The nitrogen cycle is a fundamental biogeochemical process that moves nitrogen through Earth’s atmosphere, land, and oceans. This cycle is essential for all life, as nitrogen is a building block of proteins and nucleic acids. For millennia, the natural nitrogen cycle maintained a delicate balance within ecosystems, but human activities have significantly altered this global process. These alterations have profound and widespread effects on both natural environments and human societies.
Understanding the Natural Nitrogen Cycle
Nitrogen gas (N2) makes up about 78% of Earth’s atmosphere, but in this form, it is unusable by most organisms. The process of nitrogen fixation converts atmospheric nitrogen into ammonia (NH3), a usable form. This conversion primarily occurs through biological fixation by certain bacteria in soil and root nodules of legumes, or by lightning strikes. Ammonia is then converted to nitrites (NO2-) and nitrates (NO3-) through nitrification, a two-step process by nitrifying bacteria.
Plants readily absorb nitrates from the soil, incorporating them into organic compounds (assimilation). When plants and animals die, decomposers break down their organic matter, releasing nitrogen back into the soil as ammonia through ammonification. Finally, denitrification completes the cycle, where bacteria convert nitrates back into atmospheric nitrogen gas. This natural balance ensures a continuous supply of usable nitrogen while also returning excess nitrogen to the atmosphere.
Key Human Activities Altering Nitrogen
Human activities have significantly accelerated the nitrogen cycle, by converting inert atmospheric nitrogen into reactive forms at an unprecedented scale. One of the most impactful interventions is industrial nitrogen fixation, particularly the Haber-Bosch process. This process, developed in the early 20th century, synthesizes ammonia from nitrogen and hydrogen gases under high temperatures and pressures, using a catalyst. This innovation enabled the mass production of synthetic fertilizers, crucial for global food production, supporting nearly half the world’s population. Annually, over 90 million metric tons of nitrogen fertilizer are produced this way, vastly exceeding natural nitrogen fixation rates.
The combustion of fossil fuels in vehicles, power plants, and industrial processes introduces reactive nitrogen into the atmosphere. This burning releases nitrogen oxides (NOx) that form when nitrogen and oxygen react at high temperatures. Global NOx emissions from fossil fuel burning are estimated at 33 teragrams per year, increasing rapidly in regions with growing fossil fuel use. These atmospheric nitrogen compounds eventually deposit onto land and water, altering natural ecosystems.
Agricultural practices extend beyond synthetic fertilizers in their impact. Manure from livestock, when not properly managed, can contribute nitrogen to the environment through volatilization and leaching. The cultivation of nitrogen-fixing crops, such as legumes, adds nitrogen to soils. While beneficial for soil health, widespread planting of these crops, especially in rotation, further increases fixed nitrogen in agricultural systems. Deforestation for agricultural expansion or other land uses releases nitrogen stored in soil and biomass, reducing the capacity of ecosystems to absorb nitrogen, leading to increased nitrogen runoff and gaseous emissions.
Finally, wastewater and sewage discharge are another source of human-derived nitrogen. Human and animal waste contains nitrogen compounds that, if not treated, are released into water bodies. Wastewater treatment plants utilize biological processes, including nitrification and denitrification, to remove nitrogen, but many facilities still release reactive nitrogen into the environment, contributing to nutrient loading in aquatic ecosystems.
Environmental Consequences of Altered Nitrogen
Excessive reactive nitrogen in the environment has serious consequences, particularly for water quality. Eutrophication occurs when excess nitrogen and phosphorus enter aquatic ecosystems. This overload stimulates rapid algal growth, leading to dense blooms. As these algal masses die and decompose, bacteria consume dissolved oxygen, creating hypoxic or anoxic “dead zones” where most aquatic life cannot survive. Nitrate contamination of drinking water is another health concern, particularly for infants, due to nitrogen runoff into groundwater and surface water sources.
Atmospheric pollution is another consequence of altered nitrogen cycling. Nitrogen oxides (NOx) released from fossil fuel combustion contribute to the formation of ground-level ozone, a harmful component of smog, especially in urban areas. These compounds contribute to acid rain (acid deposition), which can acidify soils and water bodies, damaging forests and aquatic life. Nitrous oxide (N2O), released from agricultural soils and industrial processes, is a potent greenhouse gas, trapping heat approximately 300 times more effectively than carbon dioxide over a 100-year period. It contributes significantly to climate change and stratospheric ozone depletion.
Changes in nitrogen availability affect soil health and ecosystem composition. Excess nitrogen can lead to soil acidification, altering nutrient balances and potentially harming soil microorganisms. This nutrient imbalance can favor fast-growing, nitrogen-loving plant species over others, reducing plant biodiversity and shifting ecosystem structure. In forested areas, chronic nitrogen additions can result in nitrogen saturation, where the ecosystem can no longer absorb nitrogen, leading to increased leaching losses and forest decline.