Nitrogen is a fundamental element for all life, serving as a building block for proteins, nucleic acids, and other organic molecules. Its availability often limits biological productivity, driving agricultural practices to supplement natural nitrogen levels. Fertilizers, particularly those rich in nitrogen, are widely applied to enhance crop yields. This practice significantly influences the Earth’s natural nitrogen cycle.
The Natural Nitrogen Cycle Explained
The natural nitrogen cycle describes the continuous movement of nitrogen through the atmosphere, soil, and living organisms. Atmospheric nitrogen (N2) makes up about 78% of the air but is largely unreactive and unusable by most organisms. Nitrogen fixation, an initial step, converts atmospheric N2 into ammonia (NH3) or ammonium (NH4+), primarily through bacteria in soil or associated with plant roots. These nitrogen-fixing microorganisms possess specialized enzymes that break the strong triple bond in N2.
Once in the soil, ammonium undergoes nitrification, a two-step process carried out by different groups of bacteria. First, nitrifying bacteria convert ammonium into nitrite (NO2-), and then other bacteria oxidize nitrite into nitrate (NO3-). Nitrate is a highly mobile form of nitrogen that plants readily absorb and assimilate into their tissues. This assimilation integrates inorganic nitrogen into organic compounds within living organisms.
Nitrogen then moves through the food web as organisms consume plants or other animals. When plants and animals die, or when animals excrete waste, decomposers like bacteria and fungi break down organic nitrogen compounds. This process, known as ammonification, releases ammonium back into the soil, making it available for nitrification or direct plant uptake. Finally, denitrification completes the cycle, where specific anaerobic bacteria convert nitrate back into gaseous nitrogen (N2) or nitrous oxide (N2O), which then returns to the atmosphere. These microbial processes ensure nitrogen’s continuous cycling.
Fertilizers as Nitrogen Inputs
Fertilizers serve as concentrated sources of nutrients, including nitrogen, applied to agricultural lands to boost crop growth. These can be broadly categorized into synthetic and organic types. Synthetic nitrogen fertilizers are industrially manufactured, primarily through the Haber-Bosch process, which converts atmospheric nitrogen gas into ammonia under high temperature and pressure. Common examples include urea, ammonium nitrate, and diammonium phosphate, all designed to deliver readily available nitrogen to plants.
Organic fertilizers are derived from natural sources such as animal manure, compost, and plant residues. The nitrogen in these materials is typically in organic forms, like proteins and amino acids, which are not immediately available for plant uptake. Soil microorganisms must first break down these complex organic compounds through ammonification, converting them into ammonium and other inorganic forms that plants can absorb. Both synthetic and organic fertilizers introduce significant amounts of nitrogen into the soil system, supplementing natural nitrogen pools.
How Fertilizers Alter Nitrogen Cycle Processes
The introduction of large quantities of nitrogen through fertilizers significantly alters the natural balance and rates of processes within the nitrogen cycle. When synthetic nitrogen fertilizers like urea or ammonium nitrate are applied, they rapidly increase the concentration of ammonium and nitrate in the soil. This sudden influx often overwhelms the soil’s natural capacity to process and retain nitrogen. An elevated ammonium concentration directly stimulates nitrifying bacteria, accelerating the rate of nitrification.
This accelerated nitrification leads to a higher production of nitrate in the soil. Unlike ammonium, nitrate carries a negative charge and is therefore highly soluble and prone to leaching through the soil profile. The increased availability of nitrate can also enhance denitrification rates, particularly in waterlogged or anaerobic soil conditions. This means more nitrate is converted back into gaseous nitrogen forms like N2 and nitrous oxide (N2O), which then escape into the atmosphere. The sudden abundance from fertilizers can shift microbial activity and composition.
Environmental Consequences of Nitrogen Imbalance
The alterations to the nitrogen cycle due to fertilizer use lead to several significant environmental consequences. One primary concern is nitrate leaching, where excess nitrate moves downward through the soil profile and enters groundwater. This can contaminate drinking water sources, posing health risks to humans, particularly infants, due to a condition known as methemoglobinemia, or blue baby syndrome. Nitrate can also be transported through runoff into surface waters, including rivers, lakes, and coastal areas.
The influx of excess nitrogen into aquatic ecosystems fuels a process called eutrophication. This involves the rapid growth of algae and aquatic plants, often forming dense algal blooms that block sunlight from reaching submerged vegetation. As these blooms die and decompose, bacteria consume large amounts of dissolved oxygen, creating hypoxic or anoxic “dead zones” where marine life cannot survive. A notable example is the large dead zone that forms annually in the Gulf of Mexico, largely driven by nitrogen runoff from the Mississippi River basin.
Another substantial environmental impact is the emission of nitrous oxide (N2O) into the atmosphere. As a potent greenhouse gas, N2O has a global warming potential approximately 265 times greater than carbon dioxide over a 100-year period, contributing significantly to climate change. Increased denitrification rates in fertilized soils are a major source of these emissions. Additionally, ammonia volatilization, where ammonium from fertilizers converts to gaseous ammonia (NH3) and escapes into the atmosphere, contributes to air pollution. Atmospheric ammonia can react with other pollutants to form fine particulate matter, which poses respiratory health risks, and can also contribute to acid rain when deposited back onto land and water.