The nitrogen cycle governs the movement of nitrogen through the atmosphere, soil, water, and living organisms. This element is a component of all amino acids, proteins, and the nucleic acids that form DNA. Although nitrogen gas (\(\text{N}_{2}\)) makes up approximately 78% of the atmosphere, it exists in an unreactive state that most organisms cannot use directly. For life to access this abundant reservoir, it must be converted into biologically available forms, a process that historically occurred at a slow, natural pace. Human activities have dramatically accelerated and altered the rate and pathways of this conversion, leading to a significant global disruption of the planet’s nitrogen balance.
How Nitrogen Moves Naturally
The natural cycle is regulated primarily by specialized microorganisms that convert nitrogen into usable compounds and back again. The process begins with nitrogen fixation, where certain bacteria convert atmospheric \(\text{N}_{2}\) into ammonia (\(\text{NH}_{3}\)) or ammonium (\(\text{NH}_{4}^{+}\)), the first forms accessible to plants. Lightning also contributes a small amount of fixed nitrogen in the form of nitrates.
Following fixation, a two-step process called nitrification occurs, where different bacteria first convert ammonia into nitrites, and then into nitrates (\(\text{NO}_{3}^{-}\)), which are easily assimilated by plants. When plants and animals die, decomposers convert the organic nitrogen back into ammonia through ammonification. The cycle is completed by denitrification, a process carried out by other bacterial species that convert nitrates back into \(\text{N}_{2}\) gas, releasing it into the atmosphere and closing the loop.
Agricultural Fertilizer Overload
The industrial production of synthetic fertilizers is a major human alteration to the nitrogen cycle. The Haber-Bosch process, developed in the early 20th century, allows for the mass synthesis of ammonia by combining atmospheric nitrogen and hydrogen under intense heat and pressure. This method artificially bypasses the slow, natural bacterial fixation step, creating vast quantities of biologically available nitrogen used in fertilizers like urea and ammonium nitrate. This industrial process now fixes more atmospheric nitrogen than all natural terrestrial processes combined.
Excess nitrogen from these fertilizers is often not absorbed by crops and is lost from agricultural fields through runoff and leaching. The water-soluble forms, particularly nitrates, flow into streams, rivers, and coastal waters. This influx of nutrients drives eutrophication, causing explosive growth of algae, or algal blooms.
When these algal blooms die, their decomposition by bacteria consumes vast amounts of dissolved oxygen in the water. This oxygen depletion creates hypoxic zones, commonly known as oceanic “dead zones,” where marine life cannot survive. The dead zone in the Gulf of Mexico, fueled by agricultural runoff from the Mississippi River basin, exemplifies these consequences.
Emissions from Burning Fossil Fuels
The combustion of fossil fuels in vehicles and power plants injects nitrogen oxides (\(\text{NO}_{\text{x}}\)) directly into the atmosphere. This occurs because the high temperatures generated during combustion cause the normally inert nitrogen (\(\text{N}_{2}\)) and oxygen (\(\text{O}_{2}\)) gases in the air to chemically react. This reaction forms nitric oxide (\(\text{NO}\)) and nitrogen dioxide (\(\text{NO}_{2}\)), collectively termed \(\text{NO}_{\text{x}}\).
These airborne nitrogen compounds contribute to two distinct atmospheric pollution problems. Nitrogen oxides react with volatile organic compounds (VOCs) in the presence of sunlight to form ground-level ozone, a primary component of smog that poses respiratory health risks. They also react with water and oxygen in the atmosphere to form nitric acid.
When this nitric acid is deposited on the Earth’s surface through rain, snow, or fog, it is known as acid deposition or acid rain. This deposition can acidify soils and freshwater bodies, damaging forests and aquatic ecosystems. The atmospheric deposition of \(\text{NO}_{\text{x}}\) also provides an additional source of nitrogen that contributes to nutrient overloading and eutrophication in distant water bodies.