Nitrogen serves as a fundamental building block for all living organisms, playing an important role in DNA, proteins, and plant growth. It is naturally abundant, comprising about 78% of Earth’s atmosphere. While nitrogen is essential, an excess of this element, particularly originating from soil, poses a significant environmental challenge to aquatic ecosystems. This surplus nitrogen can disrupt the delicate balance of water bodies, leading to various forms of pollution.
Sources of Nitrogen Accumulation in Soil
Human activities primarily drive the excess accumulation of nitrogen in soil. Agricultural practices contribute significantly, as synthetic fertilizers and animal manure introduce nitrogen compounds into the soil beyond what crops can fully absorb. Within the soil, microorganisms convert ammonium (NH4+) into nitrate (NO3-) through nitrification. Nitrate is highly mobile and does not bind well to soil particles, making it susceptible to movement. While some nitrogen returns to the atmosphere as gas through denitrification, this process is often incomplete, leaving excess nitrogen in the soil.
Atmospheric deposition also adds nitrogen to soil. Nitrogen oxides (NOx) from fossil fuel combustion and ammonia (NH3) from agricultural emissions settle onto land, contributing to the soil’s nitrogen load. Wastewater and septic systems also release nitrogen into surrounding soil environments. Improperly managed or failing systems allow nitrogen compounds, such as ammonium and nitrate, to seep into the ground, increasing soil nitrogen levels.
Pathways of Nitrogen Movement to Water
Nitrogen moves from the soil into aquatic environments through several processes. Leaching is a primary pathway, where soluble nitrogen, predominantly nitrate, dissolves in water from rain or irrigation. Because nitrate carries a negative charge, it does not readily adhere to soil particles, allowing it to move freely with infiltrating water. This water then seeps downwards through the soil profile into groundwater. Groundwater often connects to surface water bodies like rivers, lakes, and oceans, carrying dissolved nitrate. Factors like heavy rainfall, excessive irrigation, and permeable sandy soils increase the potential for nitrate leaching.
Surface runoff is another significant transport mechanism. During rainfall or irrigation, water flowing over the land picks up nitrogen, including dissolved compounds and particulate nitrogen attached to eroded soil particles. This runoff directly carries nitrogen into nearby rivers, lakes, and coastal waters.
Subsurface drainage systems, such as agricultural tile drains, expedite nitrogen movement from fields into waterways. These systems remove excess water from the soil but can inadvertently channel nitrogen directly into drainage ditches and streams. This bypasses natural filtration processes in the soil, allowing nitrogen to enter aquatic environments more rapidly.
Consequences for Aquatic Ecosystems
Once excess nitrogen enters aquatic environments, it triggers ecological impacts. Eutrophication is a common outcome, where surplus nitrogen acts as a nutrient, stimulating rapid growth of algae and other aquatic plants. These dense growths are often referred to as algal blooms.
Following these blooms, dead algae and plants sink and decompose. This decomposition, carried out by bacteria, consumes vast amounts of dissolved oxygen. This leads to low oxygen (hypoxia) or no oxygen (anoxia), creating “dead zones” where most aquatic life cannot survive.
Excess nitrogen can also fuel the proliferation of specific types of algae that produce toxins, resulting in harmful algal blooms (HABs). These toxins can be detrimental to aquatic animals and pose health risks to humans and pets consuming contaminated water or seafood. These disruptions alter aquatic food webs, reduce species diversity, and disrupt the balance of freshwater and marine ecosystems.