Unlocking the nitrogen cycle in glacial forelands: an isotopic perspective

Glacial forelands are expanding worldwide due to glacier shrinkage1, exposing new areas prone to the development of post-glacial ecosystems2,3. Nitrogen (N) is generally considered as a (co-)limiting nutrient in alpine regions, and deposition of atmospheric N, mainly emitted due to fossil fuel combustion, has for long been admitted as the main source of N4. However, other N sources such as glacial meltwaters5, long-range transport of fertilizers6 or bedrock erosion7 have recently been suspected of playing a more significant role than previously thought and could drive the establishment of pioneer microbial and plant communities in glacial forelands. Here, we show the isotopic composition and concentration of nitrate (δ15N, δ18O, Δ17O) and ammonium (δ15N) in glacial meltwaters, soils and plants from three glacial forelands in the French Alps. Samples were collected along transects expanding from the glacier front to areas deglaciated around 60 years ago. We find that the contribution of atmospheric deposition to the nitrate pool in soils decreases as time since deglaciation increases, but never exceeds 40%, not even at the glacier front where soils are entirely mineral with no detectable nitrification enzymatic activity. This pattern suggests that bedrock nitrogen and glacial meltwaters are the main N sources in post-glacial ecosystems and calls for a better quantification of those inputs.     (1)             Hugonnet, R.; McNabb, R.; Berthier, E.; Menounos, B.; Nuth, C.; Girod, L.; Farinotti, D.; Huss, M.; Dussaillant, I.; Brun, F.; Kääb, A. Accelerated Global Glacier Mass Loss in the Early Twenty-First Century. Nature 2021, 592 (7856), 726–731. https://doi.org/10.1038/s41586-021-03436-z. (2)             Bosson, J. B.; Huss, M.; Cauvy-Fraunié, S.; Clément, J. C.; Costes, G.; Fischer, M.; Poulenard, J.; Arthaud, F. Future Emergence of New Ecosystems Caused by Glacial Retreat. Nature 2023, 620 (7974), 562–569. https://doi.org/10.1038/s41586-023-06302-2. (3)             Ficetola, G. F.; Marta, S.; Guerrieri, A.; Gobbi, M.; Ambrosini, R.; Fontaneto, D.; Zerboni, A.; Poulenard, J.; Caccianiga, M.; Thuiller, W. Dynamics of Ecological Communities Following Current Retreat of Glaciers. Annu. Rev. Ecol. Evol. Syst. 2021, 52(1), 405–426. https://doi.org/10.1146/annurev-ecolsys-010521-040017. (4)             Holtgrieve, G. W.; Schindler, D. E.; Hobbs, W. O.; Leavitt, P. R.; Ward, E. J.; Bunting, L.; Chen, G.; Finney, B. P.; Gregory-Eaves, I.; Holmgren, S.; Lisac, M. J.; Lisi, P. J.; Nydick, K.; Rogers, L. A.; Saros, J. E.; Selbie, D. T.; Shapley, M. D.; Walsh, P. B.; Wolfe, A. P. A Coherent Signature of Anthropogenic Nitrogen Deposition to Remote Watersheds of the Northern Hemisphere. Science 2011, 334 (6062), 1545–1548. https://doi.org/10.1126/science.1212267. (5)             Saros, J. E.; Rose, K. C.; Clow, D. W.; Stephens, V. C.; Nurse, A. B.; Arnett, H. A.; Stone, J. R.; Williamson, C. E.; Wolfe, A. P. Melting Alpine Glaciers Enrich High-Elevation Lakes with Reactive Nitrogen. Environ. Sci. Technol. 2010, 44 (13), 4891–4896. https://doi.org/10.1021/es100147j. (6)             Hundey, E. J.; Russell, S. D.; Longstaffe, F. J.; Moser, K. A. Agriculture Causes Nitrate Fertilization of Remote Alpine Lakes. Nat. Commun. 2016, 7 (1), 10571. https://doi.org/10.1038/ncomms10571. (7)             Houlton, B. Z.; Morford, S. L.; Dahlgren, R. A. Convergent Evidence for Widespread Rock Nitrogen Sources in Earth’s Surface Environment. Science 2018, 360 (6384), 58–62. https://doi.org/10.1126/science.aan4399.