4.7 Article

Geomorphic control on the δ15N of mountain forests

Journal

BIOGEOSCIENCES
Volume 10, Issue 3, Pages 1693-1705

Publisher

COPERNICUS GESELLSCHAFT MBH
DOI: 10.5194/bg-10-1693-2013

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Funding

  1. National Environmental Research Council (NERC), UK [1228.0407]

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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 C-14 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 similar to 6% variability in the stable isotopic composition (delta N-15) of soil and plants which was not related to soil C-14 age or climatic conditions. Instead, delta N-15 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 delta N-15, and suggest particulate nitrogen export is a major, yet underappreciated, loss term in the nitrogen budget of mountain forests.

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