Nitrogen is foundational to life, serving as a primary building block for DNA, amino acids, and proteins necessary for plant growth and crop yield. Modern agriculture relies heavily on synthetic nitrogen fertilizers to maximize productivity and feed billions of people. The application of these fertilizers introduces massive amounts of reactive nitrogen, fundamentally altering the planet’s natural nitrogen cycle and causing consequences that ripple through soil, water, and the atmosphere.
The Natural Nitrogen Cycle
The natural nitrogen cycle is a complex biogeochemical process that converts inert nitrogen gas ($\text{N}_2$), which makes up about 78% of the atmosphere, into forms that living organisms can use. This cycling is mediated by diverse communities of soil microbes. The process begins with nitrogen fixation, where specialized bacteria, such as Rhizobium, convert atmospheric $\text{N}_2$ into ammonia ($\text{NH}_3$), making it available for plants.
The cycle continues with ammonification, where bacteria and fungi break down organic matter to release nitrogen back into the soil as ammonium ($\text{NH}_4^+$). Next, nitrifying bacteria perform nitrification, converting ammonium first into nitrite ($\text{NO}_2^-$), and then quickly into nitrate ($\text{NO}_3^-$). Nitrate is the form of nitrogen most easily absorbed by plant roots, a process called assimilation.
Nitrogen is returned to the atmosphere through denitrification, performed by microbes that use nitrate instead of oxygen for respiration in low-oxygen conditions. These denitrifying bacteria convert the nitrate back into nitrogen gas ($\text{N}_2$) or nitrous oxide ($\text{N}_2\text{O}$), completing the loop. The introduction of synthetic fertilizers bypasses the slow, natural fixation process, disrupting this established equilibrium.
Immediate Soil Impacts of Fertilizer Application
Synthetic nitrogen fertilizer, often applied as ammonia or nitrate, floods the soil with readily available nutrients, creating an immediate chemical shift. This high concentration of nitrogen acts as a massive energy source for native soil microbial communities. The influx accelerates nitrification, as nitrifying bacteria rapidly convert the introduced ammonium into nitrate.
This rapid conversion releases hydrogen ions ($\text{H}^+$) as a byproduct, which leads to soil acidification over time. The resulting lower soil pH negatively impacts the availability of other nutrients and alters the composition of the native soil microbiota. Furthermore, the high concentration of soluble salts in the fertilizer can cause osmotic stress, harming the organisms that mediate the natural cycle. Excessive use of fertilizer changes the soil environment, sometimes selecting for different microbial populations.
Water Pollution from Nitrogen Runoff
When more nitrogen is applied than crops can absorb, the excess moves out of the soil and into aquatic ecosystems. This movement occurs through two primary pathways: surface runoff and leaching. Surface runoff carries dissolved nitrogen compounds into rivers, streams, and lakes, especially during heavy rainfall. Leaching involves the downward movement of soluble nitrate ($\text{NO}_3^-$) through the soil profile into groundwater, which eventually flows into surface water bodies.
Excessive nutrient loading results in eutrophication, the over-enrichment of water bodies by nitrogen and phosphorus compounds. This nutrient spike triggers the rapid growth of algae and phytoplankton, commonly called an algal bloom. When these populations die, their decomposition by bacteria consumes vast amounts of dissolved oxygen.
This process creates anoxic (zero oxygen) or hypoxic (low oxygen) conditions, which cannot support most aquatic life. These areas are referred to as “dead zones” because marine organisms cannot survive. A well-known example is the large, annually forming dead zone in the Gulf of Mexico, fueled by nitrogen runoff carried by the Mississippi River.
Atmospheric Emissions and Greenhouse Gases
The disruption of the nitrogen cycle results in the release of gaseous nitrogen compounds that impact the atmosphere. The primary concern is the production of nitrous oxide ($\text{N}_2\text{O}$), a powerful greenhouse gas. While naturally produced during nitrification and denitrification, synthetic fertilizer application significantly increases its production.
Nitrous oxide has a warming potential approximately 300 times greater than carbon dioxide ($\text{CO}_2$) over a 100-year period, contributing significantly to global warming. $\text{N}_2\text{O}$ is a long-lived gas that persists in the atmosphere for an average of 114 years. Once it reaches the stratosphere, nitrous oxide also acts as an ozone-depleting substance, contributing to the thinning of the ozone layer.
Nitrogen fertilizer use also leads to the emission of nitrogen oxides ($\text{NO}_x$) during manufacturing and from volatilization following application. These compounds are precursors to ground-level ozone, a component of smog. They also contribute to acid rain, demonstrating the atmospheric consequences of excess nitrogen use.