Geomorphic control on the δ15N of mountain forests

Hilton, R.G.; Galy, A.; West, A.J.; Hovius, N.; Roberts, G.G.

Abstract

Mountain forests are subject to high rates of physical erosion which can export particulate nitrogen from ecosystems. However, the impact of geomorphic processes on nitrogen budgets remains poorly constrained. We have used the elemental and isotopic composition of soil and plant organic matter to investigate nitrogen cycling in the mountain forest of Taiwan, from 24 sites with distinct geomorphic (topographic slope) and climatic (precipitation, temperature) characteristics. The organic carbon to nitrogen ratio of soil organic matter decreased with soil 14C age, providing constraint on average rates of nitrogen loss using a mass balance model. Model predictions suggest that present day estimates of nitrogen deposition exceed contemporary and historic nitrogen losses. We found ∼6‰ variability in the stable isotopic composition (δ15N) of soil and plants which was not related to soil 14C age or climatic conditions. Instead, δ15N was significantly, negatively correlated with topographic slope. Using the mass balance model, we demonstrate that the correlation can be explained by an increase in nitrogen loss by non-fractioning pathways on steeper slopes, where physical erosion most effectively removes particulate nitrogen. Published data from forests on steep slopes are consistent with the correlation. Based on our dataset and these observations, we hypothesise that variable physical erosion rates can significantly influence soil δ15N, and suggest particulate nitrogen export is a major, yet underappreciated, loss term in the nitrogen budget of mountain forests.

Citation

Hilton, R., Galy, A., West, A., Hovius, N., & Roberts, G. (2013). Geomorphic control on the δ15N of mountain forests. Biogeosciences, 10, 1693-1705. https://doi.org/10.5194/bg-10-1693-2013

• Journal Article Type: Article
• Acceptance Date: Feb 14, 2013
• Publication Date: Mar 13, 2013
• Deposit Date: Feb 15, 2013
• Publicly Available Date: Apr 13, 2015
• Journal: Biogeosciences
• Print ISSN: 1726-4170
• Electronic ISSN: 1726-4189
• Publisher: European Geosciences Union
• Peer Reviewed: Peer Reviewed
• Volume: 10
• Pages: 1693-1705
• DOI: https://doi.org/10.5194/bg-10-1693-2013
• Public URL: https://durham-repository.worktribe.com/output/1466524