The nitrogen cycle represents one of the most important nutrient cycles found in ecosystems (Figure 9s-1). Nitrogen is used by living organisms to produce a number of complex organic molecules like proteins. The store of nitrogen found in the atmosphere, where it exists as a gas (mainly N₂), plays an important role for life. This store is about one million times larger than the total nitrogen contained in living organisms. Other major stores of nitrogen include organic matter in soil and the oceans. Despite its abundance in the atmosphere, nitrogen is often the most limiting nutrient for plant growth. This problem occurs because most plants can only take up nitrogen in two solid forms: ammonium ion (NH₄⁺ ) and the ion nitrate (NO₃^- ). Most plants obtain the nitrogen they need as inorganic nitrate from the soil solution. Ammonium is used less by plants for uptake because in large concentrations it is extremely toxic. Animals receive the required nitrogen they need for metabolism, growth, and reproduction by the consumption of living or dead organic matter containing molecules composed partially of nitrogen.

Figure 9s-1: Nitrogen cycle.

 

In most ecosystems nitrogen is primarily stored in living and dead organic matter. This organic nitrogen is converted into inorganic forms when it decomposes. Decomposers, found in the upper soil layer, chemically modify the nitrogen found in organic matter from ammonia (NH₃ ) to ammonium salts (NH₄⁺ ). This process is known as mineralization and it is carried out by a variety of bacteria, and fungi.

Almost all of the nitrogen found in any land ecosystem originally came from the atmosphere. Significant amounts enter the soil in rainfall or through the effects of lightning. The majority, however, is fixed within the soil by specialized micro-organisms like bacteria. Members of the bean family (legumes) and some other kinds of plants fix nitrogen for plant use.  Scientists estimate these bacteria add about 140 million tons of nitrogen to ecosystems every year.

The activities of humans have severely altered the nitrogen cycle. Some of the major processes involved in this alteration include:

	The application of nitrogen fertilizers to crops has caused increased rates of denitrification and leaching of nitrate into groundwater. The additional nitrogen entering the groundwater system eventually flows into streams, rivers, lakes, and estuaries. In these systems, the added nitrogen can lead to algae growth.
	Livestock ranching. Livestock release a large amounts of ammonia into the environment from their wastes. This nitrogen enters the soil system and then the hydrologic system through leaching, groundwater flow, and runoff.
	Sewage waste and septic tank leaching.