What makes nitrogen usable

Nitrogen is in the soil under our feet, in the water we drink, and in the air we breathe. Nitrogen is important to all living things, including us. It plays a key role in plant growth: too little nitrogen and plants cannot thrive, leading to low crop yields; but too much nitrogen can be toxic to plants [ 1 ]. Nitrogen is necessary for our food supply, but excess nitrogen can harm the environment. The delicate balance of substances that is important for maintaining life is an important area of research, and the balance of nitrogen in the environment is no exception [ 2 ].

When plants lack nitrogen, they become yellowed, with stunted growth, and produce smaller fruits and flowers. Farmers may add fertilizers containing nitrogen to their crops, to increase crop growth. Without nitrogen fertilizers, scientists estimate that we would lose up to one third of the crops we rely on for food and other types of agriculture.

But we need to know how much nitrogen is necessary for plant growth, because too much can pollute waterways, hurting aquatic life. Nitrogen is a key element in the nucleic acids DNA and RNA , which are the most important of all biological molecules and crucial for all living things.

DNA carries the genetic information, which means the instructions for how to make up a life form. When plants do not get enough nitrogen, they are unable to produce amino acids substances that contain nitrogen and hydrogen and make up many of living cells, muscles and tissue. Without amino acids, plants cannot make the special proteins that the plant cells need to grow.

Without enough nitrogen, plant growth is affected negatively. With too much nitrogen, plants produce excess biomass, or organic matter, such as stalks and leaves, but not enough root structure. In extreme cases, plants with very high levels of nitrogen absorbed from soils can poison farm animals that eat them [ 3 ].

Excess nitrogen can also leach—or drain—from the soil into underground water sources, or it can enter aquatic systems as above ground runoff. This excess nitrogen can build up, leading to a process called eutrophication. Eutrophication happens when too much nitrogen enriches the water, causing excessive growth of plants and algae.

When the phytoplankton dies, microbes in the water decompose them. Organisms in the dead zone die from lack of oxygen. These dead zones can happen in freshwater lakes and also in coastal environments where rivers full of nutrients from agricultural runoff fertilizer overflow flow into oceans [ 4 ].

Can eutrophication be prevented? People who manage water resources can use different strategies to reduce the harmful effects of algal blooms and eutrophication of water surfaces. They can re-reroute excess nutrients away from lakes and vulnerable costal zones, use herbicides chemicals used to kill unwanted plant growth or algaecides chemicals used to kill algae to stop the algal blooms, and reduce the quantities or combinations of nutrients used in agricultural fertilizers, among other techniques [ 5 ].

But, it can often be hard to find the origin of the excess nitrogen and other nutrients. Once a lake has undergone eutrophication, it is even harder to do damage control. When nitrogen nutrients have served their purpose in plants and animals, specialized decomposing bacteria will start a process called ammonification , to convert them back into ammonia and water-soluble ammonium salts. After the nutrients are converted back into ammonia, anaerobic bacteria will convert them back into nitrogen gas, during a process called denitrification.

The whole process starts over after release. A schematic representation of the nitrogen cycle is shown here: Nitrogen as a limiting factor Although the nitrogen conversion processes often occurs and large quantities of plant nutrients are produced, nitrogen is often a limiting factor for plant growth.

Water flowing across the soil causes this error. Nitrogen nutrients are water-soluble and as a result they are easily drained away, so that they are no longer available for plants. The annamox reaction In researchers at the Gist-Brocades in Delft, The Netherlands, discovered a new reaction to be added to the nitrogen cycle; the so-called annamox reaction. The act of breaking apart the two atoms in a nitrogen molecule is called "nitrogen fixation".

Plants get the nitrogen that they need from the soil, where it has already been fixed by bacteria and archaea. Bacteria and archaea in the soil and in the roots of some plants have the ability to convert molecular nitrogen from the air N 2 to ammonia NH 3 , thereby breaking the tough triple bond of molecular nitrogen. Such organisms are called "diazotrophs". From here, various microorganisms convert ammonia to other nitrogen compounds that are easier for plants to use.

In this way, plants get their nitrogen indirectly from the air via microorganisms in the soil and in certain plant roots. However, nitrogen in excess of plant demand can leach from soils into waterways. The nitrogen enrichment contributes to eutrophication. Another problem can occur during nitrification and denitrification. When the chemical process is not completed, nitrous oxide N 2 O can be formed.

This is of concern, as N 2 O is a potent greenhouse gas — contributing to global warming. A balance of nitrogen compounds in the environment supports plant life and is not a threat to animals. It is only when the cycle is not balanced that problems occur.

Organic forms are a very diverse group of nitrogen-containing organic molecules including simple amino acids through to large complex proteins and nucleic acids in living organisms and humic compounds in soil and water. Scientists make observations and develop their explanations using inference, imagination and creativity.

Often they use models to help other scientists understand their theories. The nitrogen cycle diagram is an example of an explanatory model.