Core Concepts
Nitrogen is a key nutrient essential for all life. This article explores the significance of nitrogen for our ecosystem and the potential adverse effects of having excess within an ecosystem. Lastly, this article will explain how nitrogen travels from a landscape to nearby surface water and groundwater.
Introduction
Nitrogen is essential for all living organisms. It has a few different forms depending on the surrounding environment. Plants are the most common source of nitrogen for other animals (such as humans and livestock). The common form of nitrogen available for plant uptake are ammonium and nitrate. Ammonium has the chemical formula NH4+ and nitrate has the chemical formula NO3–. Nitrate is typically more common in soils because of the presence of oxygen. In aerobic (oxygen-containing) soils, ammonium is typically converted to nitrate. It is a dominant source of nitrogen for plants and is necessary for growth. Nitrogen predominately enters the soil through chemical fertilizers, manure applications, organic matter decomposition, and nitrogen-fixing bacteria. NO3– is primarily produced by soil microorganisms through a process called nitrification. This process is key for converting nitrogen into a plant available form. Nitrification is when ammonia is oxidized into nitrite and nitrate. Due to nitrate’s stability, it is found more abundantly than nitrite. The oxidation of nitrite to nitrate is driven by soil microorganisms.
Balancing Productivity with Sustainability
Although, nitrate is essential for plants, there is a balance between the need for plant growth and the risks of pollution. Excess nitrate in the environment can create problems for nearby bodies of water including both surface water and groundwater. Nitrate serves as a key nutrient for aquatic plants, specifically, algae. If nitrate exists in a body of water in high amounts, rapid algae growth can occur. This rapid algae growth actually causes negative side effects for the health of the environment. Algae has a relatively short life span of a few days to a couple months and when this algae starts to decompose, oxygen levels decrease. Essentially, the algae has used up all the available dissolved oxygen in the water. Microbes use oxygen in order to decompose algae and if a large amount of algae has grown, a large amount of the oxygen in the body of water will be used to break it down. This is referred to as hypoxia and can lead to fish kills, who also rely on dissolved oxygen to survive. A recent example of a large fish kill caused by algal bloom happened in Iowa in March 2024. A nitrogen fertilizer spill led to a fish kill of nearly 750,000 fish.
There are also side effects for human health if nitrate enters surface or groundwater resources used for drinking water supplies. Nitrate levels in drinking water can affect all ages of the population. It can affect babies through blue baby syndrome, a condition that leads to a lack of oxygen in the blood. It can cause birth defects, or increase the risk of cancer. Nitrate is typically found in higher levels in the rural Midwest than other regions due to the significant amount of agriculture in this region. Private wells are more at risk than public drinking water systems because they are not federally regulated. So, homeowners are responsible for making sure their well meets drinking water standards.
Sources of Nitrate
Agricultural
The three main sources of environmental nitrate are agricultural, industrial, and urban. Agricultural sources include systems that utilize synthetic fertilizers and livestock (i.e., cows, pigs, and sheep) manure. In agricultural landscapes, monocrops like corn and soybeans often need large nitrogen inputs from fertilizer. Estimating the proper amount of fertilizer needed each growing season depends on a detailed nutrient management plan. Fertilizer needs are based on the crop being grown, the growth stage of the crop, and the current soil. Not having an effective nutrient management plan can lead to excess fertilizer within the agricultural landscape.
Industrial
Industrial sources include the production of synthetic fertilizers and discharge from industrial wastewater treatment plants. The most common nitrogen fertilizer produced is urea. Synthetic nitrogen fertilizer production can lead to nitrogen discharge to nearby waterways in the form of ammonia. Not only can the fertilizer manufacturing process impact water resources, but it can lead to atmospheric pollution in the form of nitrous oxide. Wastewater treatment plants contribute to nitrogen levels in waterways depending on how they deal with the water effluent and the sludge that is produced throughout the process. If the treated water effluent is not used for irrigation, it is typically just discharged to a nearby surface water body. Additionally, if the sludge (also named biosolids) are not land applied as organic fertilizer, they are just sent to the landfill.
Urban
Urban sources can be discharges from residential wastewater treatment plants, personal septic systems, and urban stormwater. Residential wastewater treatment plants contribute to nitrogen levels in the environment similarly to industrial wastewater treatment plants. Personal septic tank systems act as an on-site wastewater treatment system. So, they also contribute to nitrogen in the environment similarly to larger scale wastewater treatment plants. This is a concern in rural areas that have a high density of private wells because septic tanks can contribute to increased nitrogen in private drinking water wells. Urban stormwater is also an important source of nitrogen to the environment. Due to the amount of impervious surfaces that exist in urban areas, storm events can produce runoff that contain nutrients (such as nitrogen) and sediments.
The agricultural, industrial, and urban sources listed above are considered anthropogenic sources due to the influence of human activity. However, natural sources of nitrate exist as well. Processes like plant decay, decomposition of other organic material, and biological fixation of nitrogen gas can also contribute to nitrate levels.
Transport
Nitrate moves from land to water through three main processes: surface runoff, subsurface flow, and leaching. These contribute to nitrate in both surface water and groundwater. If a rain event occurs and the rainfall rate exceeds the soil’s ability to let water in, then surface runoff is produced because the soil can’t accept that much water at once. This runoff is called infiltration excess. Or, if the soil becomes saturated, then surface runoff will also be produced from the excess water. This runoff is called saturation excess. Subsurface flow is the transport of nitrate through the unsaturated zone. In this process, nitrate moves through soil pores due to a mixture of gravity and capillary forces (such as adhesion and cohesion). A heavy rainfall may also facilitate the leaching of nitrate by saturating the soil, causing nitrate to gravitate downwards. Leaching is primarily driven through advection (the bulk flow of water) and dispersion (the spread of a chemical). A visual of these three transport processes is shown below. Once nitrate enters the groundwater from leaching, it can then be transported to nearby surface water bodies through groundwater flow. These processes not only increase nitrogen levels in waterbodies, but they increase other pollutants such as phosphorus, pharmaceuticals, and pesticides.
Managing Nitrogen Pollution
There are ways we can manage nitrogen’s impact in the environment depending on the source. In agriculture, there are best management practices (BMPs) that have been developed to reduce the load of nitrogen on watersheds. Some of these practices focus on limiting erosion like cover cropping to maintain soil cover or reducing tillage to limit soil disturbance. Practices that limit erosion such as these reduce the potential for surface runoff. Others rely on precision agriculture like optimizing nutrient applications or irrigation amount and frequency. This can help limit leaching by reducing excess fertilizer and water inputs. Landowners that have a stream on their property may also install a strip of vegetation along the streambank to prevent surface runoff and subsurface flow from entering a stream. This is called a riparian buffer zone. Industry has methods of reducing nitrogen through a biological process called denitrification. This is when microorganisms convert nitrate to nitrogen gas. This is a common practice in wastewater treatment facilities. In urban settings, stormwater basins are installed or wetlands are constructed to store runoff. These also reduce contaminants through plant uptake and facilitate denitrification. Or, urban systems can create more permeable surfaces like green roofs or permeable pavements to lower surface runoff.
Conclusion
Overall, while nitrogen is essential for life, there needs to be a balance between productivity and environmental protection to help sustain our ecosystem for the future. Awareness, education, and policy can help achieve this balance and ensure sustainable resource management.