The nitrogen cycle governs the movement of nitrogen through Earth’s atmosphere, land, and oceans. This cycle is essential for all life, as nitrogen is a basic component of proteins, nucleic acids, and other organic molecules. For millennia, natural processes maintained a delicate balance.
The Natural Nitrogen Cycle
Nitrogen gas (N2) comprises about 78% of Earth’s atmosphere, but most organisms cannot directly utilize it. Nitrogen fixation converts atmospheric N2 into reactive forms like ammonia (NH3) or ammonium (NH4+), primarily through specialized microorganisms. These microbes, residing in soil or in symbiotic relationships with plants, make nitrogen biologically available.
Ammonia and ammonium are transformed through nitrification, a two-step process by bacteria. First, ammonia is oxidized to nitrite (NO2-), and then nitrite is further oxidized to nitrate (NO3-). Plants absorb nitrate and ammonium from the soil through assimilation, incorporating them into their tissues to build organic compounds.
When plants and animals decompose, ammonification occurs, converting organic nitrogen back into ammonia or ammonium. The final stage, denitrification, returns nitrogen gas to the atmosphere. Denitrifying bacteria convert nitrates back into gaseous N2, completing the cycle and maintaining the atmospheric nitrogen reservoir.
Human Activities Increasing Reactive Nitrogen
Human activities have significantly altered the natural nitrogen cycle by increasing reactive nitrogen. Industrial nitrogen fixation, primarily the Haber-Bosch process, is the largest human input. This process synthesizes ammonia for synthetic fertilizers and now rivals or exceeds natural nitrogen fixation globally.
Agricultural practices also increase reactive nitrogen. Synthetic fertilizers introduce large quantities of ammonium and nitrate into soils, often exceeding crop uptake. Cultivating nitrogen-fixing crops like soybeans and alfalfa enhances natural fixation. Livestock manure also adds to the nitrogen load.
Combustion of fossil fuels in vehicles, power plants, and industrial facilities releases nitrogen oxides (NOx) into the atmosphere. These gases form when nitrogen and oxygen react at high temperatures, creating new reactive nitrogen compounds. Atmospheric deposition can carry these compounds far from their source.
Land-use changes also alter the nitrogen cycle. Deforestation reduces forests’ capacity to cycle nitrogen, while urbanization introduces nitrogen from wastewater and runoff. These activities lead to an unprecedented accumulation of reactive nitrogen.
Impacts on Aquatic and Terrestrial Ecosystems
Excess reactive nitrogen impacts aquatic ecosystems, causing eutrophication. This over-enrichment of water bodies triggers rapid algal growth, known as algal blooms. As blooms decompose, bacteria consume oxygen, creating hypoxic “dead zones” where marine life cannot survive. The Gulf of Mexico’s recurrent dead zone is fueled by nutrient runoff from the Mississippi River basin.
Elevated nitrogen levels also contaminate drinking water. Nitrates from agricultural runoff and wastewater leach into groundwater and surface water. High nitrate concentrations pose health risks, especially for infants, causing methemoglobinemia (“blue baby syndrome”), which impairs oxygen transport. Many regions struggle to meet drinking water standards due to nitrate pollution.
Excessive nitrogen deposition also acidifies terrestrial soils. Deposited nitrogen compounds convert to nitric acid, lowering soil pH. This acidification mobilizes toxic metals, leaches essential nutrients, and reduces phosphorus availability, affecting plant growth and soil health. Such changes alter plant communities, favoring nitrogen-loving species.
Chronic nitrogen enrichment changes biodiversity by disrupting the competitive balance among plant species. Fast-growing, nitrogen-responsive species outcompete slower-growing native plants, reducing diversity in sensitive habitats. Soil microbial communities are also impacted, altering nutrient cycling and decomposition. These shifts can have cascading effects throughout the food web.
Atmospheric and Climate Implications
The altered nitrogen cycle also has significant atmospheric and climate implications. Nitrous oxide (N2O), a potent greenhouse gas, is released from agricultural soils through denitrification and nitrification, especially after fertilizer application. N2O has a global warming potential 265 times greater than carbon dioxide over 100 years. It also contributes to stratospheric ozone depletion.
Nitrogen oxides (NOx) from fossil fuel combustion contribute to ground-level ozone (smog) formation. Ground-level ozone is a harmful air pollutant that damages lung tissue, exacerbates respiratory diseases, and harms vegetation. NOx emissions also cause acid rain. When nitrogen oxides react with atmospheric water, oxygen, and other chemicals, they form nitric acid. This acid falls as acid rain, acidifying lakes and streams, damaging forests, and corroding infrastructure.