Nitrogen cycle

Nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification.

Overview

The atmosphere is the largest reservoir of nitrogen, containing over 78% of nitrogen gas (N2). However, this form of nitrogen is unavailable for use by most organisms, as it is largely inert. To become part of the biosphere, nitrogen gas must first be converted to more reactive forms such as ammonium (NH4+), nitrate (NO3-), or organic nitrogen (e.g. urea (NH2)2CO). This process is called nitrogen fixation.

Nitrogen fixation

Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into ammonia (NH3). This process is carried out by nitrogen-fixing bacteria, such as Azotobacter, and by certain types of cyanobacteria. Some nitrogen-fixing bacteria form symbiotic relationships with plants, such as legumes, providing them with a source of nitrogen in exchange for carbohydrates.

Ammonification

Ammonification or mineralization is performed by bacteria to convert organic nitrogen into ammonia. The ammonia is then converted into ammonium ions (NH4+), which are suitable for uptake by plants.

Nitrification

Nitrification is the process by which ammonium ions are converted into nitrites (NO2-) by bacteria (such as Nitrosomonas) and then into nitrates (NO3-) by another group of bacteria (such as Nitrobacter).

Denitrification

Denitrification is the process by which nitrates are converted into nitrogen gas (N2), completing the nitrogen cycle. This process is performed by a variety of microbial species in the absence of oxygen, often in waterlogged soils.

Human impact

Human activities such as burning fossil fuels, using synthetic nitrogen fertilizers, and cultivating legumes have dramatically altered the nitrogen cycle. These changes have increased the availability of biologically active nitrogen in many ecosystems, leading to a range of environmental problems including eutrophication of water bodies and global warming.

See also

• Carbon cycle
• Oxygen cycle
• Phosphorus cycle
• Sulfur cycle

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