The Nitrogen Cycle

The nitrogen cycle involves the movement of nitrogen atoms between biotic and abiotic components in the ecosystem.

Most of the ecosystem's nitrogen is stored in the atmosphere as a gas, N2. But only bacteria posses the necessary enzymes to convert nitrogen gas into a usable form by other organisms, a term called nitrogen fixation. Therefore nitrogen is a very limited nutrient in the ecosystem.

Once bacteria in the soil convert the gas into ammonia, plants can the convert the nitrogen into proteins, DNA, and other organic molecules, which are consumed by herbivores and broken down into amino acids.

These can be reassembled into usable proteins for growth and development and for nutrition for consumers. Simultaneously, herbivores and consumers are returning some nitrogen back to the soil as waste products.

Both dead organisms and waste products contain nitrogen in the form of protein, urea, and uric acid. Decomposers like bacteria and fungi can break down these molecules releasing ammonia back into soil, which can be absorbed by plants or converted into nitrates by nitrifying bacteria.

These nitrates can also be either absorbed by plants or in the absence of oxygen be converted into nitrogen gas by denitrifying bacteria.

Human also input nitrogen into the ecosystem through the use of agricultural fertilizers. However, when used at the wrong time or in large quantities, excess nitrogen can flow into aquatic ecosystems and increase growth of bacteria, algae, and aquatic plants, a process called eutrophication.