δN of lichens reflects the isotopic signature of ammonia source.

Although it is generally accepted that δN in lichen reflects predominating N isotope sources in the environment, confirmation of the direct correlation between lichen δN and atmospheric δN 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, δN and chlorophyll a fluorescence, were measured. Under the experimental conditions, δN in lichens quantitatively responded to the δN of released gaseous ammonia (NH). 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 NH, 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 NH, then NH, with lowest sensitivity to NO. Moreover, these results confirm that lichen δN can be used to indicate the δN of atmospheric ammonia, providing a suitable tool for the interpretation of the spatial distribution of NH sources in relation to their δN signal.