Principles and Limitations of Stable Isotopes in Differentiating Organic and Conventional Foodstuffs: 1. Plant Products

Among the lighter elements having two or more stable isotopes (H, C, N, O, S), delta N-15 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 delta N-15 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 delta N-15 signatures of plants are N fertilizers, biological N-2 fixation, plant organs and plant age. Correlations between mode of production and delta C-13 (except greenhouse tomatoes warmed with natural gas) or delta S-34 signatures have not been established, and delta H-2 and delta O-18 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 delta N-15 signatures of organic and conventionally produced plant products, delta N-15 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 delta N-15 to differentiate mode of production will depend on the establishment of databases that have been verified for individual plant products.