Urbanization reshapes natural landscapes on a global scale. This transformation profoundly influences various ecological systems, including the nitrogen cycle. The nitrogen cycle is a fundamental biogeochemical process that governs the movement of nitrogen through Earth’s atmosphere, land, and oceans. Understanding how urbanization interacts with the nitrogen cycle highlights how human activities reshape planetary processes.
The Natural Nitrogen Cycle
Nitrogen is an essential element for all life forms, playing a part in proteins and DNA. While atmospheric nitrogen (N2) makes up about 78% of the air, it is largely unusable by most organisms in this gaseous form. Specialized processes convert this inert gas into reactive forms that living things can absorb.
The nitrogen cycle begins with nitrogen fixation, where bacteria convert atmospheric N2 into ammonia (NH3), a compound plants can use. This ammonia then undergoes nitrification, where bacteria transform it into nitrites and subsequently into nitrates.
Plants absorb these nitrates and ammonium through assimilation, incorporating nitrogen into their tissues. When plants and animals die, decomposers facilitate ammonification, returning nitrogen from organic matter back into ammonia. The cycle concludes with denitrification, where specific bacteria convert nitrates back into atmospheric N2, completing the loop.
Sources of Urban Nitrogen
Urban areas introduce substantial amounts of reactive nitrogen into the environment through several human-derived activities. A primary source is the combustion of fossil fuels from vehicles, power plants, and industrial operations, which releases nitrogen oxides (NOx) into the atmosphere. These emissions can be 20-30% higher than those from other human activities combined.
Wastewater and sewage also contribute significantly, carrying high levels of nitrogen compounds like ammonia and nitrates. Another major contributor is the widespread use of synthetic fertilizers for lawns, gardens, and urban green spaces. Runoff and leaching from these fertilized areas can carry nitrogen into waterways.
Changes in land use alter soil nitrogen dynamics. Industrial activities within urban centers also release nitrogen compounds.
How Urbanization Alters Nitrogen Processes
The influx of nitrogen and infrastructure changes in urban environments directly modify the natural nitrogen cycle. Atmospheric deposition increases as NOx from combustion settles on land and water, altering the chemistry of soil and water bodies. Urban areas are recognized as hotspots for atmospheric nitrogen deposition, with rates significantly higher than surrounding rural areas.
Urbanization can also accelerate microbial processes like nitrification and denitrification. Altered soil moisture, temperature, and organic matter content in urban soils create conditions that promote these transformations. This can lead to increased emissions of nitrous oxide (N2O), a potent greenhouse gas, or greater nitrate leaching into water systems.
Impervious surfaces like roads and buildings increase stormwater runoff, which bypasses natural infiltration into soil. This runoff rapidly transports nitrogen from fertilizers, pet waste, and atmospheric deposition directly into rivers and lakes.
Urbanization often involves the removal of wetlands and native vegetation. These natural ecosystems play a role in absorbing and processing nitrogen, and their loss reduces the environment’s ability to manage nitrogen loads. Ultimately, these alterations can lead to nitrogen saturation, where ecosystems become overloaded and cannot absorb or use the excess nitrogen.
Ecological and Health Outcomes
The altered nitrogen cycle in urban areas leads to several environmental and human health consequences. Excess nitrogen in water bodies often causes eutrophication, triggering rapid growth of algae. These algal blooms deplete oxygen in the water as they decompose, creating “dead zones” that harm aquatic life in rivers, lakes, and coastal areas.
Nitrogen oxides contribute to air quality degradation by forming ground-level ozone and fine particulate matter. These pollutants contribute to smog and can worsen respiratory problems in humans.
When nitrogen oxides combine with water vapor in the atmosphere, they form nitric acid, leading to acid rain. Acid rain can acidify soils and water bodies, damaging forests and aquatic ecosystems.
Increased emissions of nitrous oxide (N2O) from urban soils contribute to climate change. N2O is a powerful greenhouse gas with a warming potential nearly 300 times that of carbon dioxide over a 100-year period. Changes in soil and water chemistry due to excess nitrogen can favor the growth of nitrogen-tolerant plant species, potentially reducing overall biodiversity by outcompeting native flora. Air pollution from nitrogen compounds contributes to respiratory illnesses, and high nitrate levels in drinking water can cause methemoglobinemia, commonly known as “blue baby syndrome,” in infants.