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Denitrification is reduction of oxidized nitrogen compounds to gaseous products through the sequence nitrate, nitrite, nitric oxide, nitrous oxide and molecular nitrogen. In many microorganisms it supports anaerobic respiration when oxygen is scarce. Different organisms can perform different portions of the sequence; release of nitrous oxide does not imply complete conversion to molecular nitrogen.

Denitrification removes biologically available nitrogen from an ecosystem, whereas nitrification oxidizes reduced nitrogen. Disappearance of nitrate alone cannot distinguish gaseous loss from uptake by plants. Measurements using isotope labelling and mass spectrometry can trace nitrogen into both nitrogen gas and nitrous oxide, connecting the nitrogen cycle to an experimentally measured gas balance.^[ref]

The nitrogen cycle connects the movement and chemical transformation of nitrogen in the atmosphere, organisms, soils and water. Nitrogen fixation converts molecular nitrogen into reduced forms; nitrogen assimilation incorporates available nitrogen into cellular compounds; ammonification returns organic nitrogen to ammonium during breakdown.

Nitrification and denitrification connect further oxidized and reduced forms under different environmental conditions. The distinction matters because a large atmospheric reservoir does not make all of its nitrogen directly available to every plant: biological access depends on the chemical form and on transformations carried out by other organisms.^[ref]

Timothy T. Bergsma developed field measurements of nitrogen gas losses from soil. His 2001 study measured both nitrous oxide and dinitrogen using enrichment with the stable isotope nitrogen-15 and mass spectrometry. Modification of the instrument's head amplifier enabled analysis of nitrous oxide as well as dinitrogen.^[ref]

Measuring both gases matters for denitrification: a soil can lose nitrogen without releasing the same fraction as nitrous oxide. An isotope label helps distinguish newly generated gas from the large atmospheric background of dinitrogen, making the chemical partition of the loss experimentally accessible.