Nitrogen Cycle 

The Changing Nitrogen Cycle

Credit: UCAR Center for Science Education

Plants and animals could not live without the essential element, nitrogen. It makes up many biological structures and processes such as cells, amino acids, proteins, and even DNA. It is also necessary for plants to produce chlorophyll, which they use in photosynthesis to make their food and energy.

Nitrogen forms simple chemicals called amino acids, the essential building blocks of all proteins, enzymes, and especially DNA.  It helps plants use carbohydrates to gain energy, like certain foods we eat help us to gain energy. Nitrogen controls how plants take their form and how they function inside, and nitrogen helps plants make the protein that helps them grow strong and healthy. Humans and animals benefit from eating vegetables and plants that are rich in nitrogen because proteins are passed on to humans and animals when they eat vegetables and plants.

We might commonly think of Earth as having an oxygen-dominated atmosphere, but in reality, the molecule makes up a little less than 20% our air. Most of what surrounds us is nitrogen, at 78 percent in the form of diatomic nitrogen gas, the gas itself is very unreactive. plants and animals simply cannot absorb the gas directly from the atmosphere. Nitrogen, in the forms of Nitrates (NO3), Nitrites (NO2), and Ammonium (NH4), is a nutrient needed for plant growth. Plants take up nitrogen in forms of nitrate ( NO3-) and ammonium ( NH4+ ). Most plants thrive on equal amounts of these ions but nitrates are more quickly available to plants because they move through the soil solution, whereas ammonium ions become fixed or held on to clay particles, called colloids, because of their positive charge.

How Plants Take Up Nitrogen

The nitrogen cycle involves certain processes that change nitrogen into different forms. Unfortunately, these forms of nitrogen are not always used by plants because they either get onto clay particles in soil, they leach into the groundwater because they cannot be absorbed by the soil, or they change into nitrogen gases that escape into Earth's atmosphere. So how does nitrogen change states from N2 in the air to these other states so that they are accessible by the Biosphere? Luckily there are specific kinds of microorganisms living in the soil that can convert gaseous forms of nitrogen into inorganic nitrogen that plants can use.

Specialized bacteria in soil (and certain types of algae in water) can fix nitrogen. These bacteria that cling to roots within the soil convert (or "fix") this inorganic nitrogen into organic forms (ammonia and nitrate ions) that plants can absorb. This process of converting nitrogen to a “biologically available” form - in other words, converting nitrogen gas to a form that plants can use - is referred to as nitrogen fixation. Lightning strikes also result in some nitrogen fixation by splitting the nitrogen molecule into free nitrogen, which immediately reacts with oxygen in the air to form nitrogen oxides. Some of these nitrogen oxide gases dissolve in rainwater and eventually percolate into the soil (Pedosphere). The nutrients needed for plant growth are drawn from the soil from the roots to the leaves. Therefore, any organism (including humans) consuming the nuts, leaves, seeds, roots, tubercles, or fruits of plants can digest this organic form of nitrogen. The Nitrogen Cycle is this process of moving nitrogen among plants, animals, bacteria, the atmosphere, and soil in the ground. This cycle is continuous.

Human activities have a large impact on global nitrogen cycles. In agriculture, soils are generally not rich enough in fixed nitrogen to sustain repetitive crop yields year after year; as a result, farmers use compost heaps or add industrially mass-produced fertilizers such as ammonium nitrate (containing high amounts of organic nitrogen), to enhance the soil.

The illustration above shows how nitrogen travels through the living and non-living parts of the Earth system. Credit: NCAR 

What Happens When Plants Don't Get Enough Nitrogen: 

Plants deficient in nitrogen have thin, spindly stems and their growth is stunted. Their older leaves turn yellowish-green from the lack of chlorophyll produced in the leaves (chlorosis), while newer leaves are supplied with the available nitrogen and sufficient chlorophyll.

What Happens When Plants Get Too Much Nitrogen: 

Plants that get too much nitrogen have a lot of foliage (leaf) growth but are not strong. Plants that are not strong can get diseases more easily, can be bothered more by bugs, and can eventually fall over and die. An excess amount of nitrogen in plants can affect the amount of sugar and vitamins in fruits and vegetables, making them taste different. More importantly, excess nitrogen can build up in plant tissues causing toxicity (poisoning) in livestock and in small children who eat nitrogen-rich, leafy vegetables. As we produce synthetic fertilizers, burn fossil fuels, grow legumes such as soybeans as a crop (which fix nitrogen), and clear, burn, and drain wetlands, we release nitrogen in forms that plants use.  We have made the amount of biologically available nitrogen through human activity much greater than the nitrogen fixed by bacteria, algae, and lightning.

