Compound-specific δ15N and δ13C analysis of amino acids for discrimination between organically and conventionally grown wheat

The application of nitrogen stable isotope ratio (15N/14N) analysis to discriminate between organic and conventional plant products is based on the fact that the different types of fertilizers used 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 often statistically higher than for those grown using synthetic 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 focusing on individual chemical compounds provide a means of obtaining a more in-depth understanding. Here we present a study deploying compound-specific nitrogen and carbon isotope analysis of amino acids to discriminate between organically and conventionally grown plants. Amino acids are 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 plant available N in the soil. We focused on wheat samples grown using synthetic nitrogen fertilizers, animal manures or green manures from nitrogen fixing legumes. The measurement of amino acid δ15N and δ13C values after protein hydrolysis and derivatization was carried out using gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Our results demonstrate that the combination of δ15N and δ13C values of glutamic acid and glutamine can improve the discrimination between conventional and organic wheat compared to stable isotope analysis of bulk tissue.