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Rights: University of Waikato
Published 30 July 2013 Referencing Hub media

Dr Selai Letica explains the process of denitrification. This is the phase of the nitrogen cycle where nitrate is converted back to atmospheric gaseous nitrogen (N2).

Jargon alert: Dinitrogen is the scientific name for atmospheric gaseous nitrogen, also known as N2



The pathway that I deal with fundamentally would be the process of denitrification, so that comes in after nitrogen is transformed to nitrate. Any nitrate that’s not lost to leaching has the potential to be lost to the process of denitrification.

Denitrification is a microbially mediated process where nitrate is used as a form of energy for denitrifiers – these are soil bacteria – and they reduce it into various forms of gaseous emissions. What happens is nitrate that’s left over from plant uptake can be denitrified when soil conditions become appropriate, and that generally means that we have conditions of low oxygen or no oxygen, and there has to be a carbon source, so if we were to think about that in a farm setting, that is generally when a soil is very wet, so after a really high rainfall event or a high irrigation event, and also if soils are compacted. So those types of combinations often make a soil very vulnerable to the process of denitrification.

What happens during that process is that the microbes use the nitrate in place of oxygen to get their energy to grow and multiply, and along that pathway, they secrete various enzymes to break down that nitrate. And those different gaseous forms of nitrogen can be lost at any stage along that reductive pathway before the nitrate is fully reduced to dinitrogen. One of those gaseous forms of nitrogen is nitrous oxide.

Dr Selai Letica, AgResearch, Invermay
Professor Louis Schipper
James Lomax