Among the lighter elements having two or more stable isotopes (H, C, N, O, S), δ15N appears to be the most promising isotopic marker to differentiate plant products from conventional and organic farms. Organic plant products vary within a range of δ15N values of +0.3 to +14.6%, while conventional plant products range from negative to positive values, i.e. −4.0 to +8.7%. The main factors affecting δ15N signatures of plants are N fertilizers, biological N2fixation, plant organs and plant age. Correlations between mode of production and δ13C (except greenhouse tomatoes warmed with natural gas) or δ34S signatures have not been established, and δ2H and δ18O are unsuitable markers due to the overriding effect of climate on the isotopic composition of plant-available water. Because there is potential overlap between the δ15N signatures of organic and conventionally produced plant products, δ15N has seldom been used successfully as the sole criterion for differentiation, but when combined with complementary analytical techniques and appropriate statistical tools, the probability of a correct identification increases. The use of organic fertilizers by conventional farmers or the marketing of organic produce as conventional due to market pressures are additional factors confounding correct identification. The robustness of using δ15N to differentiate mode of production will depend on the establishment of databases that have been verified for individual plant products.
Inácio, C., Chalk, P., Magalhães, A., Principles and Limitations of Stable Isotopes in Differentiating Organic and Conventional Foodstuffs: 1. Plant Products, Critical Reviews in Food Science & Nutrition, Volume: 55 Issue: 9 Pages: 1206-1218 Published: 2015.