Source attribution of agriculture-related deposition by using total nitrogen and δ¹⁵N in epiphytic lichen tissue, bark and deposition water samples in Germany

Abstract

Compared with physico-chemical deposition measurement methods, lichens are able to identify the long-term overall effects of high N pollution concentrations in the air. In addition, the natural abundances of the stable isotope of N, ¹⁵N, are being widely used in research on N cycling in ecosystems. They can also be used as instruments for source attribution. In this study, epiphytic lichens were tested to determine whether their respective N content and δ¹⁵N ratios can be used to estimate N deposition rates and to locate various sources of N compounds. Epiphytic lichen and bark samples were collected from around various deposition measurement field stations at different sites in the western part of Germany. The N content of epiphytic lichens reflects the species-specific, agriculture-related circumstances of N deposition at various sites in Germany. At the same time, δ¹⁵N signatures of the different investigated epiphytic lichen species and bark samples are highly depleted in ¹⁵N under high ammonium deposition. The different surface types of lichens and barks exhibit different concentrations of N and δ¹⁵N ratios, despite being exposed to similar N deposition rates. The verification of highly negative δ¹⁵N ratios at sites with local and regional emitters shows that source attribution is possible by comparing different δ¹⁵N signatures in areas with a wide range of different N deposition types and the corresponding differences in δ¹⁵N among various source N pools. Especially nitrophytic lichens can support the on-site instrumentation measuring N deposition by qualification and quantification.

Keywords:

• agriculture deposition
• bark
• deposition water
• isotope ecology
• lichen
• nitrogen-15
• Germany

Acknowledgements

This work was funded by the German Federal Environmental Foundation (DBU). This support is gratefully acknowledged. Deposition data were provided by the Research Institute for Forest Ecology and Forestry – Rhineland-Palatinate, the Agricultural Testing and Research Institute, Speyer, the State Agency for Nature, Environment and Consumer Protection, North Rhine-Westphalia and the Northwest German Forest Research Station, Lower Saxony. This work has benefited from the helpful comments of Gerald Goehler. The authors also thank Dorothee Krieger and Bernhard Backes for helping in the laboratory (University of Trier) and Dr Jens Dyckmans (Centre for Stable Isotope Research and Analysis, Goettingen) for tips on analysing isotopes in the deposition water samples.