Discrimination between organic and conventional flour using compound-specific amino acid δ15N analysis

The application of nitrogen stable isotope ratio 15N/14N analysis to discriminate between organic and conventional cultivation is based on the fact that the different types of fertilizer used on the soil influence the nitrogen isotopic composition of crops. The δ15N value of plants grown with the use of fertilizers of animal origin and vegetable compost is statistically higher than for those grown using mineral fertilizers. δ15N values can therefore be used as an indicator of organic or conventional management systems. Traditional techniques rely on measurement of bulk samples, but emerging methods using individual chemical compounds provide a means of obtaining a more in-depth understanding. In this work we present a study of compound-specific nitrogen isotope analysis of amino acids to discriminate between organically and conventionally grown plants. Amino acids are indeed the dominant nitrogen-bearing biomolecules in plants and the pattern of isotopic fractionation during synthesis of these compounds records a range of information about the growth environment, such as the form of assimilable N available in the soil. Our investigation was based on flour samples grown with synthetic nitrogen fertilizer animal manure and green manure from atmospheric nitrogen fixing legumes. The measurement of amino acid δ15N values after protein hydrolysis and N-acetylisopropyl derivatization was carried out using gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Our results demonstrate that the molecular amino acid δ15N isotope profile is different in samples, depending on the relative source of nitrogen.