Nitrogen is another nutrient that is essential for aquatic plant growth, but unlike Phosphorus, most Nitrogen enters surface water bodies in a dissolved form, independent of sediment.
Total nitrogen is the sum of the different forms found naturally in the water, including nitrate, nitrite and ammonia. Nitrogen enters surface waters naturally through the air and surface runoff, or through human activities such as wastewater discharges and agricultural practices. Elevated concentrations can result in the excessive growth of algae and aquatic plants.
Nitrogen is a component of the Alberta River Nutrient Index, which rates river water quality, based on nutrients and related variables – including phosphorus, nitrogen, pH, and dissolved oxygen – in an annual series of water samples.
Based on the Surface Water Quality Guidelines for Use in Alberta, the following has been established as the Total Nitrogen Water Quality Objective (WQO) for the SSRSB:
In general, rating (condition) of the Total Nitrogen indicator is based on the number of times that recorded measurements over a one year period “exceeded” the Water Quality Objective (WQO) at that location.
There are three important forms of Nitrogen: Nitrate, Nitrite, and Ammonia
The nitrate ion is a polyatomic ion with the molecular formula NO−3 and consists of one central nitrogen atom surrounded by three identical oxygen atoms in a trigonal planar arrangement. Nitrate is the most soluble form and does not bind to soil particles or form insoluble compounds with other elements in the soil. This means nitrates can easily enter surface waterbodies through runoff or percolate deep into the ground and contaminate groundwater. Transport of large amounts of nitrate to surface waters is a concern because it is rapidly taken up by aquatic plants and can lead to eutrophication in the same way phosphorus can.
Furthermore, although nitrate is much less toxic than ammonia or nitrite, it still can be toxic to some forms of aquatic life. Ammonium is the ionic form of ammonia and unlike nitrate it does bind to soil particles and therefore is a more stable form of nitrogen in soil.
However, concentrations of ammonium in the soil are generally quite low because it is quickly converted to nitrate. An exception is when a large volume of ammonium fertilizer or manure is applied to a field just before an intense precipitation event, which can wash concentrated ammonium into surface waterbodies. Some of this ammonium will then be converted to ammonia, which is toxic to fish and other aquatic life.
The nitrite ion has the chemical formula NO2−. Nitrite is the intermediate form of nitrogen between ammonium and nitrate and because the conversion process is rapid it does not accumulate in the soil. Like nitrate, nitrite does not bind to soil particles and is easily carried into surface water bodies by runoff.
Through the use of nitrogen fertilizers and manure application for crop production, the agricultural sector is a large contributor of non-point source nitrate to surface and ground waters. Limits have been put in place on the amount of nitrogen in manure that can be applied to crops to try to reduce the amount being carried by runoff into surface waters or leaching into groundwater supplies.
However, when intensive livestock operations produce large amounts of manure and it is used as fertilizer, correct application for nitrogen requirements still results in excess phosphorus being applied to crops. This is because unlike commercial fertilizers where both nitrogen and phosphorus concentrations can be matched to crop requirements, the ratio of nitrogen to phosphorus in manure is fixed. Therefore, manure applications can lead to a build up of phosphorus in the soil and increased phosphorus inputs to surface waters through runoff. As a result, efforts to apply phosphorus concentrations that more closely match crop requirements and to minimize soil erosion and runoff are very important for avoiding phosphorus contamination of waterbodies.
Ammonia is a form of nitrogen produced by the decomposition of organic material. If found in water, ammonia can be highly toxic to aquatic organisms. In most well-oxygenated waters, ammonia is converted quickly to non-toxic nitrate, a nutrient necessary for plant growth. Ammonia can be discharged by municipal and industrial wastewater effluents.
Ammonia gas can be lost when manure or ammonia-based fertilizers are applied to fields under warm, dry conditions and are not mixed into the soil. Although most of the ammonia is re-deposited within a few hundred metres, it still may not be used by the intended crops and may runoff into surface water bodies. Burning biomass such as fossil fuels and forests, intensive livestock operations, and industrial activities can all cause nitrogen and phosphorus-bearing particles to become airborne and release nitrogen gasses into the atmosphere.