Innovative Tools for Nitrogen Fertilization Traceability in Organic Farming Products: A Fennel Case Study

Food fraud is a matter of great interest, particularly when organic productions are involved. Therefore, policymakers and institutions are asked to introduce new effective official control methods, not only based on documentary compliance to EU regulations. Lately, an integrated approach based on the use of isotopic, chemical and biological data treated by chemometrics has been suggested to authenticate organic products. The present research aims to validate the beforementioned integrated approach for the reliable classification of organically and conventionally grown fennel productions obtained by applying different treatments at diverse percentages of mineral fertilizers. Moreover, to the best of our knowledge, for the first time, an N-fixing plant (alfalfa), as an agroecological service crop in intercropping fennel, was introduced. Physicochemical parameters (fresh weight, total soluble solids, total acidity, consistency, Commission Internationale de l’Eclairage (CIE) lightness (L*), green-red (a*), blue-yellow (b*) color components), total phenolic content, in vitro antioxidant activity, total and inorganic N, and stable isotope ratios (15N/14N, 13C/12C, and 34S/32S) analyses were performed over a two-year field trial and further multivariate discriminant analysis was performed. The physicochemical parameters were not affected by the differential growing practice while antioxidant activity showed higher values in the organic treatments. The conventional treatments increased the NO3− values over 440% compared to the organic ones and the lowest values were found when alfalfa was used for intercropping. δ(15N) of fennel, δ(34S) of fennel and δ(15N) of leaves showed great discriminatory potential, with respect to the applied agronomic practice. However, these parameters alone were not sufficient as biomarkers for differentiating cultivation methods. Instead, the proposed innovative tool demonstrated that only a comprehensive multivariate approach was able to achieve a correct classification of grouped cases. Furthermore, the discrimination between organic and conventional horticultural products showed feasible results, even when N-fixing plants were incorporated in organic practice, thus avoiding misinterpretations solely based on the isotopic N fingerprint. These promising findings highlight the potential of this method and suggest that further research should extend its application to other horticultural crops to confirm and expand upon the current results.