Nitrogen, a fundamental element for all known life forms, cycles naturally through Earth’s atmosphere, land, and water. It forms a crucial component of proteins, nucleic acids, and other organic molecules that drive biological processes. The natural nitrogen cycle involves processes like nitrogen fixation, nitrification, and denitrification, which maintain a delicate balance of nitrogen forms in ecosystems. This intricate system ensures nitrogen is converted into usable forms for plants and then returned to the atmosphere. Before significant human intervention, this cycle operated in a relatively closed loop, recycling nitrogen efficiently within ecosystems.
Human activities have profoundly altered this natural balance, significantly increasing the amount of reactive nitrogen introduced into the biosphere. Reactive nitrogen refers to forms of nitrogen that are biologically, chemically, or radiatively active, unlike the inert nitrogen gas that makes up most of the atmosphere. Over the past century, human actions have more than doubled the annual transfer of nitrogen into these biologically available forms. This substantial increase has far-reaching effects on environmental systems globally.
Agricultural Contributions
Agricultural practices are a leading source of human-added nitrogen to the environment. A major contributor is the widespread use of synthetic nitrogen fertilizers. These fertilizers are primarily produced through the Haber-Bosch process, an industrial method developed in the early 20th century. This process converts inert atmospheric nitrogen gas into ammonia under high temperature and pressure using an iron-based catalyst.
This industrial fixation annually produces over 90 million metric tons of nitrogen fertilizer, boosting global food production. The application of these fertilizers introduces vast quantities of reactive nitrogen into soils, which can then leach into water systems if not fully absorbed by crops.
Another significant agricultural source is the management of manure and livestock operations. Animal waste, particularly from concentrated animal feeding operations (CAFOs), contains substantial amounts of nitrogen. When manure is applied to land or stored, ammonia volatilization occurs, releasing nitrogen gas into the atmosphere. Runoff from these operations can also carry nitrogen compounds into waterways. More than half of the nitrogen content in manure can be lost as ammonia under warm, aerated conditions, impacting both air quality and aquatic ecosystems.
The cultivation of certain crops, particularly legumes, also contributes to nitrogen additions. Legumes, such as soybeans, alfalfa, and clover, have a symbiotic relationship with specific bacteria in their root nodules that can convert atmospheric nitrogen into a usable form, a process known as biological nitrogen fixation. While this natural process is beneficial for soil fertility, the large-scale cultivation of these crops in monocultures can introduce more fixed nitrogen into agricultural systems than would occur naturally.
Combustion and Energy Production
High-temperature burning processes, particularly those involving fossil fuels, release substantial amounts of nitrogen into the atmosphere. The combustion of fossil fuels like coal, oil, and natural gas in power plants, industrial facilities, and vehicles is a major source of nitrogen oxides (NOx) emissions. During these processes, high temperatures cause atmospheric nitrogen gas to react with oxygen, forming nitric oxide (NO) and nitrogen dioxide (NO2), collectively known as NOx. These compounds are then released into the air.
Thermal NOx, formed from atmospheric nitrogen and oxygen, is the dominant component of nitrogen oxides in most combustion chambers. Its formation rate increases significantly with temperature. These emissions contribute to air pollution and various environmental issues.
Biomass burning, including the combustion of forests, grasslands, and agricultural residues, also releases various nitrogen compounds into the atmosphere. This type of burning emits nitrogen oxides (NOx), ammonia (NH3), and nitrous oxide (N2O). While this nitrogen was originally absorbed by plants, human-induced biomass burning accelerates its release. Biomass burning contributes approximately 27% of total global NOx emissions and about 9% of global ammonia emissions.
Waste Management and Land Use Changes
Human activities related to waste management and land use also contribute significantly to the biosphere’s nitrogen load. Wastewater from human and animal sources contains nitrogen compounds like nitrates and ammonia. When inadequately treated or improperly disposed of, these compounds enter aquatic systems, increasing nitrogen levels. Excess nitrogen can cause eutrophication, an overgrowth of aquatic plants and algae that depletes oxygen and harms aquatic life.
Landfills, where organic matter decomposes, produce leachate. This liquid is rich in nitrogen, primarily in the form of ammonia, and can accumulate. If not properly managed, this leachate can contaminate both surface and groundwater.
Changes in land use, such as deforestation, urbanization, and wetland drainage, further impact the nitrogen cycle. Deforestation, for example, removes vegetation that would normally absorb nitrogen from the soil, leading to increased nitrogen leaching into water bodies. Urbanization and its associated activities, like stormwater runoff, can also release nitrogen into the environment. These land use changes can disrupt the natural soil nitrogen cycles, leading to the release of previously stored nitrogen into the atmosphere or water, altering the delicate balance of ecosystems.