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.