Geomorphic control on the δ&amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N of mountain forests

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

doi:10.5194/bg-10-1693-2013

Abstract

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.

Highlights

GeoscientificModel DevelopmentNitrogen (N) is essential to primary productivity in the terres-M on nitrogen budgets remains poorly constrained
Using the mass balance model, we demonstrate that the cor- ture of the dominant processes of N loss and the rates atSwohlicidh tEheayrothccur throughout non-fractioning pathways on steeper slopes, where physical catchments
Amundson et al (2003) have previously assessed the role of dataset size for the return of significant correlations between plant and bulk soil δ15N values and environmental variables. They showed that the statistical link between δ15N and site conditions were preserved both when the number of sites were similar to this study (n < 30) and with ∼ 4 times the number of sites studied in Taiwan

Summary

Introduction

GeoscientificModel DevelopmentNitrogen (N) is essential to primary productivity in the terres-M on nitrogen budgets remains poorly constrained. The organic carbon to nitrogen ratio of soil organic matter decreased with soil 14C age, providing conenhanced productivity (OEreanrteht aSl.,y2s00te1)m, and determgrionwesththaendimsopialctbioofgeaoncthhreompiosgtSreynci(cAieNbnerdceeept osasli.t,io1n98o9n; plant Matson et al, 1999; Zaehle et al, 2011). For these reasons, there straint on average rates of nitrogen loss using a mass balance have been considerable efforts to better understand the promodel. NHcedein et al, 1995; Howarth et al, 1996; Lewis et al, 1999; Saunders et al, 2006; Schlesinger et al, 2006) Δ15N was inhibit or amplify nutOriecnteeaxpnorSt (ce.ige. nHcedein et al, 1995; Howarth et al, 1996; Lewis et al, 1999; Saunders et al, 2006; Schlesinger et al, 2006)

Methods

Results

Conclusion