Nitrogen is an essential component of protein and required by all living organisms including human beings. Nitrogen is needed for our DNA, RNA and proteins and is critical to human agriculture. Nitrogen, a component of proteins and nucleic acids, is essential to life on Earth.

Although 78 percent by volume of the atmosphere is nitrogen gas, this abundant reservoir exists in a form unusable by most organisms. Through a series of microbial transformations, however, nitrogen is made available to plants, which in turn ultimately sustain all animal life.

• Nitrogen fixation, in which nitrogen gas is converted into inorganic nitrogen compounds, is mostly (90 percent) accomplished by certain bacteria and blue-green algae (see nitrogen fixation). A much smaller amount of free nitrogen is fixed by abiotic means (e.g., lightning, ultraviolet radiation, electrical equipment) and by conversion to ammonia through the Haber-Bosch process.

• Nitrates and ammonia resulting from nitrogen fixation are assimilated into the specific tissue compounds of algae and higher plants. Animals then ingest these algae and plants, converting them into their own body compounds.

• The remains of all living things and their waste products are decomposed by microorganisms in the process of ammonification, which yields ammonia. (Under anaerobic, or oxygen-free, conditions foul-smelling putrefactive products may appear, but they too are converted to ammonia in time.) Ammonia can leave the soil or be converted into other nitrogen compounds, depending in part on soil conditions.

• Nitrification, a process carried out by nitrifying bacteria, transforms soil ammonia into nitrates, which plants can incorporate into their own tissues.

• Nitrates also are metabolized by denitrifying bacteria, which are especially active in water-logged, anaerobic soils. The action of these bacteria tends to deplete soil nitrates, forming free atmospheric nitrogen.

Human Impact on the Nitrogen Cycle:

• Humans have contributed significantly to the nitrogen cycle by artificial nitrogen fertilization (primarily through the Haber process, using energy from fossil fuels to convert N2 to ammonia gas (NH3) and planting of nitrogen fixing crops. In addition, humans have significantly contributed to the transfer of nitrogen gases from Earth to the atmosphere.
• N2O has risen in the atmosphere as a result of agricultural fertilization, biomass burning, cattle and feedlots, and other industrial sources. N2O has deleterious effects in the stratosphere, where it breaks down and acts as a catalyst in the destruction of atmospheric ozone. NH3 in the atmosphere has tripled as the result of human activities.
• It is a reactant in the atmosphere, where it acts as an aerosol, decreasing air quality and clinging on to water droplets, eventually resulting in acid rain. Fossil fuel combustion has contributed to a 6 or 7 fold increase in NxOx flux to the atmosphere. NxOx actively alters atmospheric chemistry, and is a precursor of tropospheric (lower atmosphere) ozone production, which contributes to smog, acid rain, and increases nitrogen inputs to ecosystems.
• Ecosystem processes can increase with nitrogen fertilization, but anthropogenic input can also result in nitrogen saturation, which weakens productivity and can kill plants. Decreases in biodiversity can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degradation of nitrogen-poor, species diverse heath lands.