δ¹⁵N of lichens reflects the isotopic signature of ammonia source

Although it is generally accepted that δ¹⁵N in lichen reflects predominating N isotope sources in the environment, confirmation of the direct correlation between lichen δ¹⁵N and atmospheric δ¹⁵N is still missing, especially under field conditions with most confounding factors controlled. To fill this gap and investigate the response of lichens with different tolerance to atmospheric N deposition, thalli of the sensitive Evernia prunastri and the tolerant Xanthoria parietina were exposed for ten weeks to different forms and doses of N in a field manipulation experiment where confounding factors were minimized. During this period, several parameters, namely total N, δ¹⁵N and chlorophyll a fluorescence, were measured. Under the experimental conditions, δ¹⁵N in lichens quantitatively responded to the δ¹⁵N of released gaseous ammonia (NH₃). Although a high correlation between the isotopic signatures in lichen tissue and supplied N was found both in tolerant and sensitive species, chlorophyll a fluorescence indicated that the sensitive species very soon lost its photosynthetic functionality with increasing N availability. The most damaging response to the different N chemical forms was observed with dry deposition of NH₃, although wet deposition of ammonium ions had a significant observable physiological impact. Conversely, there was no significant effect of nitrate ions on chlorophyll a fluorescence, implying differential sensitivity to dry deposition versus wet deposition and to ammonium versus nitrate in wet deposition. Evernia prunastri was most sensitive to NH₃, then NH₄⁺, with lowest sensitivity to NO₃^-. Moreover, these results confirm that lichen δ¹⁵N can be used to indicate the δ¹⁵N of atmospheric ammonia, providing a suitable tool for the interpretation of the spatial distribution of NH₃ sources in relation to their δ¹⁵N signal.