Laura Nielsen for Frontier Scientists –
Nitrogen is one of the most abundant elements on Earth; nitrogen gas (N2) makes up 78% of Earth’s atmosphere. Nitrogen is also an essential element for all organisms. In order to live and grow, plants and animals need the hydrogen (H) and oxygen (O) which compose water, as well as carbon (C), nitrogen (N) and phosphorus (P). Organisms use nitrogen to make proteins, amino acids, and more. Plants use it to create chlorophyll, which they need to perform photosynthesis. However, organisms cannot use nitrogen gas (N2) directly. They need to obtain nitrogen in a different form.
Many nitrogen transformations occur because of bacteria, tiny microorganisms living in soil. The process through which nitrogen moves through the environment, changes forms, becomes utilized by organisms, and once more returns to the air or soil, is called the nitrogen cycle.
Transforming and using nitrogen
Nitrogen-fixing bacteria in soil convert nitrogen gas (N2) into nitrogen compounds during a process called nitrogen fixation. Lightning strikes can do the same, but are much more rare. Nitrogen-fixing bacteria use nitrogen gas (N2) and release Ammonium (NH4).
Meanwhile, dead plants and animals are decomposed by bacteria and fungi. The decomposers release the nitrogen present in the organisms' bodies into the soil as Ammonium (NH4) through a process called ammonification.
Ammonium (NH4) in the soil is turned into Nitrites (NO2) and Nitrates (N03) by nitrifying bacteria during a process called nitrification. These nitrogen compounds, known as fixed forms of nitrogen, are highly useful to plants. Plants assimilate or absorb Nitrites (NO2) and Nitrates (N03) through their roots (assimilation). With these nitrogen compounds, plants build chlorophyll, proteins, and amino acids and nucleic acids.
Most other organisms– including humans– get the nitrogen they need to live by eating plants.
Humans and the nitrogen cycle
Humans are affecting the nitrogen cycle by burning fossil fuels, raising livestock that create animal waste, adding nitrogen fertilizers to the environment, and dumping sewage into water. Our actions impact even remote places like the Arctic.
In the Alaskan tundra, soil can remain frozen year-round. This frozen ground, or permafrost, stores layers of ancient organisms: dead plants and even animals that have long avoided decomposition because the temperature is so cold. Things are changing, though. Earth's climate is warming, and the Arctic is warming faster than any other region.
As permafrost thaws, previously frozen organic matter decomposes. Microbes, tiny microorganisms like bacteria, become more active as thaw increases. Where once they had to rely on snow bringing trace amounts of nitrogen to the ecosystem through atmospheric deposition (snow carrying tiny particles from the air to the ground), they can now feed off newly-thawed organisms deeper underground. The increased decay of once-living organisms releases nitrogen into the Arctic ecosystem. More nutrients means more possibilities for growing plants, but has other implications as well.
Nitrogen escaping permafrost
When nitrogen that was previously frozen is released in the Arctic tundra it can be a boon to local organisms which– of course– require nitrogen to live. However, there may be more nitrogen suddenly available than local plants and bacteria can use. Some excess nitrogen is released back into the atmosphere by dentrifying bacteria which perform dentrification to turn nitrogen compounds back into nitrogen gas (N2). Other excess nitrogen moves through the snowmelt-fed ecosystem of the Arctic carried by water, which is one of the reasons why scientists are studying water tracks in the Alaskan Arctic.
A sudden influx of nitrogen in a system that normally has little nitrogen creates problems. Nitrate (NO3) is extremely mobile, moving easily with water through soil. The National Park Service sums it up: "Excess N [nitrogen] can leak out of soils into streams and lakes, where it can cause blooms of algae. In addition, more N [nitrogen] may be lost to the atmosphere as nitrous oxide, a greenhouse gas that influences global warming 300 times more than carbon dioxide, and contributes to ozone depletion in the atmosphere." When too much nitrous oxide is present in the atmosphere it causes acid rain. Thawing permafrost releases not only nitrous oxide (N2O), but also carbon dioxide (CO2) and methane (CH4), all greenhouse gasses which will increase future global warming.
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
- 'Biological Nitrogen Fixation' Stephen C. Wagner, Nature Education Knowledge Project 3(10):15 (2012)
- 'Nitrogen Cycle' Scott C. Killpack and Daryl Buchholz, University of Missouri Department of Agronomy (1993)
- 'Nitrogen in the Nation's Rain' National Atmospheric Deposition Program (2000)
- 'Permafrost Thaw and the Nitrogen Cycle Fact Sheet' Dave Schirokauer, Denali National Park and Preserve Center for Resources, Science, and Learning (2012) DENA-FS-061-2012 Denali Park, Alaska