Nitrogen is central to all known forms of life, serving as a fundamental building block for proteins and nucleic acids like DNA. Although it is the most abundant gas in the atmosphere (as dinitrogen, N2), this inert form is largely unusable by most organisms. It must first be converted into chemically accessible compounds through nitrogen fixation. The natural nitrogen cycle maintains a delicate balance, relying on three main microbial processes: fixation, nitrification, and denitrification. Fixation converts atmospheric N2 into ammonia, which is then transformed into nitrates by nitrification. Denitrification completes the cycle by returning nitrogen to the atmosphere in its gaseous form. Human intervention has dramatically accelerated the creation of reactive nitrogen (Nr), fundamentally altering global ecosystems and exceeding the planet’s natural processing capacity.
Main Human Activities That Create Reactive Nitrogen
The primary human activity producing excess reactive nitrogen is the industrial synthesis of ammonia for agricultural fertilizers. This process, known as the Haber-Bosch process, uses high heat and pressure to combine atmospheric N2 with hydrogen to form ammonia (NH3), a highly reactive compound. This invention enabled the mass production of synthetic fertilizers, which dramatically boosted crop yields and supported global population growth. Industrial fixation now generates over 100 teragrams (100 million metric tons) of nitrogen annually, rivaling the amount fixed by all natural terrestrial ecosystems combined.
A major source of reactive nitrogen is the combustion of fossil fuels in vehicles, power plants, and industrial furnaces. When fuels are burned at high temperatures, nitrogen and oxygen in the air react to form various nitrogen oxides (NOx). These compounds, such as nitric oxide (NO) and nitrogen dioxide (NO2), are chemically reactive forms of nitrogen released directly into the atmosphere. This inadvertent fixation contributes a significant portion of the reactive nitrogen burden, particularly in industrialized and urban areas.
Agricultural practices also accelerate the creation and release of reactive nitrogen beyond synthetic fertilizer use. Cultivating nitrogen-fixing crops, such as legumes like soybeans and clover, increases the rate of natural biological fixation. Concentrated livestock farming produces enormous amounts of manure, which contains high levels of nitrogen. The excess nitrogen from both synthetic fertilizers and animal waste often overwhelms the soil’s capacity to absorb it, leading to widespread environmental loss.
Impact on Water Bodies and Soil Health
Excess reactive nitrogen often enters aquatic environments through surface runoff and leaching into groundwater. This influx of nutrients drives eutrophication, the over-enrichment of water bodies. Nitrogen compounds, primarily nitrates and ammonium, fertilize aquatic plant life, particularly algae, leading to explosive growth known as algal blooms.
These blooms cloud the water, blocking sunlight and causing subaquatic vegetation to die. When the algae die, their decomposition consumes vast amounts of dissolved oxygen. This oxygen depletion creates hypoxic conditions, or “dead zones,” where most fish and aquatic life cannot survive. Coastal areas and large estuaries, such as the Gulf of Mexico, are severely impacted by these zones fed by inland agricultural runoff.
On land, excessive nitrogen fertilizer application disrupts the natural chemistry and balance of the soil. Large amounts of nitrogen compounds can lead to soil acidification over time. This change in pH causes the soil to release positively charged nutrient ions, such as calcium and magnesium. These essential cations are then leached out, reducing soil fertility and hindering healthy plant growth.
The continuous addition of nitrogen favors the growth of certain plant species that thrive in high-nitrogen conditions. This shift can crowd out native, slower-growing species, reducing the biodiversity of grasslands and forests. Nitrogen accumulation in groundwater creates a “legacy nitrogen” reservoir that can continue to leach into surface waters for decades, making restoration efforts challenging.
Nitrogen Compounds and Atmospheric Pollution
Gaseous forms of reactive nitrogen contribute to atmospheric pollution, affecting both air quality and global climate. Nitrous oxide (N2O) is a long-lived greenhouse gas primarily produced in agricultural soils when microbes convert excess nitrogen through nitrification and denitrification.
Although present in smaller concentrations than carbon dioxide, nitrous oxide is nearly 300 times more effective at trapping heat over a 100-year period. Agricultural soil management, including synthetic fertilizers and manure, accounts for the majority of human-caused N2O emissions. This output contributes to climate warming and plays a role in the destruction of the stratospheric ozone layer.
Nitrogen oxides (NOx) released from fossil fuel combustion drive the formation of ground-level ozone, a component of photochemical smog. This occurs when NOx reacts with volatile organic compounds (VOCs) in the presence of sunlight. Ground-level ozone is a respiratory irritant that causes health problems in humans and damages vegetation.
NOx also contributes to the formation of acid deposition, commonly known as acid rain. Once released, nitrogen dioxide reacts with water vapor and other chemicals to form nitric acid (HNO3). This acidic precipitation damages forests, acidifies lakes and streams, and corrodes buildings and infrastructure. Atmospheric deposition connects localized pollution sources with widespread ecosystem damage across entire regions.