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Biogeochemistry

, Volume 102, Issue 1–3, pp 195–208 | Cite as

Stable N isotope composition of nitrate reflects N transformations during the passage of water through a montane rain forest in Ecuador

  • Martin Thilo Schwarz
  • Yvonne Oelmann
  • Wolfgang Wilcke
Article

Abstract

Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO3 –N, NH4 +–N and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15–0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural 15N abundance in NO3 during the passage of rain water through the ecosystem and bulk δ15N values in soil to detect N transformations. Depletion of 15N in NO3 and increased NO3 –N fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO3 concentrations progressively decreased and were enriched in 15N but did not reach the δ15N values of solid phase organic matter (δ15N = 5.6–6.7‰). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the δ15N value of NO3 in litter leachate was smaller (δ15N = −1.58‰) than in the other quarters (δ15N = −9.38 ± SE 0.46‰) probably because of reduced mineralization and associated fractionation against 15N. Nitrogen isotope fractionation of NO3 between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO3 –N from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO3 gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream.

Keywords

Denitrification 15N natural abundance Nitrate Nitrification Terrestrial N cycling Tropical montane forest 

Abbreviations

DON

Dissolved organic nitrogen

DRY

Period between October 2006 and December 2006

INT1

Period between July 2006 and September 2006

INT2

Period between January 2007 and March 2007

LL

Litter leachate

RF

Incident rainfall

SE

Standard error

SS15

Soil solution at the 15 cm depth of the mineral soil

SS30

Soil solution at the 30 cm depth of the mineral soil

SW

Stream water

TF

Throughfall

WET

Period between April 2007 and July 2007

εamm+nit

Isotope enrichment factor during ammonification and nitrification

εdenit

Isotope enrichment factor during denitrification

Notes

Acknowledgments

We gratefully thank Nature and Culture International (NCI) in Loja, Ecuador, for providing the study area and the research station and the Instituto Forestal Areas Naturales y Vida Silvestre (INEFAN) for the study permission. We furthermore thank the Earth System Science Research Center of the Johannes Gutenberg University Mainz for improving our laboratory equipment. This study was funded by the Deutsche Forschungsgemeinschaft (DFG-FOR 816, Wi1601/8-1).

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Copyright information

© Springer Science+Business Media B.V. 2010

Authors and Affiliations

  • Martin Thilo Schwarz
    • 1
  • Yvonne Oelmann
    • 2
  • Wolfgang Wilcke
    • 1
  1. 1.Geographic InstituteUniversity of BerneBerneSwitzerland
  2. 2.Institute of Integrated Natural Sciences, GeographyUniversity of Koblenz-LandauKoblenzGermany

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