Human and climate impact on ¹⁵N natural abundance of plants and soils in high-mountain ecosystems: a short review and two examples from the Eastern Pamirs and Mt. Kilimanjaro

Abstract

Population pressure increasingly endangers high-mountain ecosystems such as the pastures in the Eastern Pamirs and the mountain forests on Mt. Kilimanjaro. At the same time, these ecosystems constitute the economic basis for millions of people living there. In our study, we, therefore, aimed at characterising the land-use effects on soil degradation and N-cycling by determining the natural abundance of ¹⁵N. A short review displays that δ¹⁵N of plant–soil systems may often serve as an integrated indicator of N-cycles with more positive δ¹⁵N values pointing towards N-losses. Results for the high-mountain pastures in the Eastern Pamirs show that intensively grazed pastures are significantly enriched in ¹⁵N compared to the less-exploited pastures by 3.5 ‰, on average. This can be attributed to soil organic matter degradation, volatile nitrogen losses, nitrogen leaching and a general opening of the N-cycle. Similarly, the intensively degraded savanna soils, the cultivated soils and the soils under disturbed forests on the foothill of Mt. Kilimanjaro reveal very positive δ¹⁵N values around 6.5 ‰. In contrast, the undisturbed forest soils in the montane zone are more depleted in ¹⁵N, indicating that here the N-cycle is relatively closed. However, significantly higher δ¹⁵N values characterise the upper montane forest zone at the transition to the subalpine zone. We suggest that this reflects N-losses by the recently monitored and climate change and antropogenically induced increasing fire frequency pushing the upper montane rainforest boundary rapidly downhill. Overall, we conclude that the analysis of the ¹⁵N natural abundance in high-mountain ecosystems is a purposeful tool for detecting land-use- or climate change-induced soil degradation and N-cycle opening.

Keywords:

• biogeochemistry
• Eastern Pamirs
• isotope ecology
• Mt. Kilimanjaro
• ¹⁵N
• nitrogen cycle
• pasturing
• soil degradation

Acknowledgements

We thank K. Vanselow (University of Erlangen-Nürnberg, Germany) and L. Sauheitl, Prof. B. Glaser and Prof. G. Gebauer (University of Bayreuth, Germany) for support during field work and δ¹⁵N measurements, respectively. Our work also greatly profited from generous logistic support provided by Prof. B. Huwe (University of Bayreuth) and Prof. Z. Svirćev and Prof. S. Marković (University of Novi Sad, Serbia). We are also very grateful to three anonymous reviewers for their useful comments to improve the manuscript and to Dr. G. Strauch for the editorial handling. This study was partly funded by the German Research Foundation (DFG: HE 2919/9-1), the VW Foundation (AZ I/81 976) and the Arbeitsgemeinschaft für Vergleichende Hochgebirgsforschung (ARGE). M. Zech also greatly acknowledges the support given by the Alexander von Humboldt Foundation.