Foliar heavy metals and stable isotope (delta C-13, delta N-15) profiles as reliable urban pollution biomonitoring tools

Anthropogenic heavy metal pollution is an important health issue in urban areas, and therefore rapid and inexpensive monitoring in time and space is desirable. This study aimed (i) to assess the suitability of Tilia cordata leaves as a valuable heavy metal bioindicator, including seasonal changes in concentrations and (ii) to evaluate the use of leaf carbon and nitrogen isotope composition (delta C-13 and delta N-15) as novel indicators of urban heavy metal pollution. Leaves were collected from three different pollution intensity locations (Bilbao, Vitoria, and Muskiz) in the Basque Country (northern Spain). Analysis of leaf heavy metals related to traffic emissions and delta C-13 and delta N-15 determinations were carried out during July-October 2018. Leaf samples from Bilbao, the most populated and traffic-intense location, showed the highest concentration of heavy metals (mainly from polluted air). Additionally, the two urban areas, Bilbao and Vitoria, showed stronger correlation between these heavy metals, indicating a traffic-related source of emissions. The source of contamination (soil or air) in relation to elements and optimal sampling time is discussed herein. On the other hand, Pearson correlation analysis revealed significant trends between leaf delta C-13 and delta N-15 and the studied heavy metals, especially Pb, Cr and Cd, supporting the hypothesis of delta C-13 and delta N-15 as tools to distinguish locations according to their heavy metal pollution levels. To our knowledge, this is the first time that delta C-13 and delta N-15 have been used as monitoring tools in heavy metal pollution and consequently more research is still needed to calibrate this tool through extensive vegetation screening.

Automated summary

The study evaluated Tilia cordata leaves as bioindicators of heavy metal pollution and found that leaves from urban areas like Bilbao had higher heavy metal concentrations related to traffic emissions. The analysis of leaf carbon and nitrogen isotopes (delta C-13 and delta N-15) showed potential as indicators of heavy metal pollution levels and could distinguish pollution sources in different locations. Further research is needed to calibrate this monitoring tool through extensive vegetation screening.