The nitrogen in synthetic fertilisers comes from atmospheric nitrogen and as little fractionation happens in the manufacturing of these fertilisers, the stable isotope ratio of the fertilisers is very close to that of atmospheric nitrogen. Fertilisers used in organic production, however, have usually been through the food chain. As the ratio of 15N to 14N tends to increase with each trophic level, this means that the nitrogen isotope ratio of organic fertilisers is much higher than that of atmospheric nitrogen.
