The Nitrogen Cycle

Why is nitrogen important? Its an essential part of many biological molecules. Proteins Nucleic acids (DNA) Chlorophyll Adenosine triphosphate (ATP: chemical energy)

Where is nitrogen found? In the air (78% of the air is nitrogen gas) The nitrogen in the air is not useful as a direct nitrogen source for most organisms. It is to tightly bonded to itself (N 2 ) The nitrogen in the air must be fixed, or put into a compound that can be used by most organisms. The nitrogen that is usable by most organisms is found in the soil or in other organisms.

Step 1: The fixation of nitrogen Pure nitrogen (N 2 ) is converted into ammonia (NH 3 ) and ammonium (NH 4 ) compounds by specialized bacteria. Most of these nitrogen fixing bacteria live on the roots of certain plants (nitrogen fixers). Legumes (beans and peas) and alders to name a few.

Step 2: Nitrification A team of bacteria convert the ammonia or ammonium to nitrates(NO 3 -).This process is called nitrification.

Step 3: Assimilation Nitrates, ammonia and ammonium are absorbed by plants through roots. This is called assimilation. The nitrogen in these compounds is used to make proteins and other biological molecules in the plants. Animals get nitrogen when they eat plants or other animals that have eaten plants.

Step 4: Ammonification The nitrogen in organic compounds is put back into the soil as ammonia or ammonium when organisms produce waste or die. This is also done by bacteria.

Step 5: Denitrification Nitrates in the soil are converted back to pure nitrogen by bacteria.

Human influences on the nitrogen cycle. Fertilizing crops with nitrogen based fertilizers, which then enter the soil and water. Ranching, during which livestock wastes are released in unnatural concentrations into the soil. Sewage and septic systems leach into rivers and groundwater.

More human impacts: Nitrogen additions to the soil can lead to changes that favor weeds over native plants, which in turn reduces species diversity and changes ecosystems. Research shows that nitrogen levels are linked with changes in grassland species, from mosses and lichens to grasses and flowers.

More human impacts: Nitrogen oxides react with water to form nitric acid, which along with sulfur dioxide is a major component of acid rain. Acid rain can damage and kill aquatic life and vegetation, as well as corrode buildings, bridges, and other structures.

More human impacts: Adding large amounts of nitrogen to rivers, lakes, and coastal systems results in eutrophication, a condition that occurs in aquatic ecosystems when excessive nutrient concentrations stimulate blooms of algae that deplete oxygen, killing fish and other organisms and ruining water quality. Parts of the Gulf of Mexico, for example, are so inundated with excess fertilizer that the water is clogged with algae, suffocating fish and other marine life.

Nitrogen is a limiting factor Under natural conditions, the amount of available nitrogen in a stream limits the growth of many organisms like algae.