The Nitrogen Cycle and the Role of Nitrates

Nitrogen is present on Earth in many forms, mostly as inert nitrogen gas in the atmosphere. Understanding the mobility of nitrogen is important in many realms, particularly when factoring in the shift in balance that human interference has caused.

It is well-established that humans have had a significant impact on the nitrogen cycle. Vitousek et al. (1997) reported that there was nearly a double of nitrogen going into the terrestrial nitrogen cycle. The main routes through which humans have increased nitrogen levels are through fossil fuel burning and the use of nitrogen-based fertilizers.

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 back into ammonia through the decomposition of dead plants and animals or their waste.

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

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

The Significance of Nitrate

Nitrate water pollution is a big concern and has been exacerbated by the anthropogenic injections of nitrogen into the cycle, for example the use of ammonia-based fertilizers. The concern over excess nitrate in water stems from both health concerns and potential eutrophication of water systems to the severe detriment of many organisms.

Being able to identify the sources of nitrate in water can therefore play an important part in managing and mitigating the pollution. Measuring the d18O and d15N of nitrate in water has been shown to provide this insight as a source tracking tool. The isotopic signatures of nitrate derived from fertilizers, manure or the atmosphere are sufficiently different that they can be distinguished.

Beta Analytic – Nitrate Source Testing

Beta Analytic performs fast, ISO 17025-accredited oxygen and nitrogen isotope measurements using an isotope ratio mass spectrometer (IRMS). Results are reported within 14 business days and can be accessed online 24/7.

Please contact the lab to get 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).

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 .