What is Nitrogen?
Nitrogen is a naturally occurring substance, with the chemical symbol N. In its gas form (N2), nitrogen makes up about 80% of the Earth's atmosphere. In other forms it is one of the most important fertilisers for plant growth. It is also found in amino acids that make up proteins, in nucleic acids (that make up DNA) and in many other organic and inorganic compounds.
This figure shows a schematic representation of the flow of nitrogen through the environment. The importance of bacteria in the cycle is immediately recognized as being a key element in the cycle, providing different forms of nitrogen compounds assimilable by higher organisms. Source: Wikipedia.
Nitrogen occurs as several forms in the environment:
Nitrogen gas (N2)
As a gas, nitrogen is relatively inactive but can be converted to other more reactive forms by certain bacteria through a process called fixation.
A highly soluble molecule made up of nitrogen and oxygen with the chemical formula NO3. It is a very important plant fertiliser but because it is highly water soluble, it leaches through soils very easily, particularly after heavy rainfall. It is one of the most common contaminants in waterways in rural and urban areas. NO3-N can be transformed to other forms of nitrogen. Sources of NO3-N include excessive application of inorganic fertilizer, septic tanks and leaking sewage systems. Nitrate also enters waterways as a result of nitrification of the ammonia in animal waste by bacteria in soil.
Nitrite-nitrogen is an ion with the chemical formula NO2. Concentrations of nitrite-nitrogen are normally low compared to nitrate-nitrogen and ammoniacal nitrogen. However, too much nitrite-nitrogen can be toxic. In drinking water it can be harmful to young infants or young livestock.
Ammoniacal nitrogen (NH4-N)
Ammoniacal nitrogen (NH4-N), also often called ‘ammonium’, covers two forms of nitrogen; ammonia (NH3)and ammonium (NH4). NH4-N can be transformed to other forms of nitrogen and is a very important plant fertiliser but is less mobile in the soil than nitrate-nitrogen. It enters waterways primarily through point source discharges, such as raw sewage or dairy shed effluent. It is toxic to aquatic life at high concentrations.
Ammonia (NH3) is a gas that reacts to form the ammonium ion (NH4) when it is dissolved in water. The balance between ammonia and ammonium in water depends on the pH and temperature of the water. Ammonia is highly toxic to aquatic life.
Total Nitrogen (TN)
Total Nitrogen is the sum of all organic and inorganic forms of nitrogen that are found in a water sample (i.e., nitrate-nitrogen (NO3-N), nitrite-nitrogen (NO2-N), ammoniacal-nitrogen (NH4-N) and organic nitrogen such as amino acids or plant tissue).
In New Zealand, total nitrogen is measured (on unfiltered water) by two different methods used by different regional councils and other agencies doing regular water testing:
One method is called TN-A where total nitrogen is measured by a procedure called “direct persulphate digestion”. The other method is called TN-K, which involves summing the separate measurements of organic nitrogen compounds, ammoniacal nitrogen (NH4-N, NH3) and oxidised nitrogen (NO3-N, NO2-N) to estimate total nitrogen.
Scientists recently found out that total nitrogen water quality measurements vary significantly, depending on the turbidity of the river water and the methodology used. Total nitrogen laboratory results were slightly higher when the TN-K method was used and lower with the TN-A method (an average of 8% less) in very turbid rivers (that is a river with Total Suspended Solids > 10 mg/L).
These new findings raise the question which method should be used by councils and whether councils may need to change the way they currently calculate total nitrogen, adjusting their methods to either TN-K or TN-A. To date, however, there is no clear answer to the question which method is ‘best practice’. Moreover, a change in methodology can cause difficulties when calculating long-term trends in turbid waters (such as flooded rivers or shallow wind-disturbed lakes prone to sediment re-suspension). We therefore recommend that until further research has been done on this subject, existing TN methods within regions should be continued to maintain sampling continuity for trend calculations.
Why is too much Nitrogen a problem?
Nitrogen is a great fertiliser but too much of it can cause aquatic weeds and algae to grow too fast. This increased plant growth can reduce oxygen in the water during night time when dead plant material decomposes. This can eventually remove the oxygen present in lakes, posing a threat to aquatic life. Nitrite-nitrogen and ammonia become toxic at high concentrations which are more likely under certain temperature and pH conditions. This can cause direct harm to fish and macroinvertebrates.
Where does Nitrogen come from?
The most common sources are wastewater treatment plants, run-off from pasture, croplands and fertilised lawns, leaky septic systems, run-off from animal manure/urine, and industrial discharges.
How to test for Nitrogen?
Water samples are collected by local authorities and sent to laboratories. Each form of nitrogen requires a different chemical test. The majority of local authorities use methods that follow the American Public Health Association (APHA) standards. Local authorities, together with the Ministry for the Environment, are currently working towards standardising sampling and testing methodologies for nutrients, including nitrogen.
Which unit is it given in?
All forms of nitrogen are measured in g/m3 (the same as mg/L) or parts per billion (ppb). 1 ppb = 0.001 g/m3.
Where do I find more information?
ANZECC & ARMCANZ (2000). Australian and New Zealand guidelines for fresh and marine water quality. Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand.
Biggs B 2000. New Zealand Periphyton Guidelines: detecting, monitoring and managing enrichment of streams. Prepared for Ministry for the Environment.
Davies-Colley, R. 2000. “Trigger” values for New Zealand rivers. Prepared for the Ministry for the Environment. NIWA Client Report: MfE002/22 May 2000.
Matheson F, Quinn J, Hickey C 2012. Review of the New Zealand instream plant and nutrient guidelines and development of an extended decision making framework: Phases 1 & 2 final report. Prepared for the Ministry of Science and Innovation Envirolink Fund.
Milne J. R, Perrie A., Robinson P. Davies-Colley R. J. (2014). National consistency in nutrient test methods: assessing the implications of changing methods on long-term water quality datasets – Part 1. Water Symposium, NZ Freshwater Sciences and NZ Hydrological Society joint annual conference, 24-28 November 2014, Blenheim.
Robinson P., Milne J.R., Perrie A., Davies-Colley R. J. (2014). National consistency in nutrient test methods: assessing the implications of changing methods on long-term water quality datasets – Part 2. Water Symposium, NZ Freshwater Sciences and NZ Hydrological Society joint annual conference, 24-28 November 2014, Blenheim