Nitrogen is fundamental to all life, forming the structure of proteins, DNA, and chlorophyll. Although abundant in the atmosphere as an inert gas, plants cannot use it in that form, making nitrogen often the nutrient that limits crop growth. To meet global population demands, the Haber-Bosch process was developed to synthesize reactive nitrogen compounds, such as ammonia, for agricultural fertilizers. The application of these synthetic fertilizers has dramatically increased crop yields worldwide. However, this human intervention alters the planet’s natural nitrogen cycle by bypassing the slow, natural processes that historically governed nitrogen availability.
Understanding the Natural Nitrogen Cycle
The natural nitrogen cycle is a complex, microbial-driven process that converts inert atmospheric nitrogen into reactive forms. The cycle begins with nitrogen fixation, where specialized bacteria convert atmospheric nitrogen gas (\(\text{N}_{2}\)) into ammonia (\(\text{NH}_{3}\)) or ammonium (\(\text{NH}_{4}^{+}\)), making it usable by plants. This energy-intensive process is the rate-limiting step for nitrogen availability in most natural ecosystems.
Nitrogen moves through the food web via assimilation. When organisms die or produce waste, the nitrogen returns to the soil as organic matter. Ammonification then occurs, where microorganisms convert this organic nitrogen back into ammonium.
The ammonium is processed through nitrification, a two-step sequence performed by soil bacteria. Nitrosomonas bacteria oxidize ammonium into nitrite (\(\text{NO}_{2}^{-}\)), and Nitrobacter bacteria convert the nitrite into nitrate (\(\text{NO}_{3}^{-}\)). The cycle concludes with denitrification, where anaerobic bacteria convert nitrate back into nitrogen gas (\(\text{N}_{2}\)), releasing it back into the atmosphere.
How Fertilizers Accelerate Soil Nitrogen Processes
Synthetic nitrogen fertilizers, such as urea, ammonium nitrate, or anhydrous ammonia, bypass the slow, energy-intensive fixation step. These products deliver an influx of highly bioavailable nitrogen, primarily as ammonium (\(\text{NH}_{4}^{+}\)) or a precursor that rapidly converts to it. This sudden abundance of substrate overwhelms the soil’s microbial communities.
The introduction of large amounts of ammonium accelerates the nitrification process. Ammonia-oxidizing bacteria respond to this excess substrate by rapidly converting ammonium into nitrate (\(\text{NO}_{3}^{-}\)). This hyper-nitrification generates more nitrate than the vegetation can absorb for growth.
Crops often utilize less than 50% of the applied fertilizer. The resulting surplus of nitrate is highly mobile because it carries a negative charge, preventing it from binding to the negatively charged clay and organic matter in the soil. This creates a soluble pool of excess nitrate primed for loss from the agricultural system.
The Movement of Excess Nitrogen into Water Systems
Unbound nitrate (\(\text{NO}_{3}^{-}\)) in the soil is easily transported out of the field through two main pathways. The first is leaching, where nitrate dissolves in water and moves downward into groundwater aquifers. High nitrate concentrations in drinking water are a public health concern, especially for infants, as it can interfere with the blood’s ability to carry oxygen (methemoglobinemia).
The second pathway is surface runoff, where rain or irrigation water washes dissolved nitrate into streams, rivers, and coastal zones. This influx of excess nitrogen drives eutrophication, or nutrient over-enrichment, in these receiving bodies of water. Nitrogen acts as a fertilizer for aquatic primary producers, causing explosive growth of algae and cyanobacteria, known as algal blooms.
When these dense blooms die, their decomposition by aerobic bacteria consumes dissolved oxygen in the water. This leads to a severe reduction in oxygen levels, known as hypoxia. Hypoxia creates “dead zones” where most fish and marine life cannot survive.
Atmospheric Effects and Nitrous Oxide Emissions
The excess nitrogen pool in the soil contributes to atmospheric pollution. When over-fertilized soils become saturated with water, they become anaerobic (oxygen-poor). In these environments, accelerated denitrification occurs as microbes use surplus nitrate instead of oxygen for respiration.
Instead of fully converting nitrate back to harmless \(\text{N}_{2}\) gas, this process frequently produces nitrous oxide (\(\text{N}_{2}\text{O}\)), which is released into the atmosphere. Nitrous oxide is a potent, long-lived greenhouse gas, possessing a global warming potential hundreds of times greater than carbon dioxide. Furthermore, \(\text{N}_{2}\text{O}\) is an ozone-depleting substance in the stratosphere.
Ammonia volatilization (\(\text{NH}_{3}\)) is another consequence, occurring when nitrogen fertilizer, particularly urea, is applied to the soil surface. Under warm, alkaline conditions, ammonium converts to ammonia gas and escapes into the air. This airborne ammonia can travel long distances before deposition, where it acts as a precursor to acid rain and contributes to fine particulate matter, posing a risk to human respiratory health.