Is δ15N a potential traceability tool in viticulture and oenology?

Stable isotope ratio analysis (SIRA) of so-called bioelements (H, C, N, O, S) is used for food authenticity control and to verify the geographical origin of food products (Kelly et al., 2005). One of the most well-known fields of application is analysis of the 2H/1H and 13C/12C ratios of wine ethanol and the 18O/16O of wine water to check whether sugar and water have been added and to verify the origin declared on the label (Dordevic et al., 2012). The 15N/14N ratio has been measured in several foods to trace geographical origin and agricultural systems (Kelly et al., 2005), but not in wine. In this study we evaluated the 15N/14N ratio (expressed as δ15N) in different matrices, namely the soil, branches and grape juices, of different oenological chains from two Italian regions (Trentino A.A. and Emilia Romagna) in order to understand whether this ratio can be used as a traceability marker. Nitrogen in grape juices indeed derives from the soil through plants, so its δ15N value is influenced by the external nitrogen source. The δ15N of a plant (δsink) is related to that of its N sources (δsource) and the δsource is the mean of the δ15N values of all potential N sources, weighted by their availability (Robinson et al., 2001). The variability of δ15N in the soil was studied at two different depths (from 20 to 30 cm and from 50 to 60 cm) in two different seasons: spring and summer. Vine branches and grape samples were collected from grapevines grown in proximity to the sampled holes in the soil. The experimental data showed that the soils were homogeneous; δ15N did not change according to the depth and there was a good correlation between the 15N/14N ratios measured in spring and summer. The δ15N values of branches showed the same trend as the soils; however, they were more negative. Therefore, δsink reflects not only δsource, but also N isotope fractionation during assimilation, N losses from plant and the mixing of different N pools within the plant (Robinson et al., 1998). The results obtained by analysing grape juices demonstrated that the δ15N of must was always slightly higher than in branches.