Is it really organic? – Multi-isotopic analysis as a tool to discriminate between organic and conventional plants

•Multi-isotopic analysis was evaluated for authentication of organic plants.•A method for compound-specific isotope analysis of plant nitrate was investigated.•Stable isotopes of nitrogen documented the use of animal manure for fertilisation.•Stable hydrogen isotopes discriminated organic and conventional cereals.•Oxygen isotopes of nitrate proved suitable for authentication of organic potatoes. Novel procedures for analytical authentication of organic plant products are urgently needed. Here we present the first study encompassing stable isotopes of hydrogen, carbon, nitrogen, oxygen, magnesium and sulphur as well as compound-specific nitrogen and oxygen isotope analysis of nitrate for discrimination of organically and conventionally grown plants. The study was based on wheat, barley, faba bean and potato produced in rigorously controlled long-term field trials comprising 144 experimental plots. Nitrogen isotope analysis revealed the use of animal manure, but was unable to discriminate between plants that were fertilised with synthetic nitrogen fertilisers or green manures from atmospheric nitrogen fixing legumes. This limitation was bypassed using oxygen isotope analysis of nitrate in potato tubers, while hydrogen isotope analysis allowed complete discrimination of organic and conventional wheat and barley grains. It is concluded, that multi-isotopic analysis has the potential to disclose fraudulent substitutions of organic with conventionally cultivated plants.