Nitrogen and phosphorus are fundamental chemical elements found throughout nature, serving as building blocks for all known life. When these nutrients enter ecosystems in excessive quantities, their natural balance can be severely disturbed, leading to significant environmental problems. Their dual nature highlights a delicate equilibrium between promoting life and causing harm.
Essential Roles in Living Organisms
Nitrogen is a core component of amino acids, the building blocks of proteins. Proteins carry out many functions within organisms, from building structural tissues like muscle and hair to acting as enzymes that facilitate biochemical reactions. Nitrogen is also part of nucleic acids (DNA and RNA), holding and transmitting genetic information. In plants, nitrogen is important for the synthesis of chlorophyll, the molecule that captures sunlight for photosynthesis.
Phosphorus plays an important role in living systems. It is a backbone component of DNA and RNA molecules, providing structural integrity. Phosphorus is also found in phospholipids, the primary constituents of cell membranes, regulating cell passage. It is an element in adenosine triphosphate (ATP), the “energy currency” of the cell, powering cellular processes. In animals, phosphorus contributes to the formation of bones and teeth.
The Natural Cycles
Nitrogen moves through the environment in a series of transformations known as the nitrogen cycle. Atmospheric nitrogen gas (N2), which makes up about 78% of the air, is unusable by most organisms. Specialized bacteria (in soil and legume roots) and lightning convert it into ammonia (NH3) via nitrogen fixation. Other bacteria convert ammonia into nitrites (NO2-) and nitrates (NO3-) through nitrification.
Plants absorb these usable forms of nitrogen from the soil and incorporate them into their tissues via assimilation. Animals obtain nitrogen by consuming plants or other animals. When organisms die or excrete waste, decomposers break down the organic nitrogen compounds back into ammonia via ammonification. Finally, denitrifying bacteria convert nitrates back into nitrogen gas, releasing it into the atmosphere and completing the cycle, often under oxygen-deficient conditions.
The phosphorus cycle differs from the nitrogen cycle because phosphorus does not have a substantial gaseous phase in the atmosphere. It moves primarily through the Earth’s crust, water bodies, and living organisms. The cycle begins with the weathering of rocks, where weathering breaks down minerals, releasing phosphates into the soil and water.
Plants absorb these phosphates from the soil or water. It then moves through the food web. When plants and animals die or excrete waste, decomposers return organic phosphorus to the soil or water. Over long geological timescales, phosphorus can settle in water bodies as sediments, forming new rocks, making the cycle much slower than the nitrogen cycle.
Human Impact and Environmental Consequences
Human activities have increased the amount of nitrogen and phosphorus entering natural ecosystems, disrupting their natural cycles. A primary source of this overload is agricultural runoff, where excess chemical fertilizers and animal manure are washed into waterways by rain or irrigation. Wastewater discharge from municipal sewer and septic systems, especially those without advanced treatment, also contributes these nutrients. Stormwater runoff from urban areas carries nutrients from lawns and impervious surfaces into local water bodies, and the burning of fossil fuels releases reactive nitrogen into the atmosphere, depositing into aquatic systems.
This nutrient influx leads to eutrophication, impacting aquatic environments. When high concentrations of nitrogen and phosphorus enter a lake, river, or coastal area, they act as a fertilizer, causing an overgrowth of algae and phytoplankton, known as an algal bloom. These dense surface blooms block sunlight from reaching underwater plants, causing them to die. As the bloom dies, the dead organic matter sinks to the bottom.
Decomposing bacteria consume much dissolved oxygen from the water. This high oxygen consumption creates areas of low oxygen, known as hypoxic zones, where dissolved oxygen levels fall below approximately 2 milligrams per liter. In severe cases, oxygen can be completely depleted, creating anoxic zones. These low-oxygen conditions make it impossible for most aquatic organisms to survive, leading to the formation of “dead zones.”
An example of this phenomenon is the Gulf of Mexico dead zone, the largest hypoxic zone in the United States. This zone forms annually off the coast of Louisiana and Texas, fueled by nutrient runoff from the Mississippi River Basin, which drains agricultural lands across the Midwest. The size of this dead zone varies seasonally, expanding in the spring and summer when agricultural runoff is highest and water stratification is strong, and shrinking in the fall.
Managing Nutrient Pollution
Addressing nutrient pollution requires a multi-faceted approach, focusing on reducing nutrient input. In agriculture, precision fertilization applies nutrients in the right amounts, at the correct time and placement, minimizing runoff. Farmers also plant cover crops to maintain year-round ground cover, preventing soil erosion and nutrient loss. Establishing buffer strips along the edges of fields, especially those bordering waterways, filter out nutrients before they reach aquatic systems.
Urban areas and wastewater treatment facilities also play an important role in mitigating nutrient pollution. Modern wastewater treatment plants are being upgraded to include advanced processes, such as enhanced biological phosphorus removal (EBPR) or chemical precipitation, to remove phosphorus. For nitrogen removal, processes like advanced denitrification or membrane bioreactors are employed to convert nitrogen compounds into inert nitrogen gas before discharge. Green infrastructure solutions like permeable pavements, green roofs, and rain gardens are used to capture, treat, and infiltrate stormwater runoff, reducing nutrient flow into local waterways from urban landscapes. Individuals can also contribute by using lawn fertilizers responsibly and disposing of pet waste, reducing their household’s nutrient footprint.