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The Nitrogen Cycle and the Role of Nitrates

Nitrogen is predominantly available as nitrogen gas (N2) in the atmosphere and present on Earth in different physical states and chemical compositions. Identifying and quantifying the mobility of nitrogen is critical for understanding how the combustion of fossil fuels and increased use of fertilizers have contributed to global changes in the ecosystem .

There is scientific evidence that humans have made a significant impact on the mobility of nitrogen that directly affect the nitrogen cycle. Scientific reports over the past two decades have examined nitrogen mobility and report nearly a doubling of nitrogen gas entering the terrestrial nitrogen cycle. This has caused increases in global N2O (g) concentrations, loss of calcium and potassium as soil nutrients, and negatively contributed to the acidification of lakes and streams.

Key Processes in the Nitrogen Cycle

The main forms of nitrogen are nitrogen gas (N2), ammonia/ammonium (NH3/NH4+), nitrate (NO3) and nitrite (NO2).

Nitrogen fixation is where nitrogen gas is converted into a form that is usable by plants or animals, like ammonia.

Assimilation is when nitrogen in the form of nitrate, nitrite, ammonia or ammonium is taken up by plants or animals.

Ammonification is the conversion of NH2 functional groups back into ammonia or ammonium ion through the decomposition of dead plants, animals or their waste.

Nitrification is the conversion of ammonia into nitrite ion and then into nitrate ion by bacteria. This is important as ammonia is toxic to many living organisms.

Denitrification is the conversion of nitrate ion back into nitrogen gas by bacteria.

Sources of Nitrates Infographics

The Significance of Nitrate

Nitrate water pollution is a great ecological concern and has been exacerbated by the anthropogenic injections of nitrogen into the cycle, primarily through the use of ammonia-based fertilizers. Excess nitrate in water poses a global health problem to humans and causes eutrophication of water systems that is detrimental to many organisms.

The ability to identify and quantify the sources of nitrate in water plays an important role in managing and mitigating pollution. Measuring the δ18O and δ15N of nitrate in water has been demonstrated as a reliable nitrogen source tracking tool. The isotopic signatures of nitrate ion derived from fertilizers, manure or the atmosphere are statistically different and distinguishable.

Beta Analytic – Nitrate Source Testing

Beta Analytic performs rapid, ISO 17025-accredited oxygen and nitrogen isotope measurements using an isotope ratio mass spectrometer (IRMS). Results can be accessed online 24/7.

Please contact the lab to obtain a sampling guide and other instructions before collecting water samples.


Khan Academy. The nitrogen cycle. [date unknown]. (Accessed December 2018).

Princeton University. The Nitrogen Cycle. [date unknown]. (Accessed December 2018).

The Environmental Literacy Council. Nitrogen Cycle. [date unknown]. (Accessed December 2018).

Bourke, S.A., et al. Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments. 2019. Hydrology and Earth System Sciences. 23, pp. 1355–1373

Ward, M. H., et al. Drinking Water Nitrate and Human Health: An Updated Review. 2018. International Journal of Environmental Research and Public Health. 15(7): 1557. 

Vitousek, P. J., et al. Human Alteration of the Global Nitrogen Cycle: Sources and Consequences. 1997. Ecological Applications. 7(3). pp. 737-750.

Vrzl, J., et al. Determination of the sources of nitrate and the microbiological sources of pollution in the Sava River Basin. 2016. Science of the Total Environment. 573. pp. 1460-1471.

This entry was posted on Thursday, February 28th, 2019 and is filed under Nitrate Test, Source Tracking in water .