Preprints
https://doi.org/10.5194/soil-2020-70
https://doi.org/10.5194/soil-2020-70

  10 Nov 2020

10 Nov 2020

Review status: a revised version of this preprint was accepted for the journal SOIL and is expected to appear here in due course.

Stable isotope signatures of soil nitrogen on an environmental-geomorphic gradient within the Congo Basin

Simon Baumgartner1,2, Marijn Bauters2,3, Matti Barthel4, Travis William Drake4, Landry Cizungu Ntaboba5, Basile Mujinya Bazirake6, Johan Six4, Pascal Boeckx2, and Kristof Van Oost1 Simon Baumgartner et al.
  • 1Earth and Life Institute, UCLouvain, Louvain-la-Neuve, 1348, Belgium
  • 2Department of Green Chemistry and Technology, Ghent University, Ghent, 9000, Belgium
  • 3Department of Environment, Ghent University, Ghent, 9000, Belgium
  • 4Department of Environmental Systems Science, Swiss Federal Institute of Technology, ETH Zurich, Zurich, 8092, Switzerland
  • 5Départment d’Agronomie, Université Catholique de Bukavu, Bukavu, DR Congo
  • 6Department of General Agricultural Sciences, University of Lubumbashi, DR Congo

Abstract. Nitrogen (N) availability can be highly variable in tropical forests on a regional and on a local scale. While environmental gradients influence N cycling on a regional scale, topography is known to affect N availability on a local scale. We compared stable isotope signatures (δ15N) of soil profiles in tropical lowland forest, tropical montane forest, and subtropical Miombo woodland within the Congo Basin as a proxy to assess ecosystem-level differences in N cycling. Furthermore, we examined the effect of surface slope angles on δ15N in the same forests to quantify local differences induced by topography. Soil δ15N profiles indicated that the N cycling in in the montane forest is more closed and dominated by organic N turnover, whereas the lowland forest and Miombo woodland experienced a more open N cycle dominated by inorganic N. Furthermore, our results show that slope angles only affects the soil δ15N signature in the Miombo forest, which is prone to erosion due to the lower vegetation cover and intense rainfalls at the onset of the wet season. Lowland forest, on the other hand, with a flat topography and protective vegetation cover, showed no influence of topography on soil N cycling. Values from the montane forest showed high variability in stable isotope signatures, but they were not constrained by topography. A pan-tropical analysis of soil δ15N values (i.e. from our study and the literature) reveals that soil δ15N is best explained by factors controlling erosion, namely mean annual precipitation, leaf area index, and slope angles. The erosive forces vary immensely between different tropical forest ecosystems and our results highlight the need of more spatial coverage of N-cycling studies in tropical forests, to further elucidate the local impact of topography on N cycling in this biome.

Simon Baumgartner et al.

 
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Simon Baumgartner et al.

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Stable isotope signatures of soil nitrogen on an environmental-geomorphic gradient within the Congo Basin Baumgartner, Simon, Bauters, Marijn, Barthel, Matti, Drake, Travis W., Ntaboba, Landry C., Bazirake, Basile M., Six, Johan, Boeckx, Pascal, and Van Oost, Kristof https://doi.org/10.5281/zenodo.4113895

Simon Baumgartner et al.

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Short summary
We compared stable isotope signatures of soil profiles in different forest ecosystems within the Congo Basin to assess ecosystem-level differences in N cycling and we examined the local effect of topography on the isotopic signature of soil N. Soil δ15N profiles indicated that the N cycling in in the montane forest is more closed, whereas the lowland forest and Miombo woodland experienced a more open N cycle. Topography only alters soil δ15N values in forests with high erosional forces.