groundwater and streams.
The runoff of surplus nitrogen into aquatic environments, such as rivers and lakes, can trigger eutrophication. This process involves the rapid overgrowth of algae, which consume large amounts of dissolved oxygen when they decompose. This creates “dead zones” that harm aquatic life. High nitrogen loads can also alter the competitive balance between plant species, potentially causing a loss of biodiversity in sensitive habitats.