Processes of the Nitrogen Cycle

N cycle

The nitrogen cycle. Image courtesy of the University of Minnesota.

Five main processes cycle nitrogen through the biosphere, atmosphere, and geosphere: 1. nitrogen fixation; 2. nitrogen uptake; 3. nitrogen mineralization; 4. nitrification; and 5. denitrification. Microorganisms, in particular, bacteria, play major roles in all of these processes. The rate at which these processes take place and cycle nitrogen from one step to another in the nitrogen cycle depends on environmental factors such as temperature and water availability.

Nitrogen Fixation
Nitrogen fixation is the process by which N2 is changed to ammonium (
N2 arrow-right NH4+) and other forms of nitrogen. This process is very important because it is the only way organisms can use the nitrogen from the atmosphere. This is done by nitrogen-fixing bacteria, lightning, and forest fires. Within the last century, humans have become an important source of fixed nitrogen. Activities such as burning fossil fuels, using synthetic fertilizers, and growing crops such as legumes all fix nitrogen. In this way, humans have more than doubled the amount of fixed nitrogen that is available to the biosphere each year.

Nitrogen Uptake
The ammonia produced by nitrogen fixation is usually quickly used to create proteins and other organic nitrogen compounds, either by plants, the bacteria on their roots, or organisms that live in the soil. When organisms nearer the top of the food chain (like us!) eat, we are using nitrogen that has been fixed initially by nitrogen fixing bacteria.

Nitrogen Mineralization
After nitrogen is incorporated into organic matter (living things), it is often converted back into inorganic nitrogen by a process called nitrogen mineralization, also known as decay. When organisms die, decomposers (such as bacteria and fungi) consume the organic matter and lead to the process of decomposition. During this process, a significant amount of the nitrogen contained within the dead organisms is converted to ammonium. Once in the form of ammonium, nitrogen is available for use by plants or is converted into nitrate through the process of nitrification.

Some of the ammonium produced by decomposition is converted to nitrate through the process of nitrification. The bacteria that carry out this process gain energy from it. Nitrification uses oxygen, so it can only happen in an oxygen-rich environment like circulating or flowing waters and the very surface layers of soil and sediments. The process of nitrification also has some important consequences. Ammonium ions are positively charged, and therefore stick to negatively charged clay & soil particles and organic matter. This prevents ammonium from being washed out of the soil by rainfall. In contrast, nitrates are negatively charged and do not stick to the soil. This means that they can be eroded from the soil and washed into surface water and/or groundwater resources, contributing to the nutrient pollution of aquatic ecosystems.

Through denitrification, nitrogen oxides (nitrites (NO2-) and nitrates (NO3-) are converted to N2 by the following process:
NO3- arrow-right NO2- arrow-right NO arrow-right N2O arrow-right N2.
Nitric oxide (NO) and nitrous oxide (N2O) are environmentally important gases. Nitric oxide contributes to smog and nitrous oxide is a greenhouse gas. Once converted to N2, this gaseous form of nitrogen is easily lost to the atmosphere. Denitrification is the only process that removes nitrogen from the biosphere, and it roughly balances the amount of nitrogen fixed by the other 4 processes described above.


Last modified: Thursday, 1 March 2012, 9:05 AM