The Nitrogen Cycle Processes:

• Fixation - Fixation is the first step in the process of making nitrogen usable by plants. Here bacteria change nitrogen into ammonium.

• Nitrification - This is the process by which ammonium gets changed into nitrates by bacteria. Nitrates are what the plants can then absorb.

• Assimilation - This is how plants get nitrogen. They absorb nitrates from the soil into their roots. Then the nitrogen gets used in amino acids, nucleic acids, and chlorophyll.

• Ammonification (or mineralization) - This is part of the decaying process. When a plant or animal dies, decomposers like fungi and bacteria turn the nitrogen back into ammonium so it can reenter the nitrogen cycle.

• Denitrification - Extra nitrogen in the soil gets put back out into the air. There are special bacteria that perform this task as well.

• Climate Program Office - NOAA

Nitrogen As a Pollutant in the Atmosphere

Nitrogen dioxide (NO2) is a gas that occurs naturally in our atmosphere, but in concentrations very low as compared to oxygen (O2) and nitrogen (N2).  NO2 is also common pollutant produced primarily during the combustion of gasoline in vehicle engines and coal in power plants. NO2 is part of a family of chemical compounds collectively called “nitrogen oxides” or “NOx”. Nitric oxide (NO) is also part of the NOx family. Together, NO and NO2 play important roles in the chemical formation of ozone near the Earth's surface, as well as contribute to smog when combined with oxygen molecules and the fumes from paint and gasoline (called Volatile Organic Compounds or VOC’s). These compounds also contribute to the production of acid rain when mixed with water vapor forming nitric acid.
 

Color bar for absolute nitrogen dioxide concentrations global images.

Using new, high-resolution global satellite maps of air quality indicators, NASA scientists tracked air pollution trends over the last decade in various regions and 195 cities around the globe. According to recent NASA research findings, the United States, Europe, and Japan have improved air quality thanks to emission control regulations, while China, India and the Middle East, with their fast-growing economies and expanding industry, have seen more air pollution.

Ozone occurs naturally in the air we breathe, but there's not enough of it to hurt us. Ozone high in the atmosphere (i.e., in the stratospheric “ozone layer”) protects us; it is like sunscreen, protecting us from harmful ultraviolet (UV) rays from the Sun. Near the ground though, ozone is a pollutant.  It damages our lungs and harms plants, including the plants we eat. Unhealthy levels of ozone form when there is a lot of NO2 in the air. NO2—and ozone—concentrations are usually highest in cities since NO2 is released into the atmosphere when we burn gas in our cars or coal in our power plants, both things that happen more in cities.

Nitrogen dioxide breaks apart in sunlight releasing free oxygen atoms to connect onto oxygen molecules forming dangerous ground-level ozone. Since sunlight is an important ingredient in the formation of high concentrations of ozone, ozone in urban areas tends to be greatest in summer when sunlight is strongest. NO2 is also unhealthy to breathe in high concentrations, such as on busy streets and highways where there are lots of cars and trucks. When driving, it is typically a good idea to keep the car windows rolled up and the car's ventilation set to “recirculate” so as to keep pollution out of the interior of the car. It is also important to reduce outdoor activities like playing or jogging if government officials warn you that air quality will be bad on a certain day.

Nitrous oxide (N20) is a powerful greenhouse gas, which traps heat near the Earth’s surface. You may have heard of it before, referred to as “laughing gas” which is a common medical treatment to decrease pain. It is produced almost entirely at the Earth's surface, about 70% from through natural processes in the Earth’s Biosphere (tiny microbes that alter nitrogen in the soils of tropical forests and in the oceans) and the rest from human activities (e.g. from farm animals, sewage, and fertilizers, as well as fossil-fuel burning). Quantities of nitrous oxide have increased since the Industrial Revolution in the Atmosphere as Earth’s climate has gotten warmer. scientists have observed an increase in N2O of about 0.3%/year since the 1950's.