04 Feb 2021

04 Feb 2021

Review status: this preprint is currently under review for the journal SOIL.

Controls on heterotrophic soil respiration and carbon cycling in geochemically distinct African tropical forest soils

Benjamin Bukombe1, Peter Fiener1, Alison M. Hoyt2, and Sebastian Doetterl3,1 Benjamin Bukombe et al.
  • 1Institute of Geography, Augsburg University, Augsburg, 86159, Germany
  • 2Max Planck Institute for Biogeochemistry, Jena, 07745, Germany
  • 3Department of Environmental System Science, ETH Zurich, Zurich, 8092, Switzerland

Abstract. Heterotrophic soil respiration is an important component of the global terrestrial carbon (C) cycle, driven by environmental factors acting from local to continental scales. For tropical Africa, these factors and their interactions remain largely unknown. Here, using samples collected along strong topographic and geochemical gradients in the East African Rift Valley, we study how soil chemistry and soil fertility, derived from the geochemical composition of soil parent material, can drive soil respiration even after many millennia of weathering and soil development.

To address the drivers of soil respiration, we incubated soils from three regions with contrasting geochemistry (mafic, felsic, and mixed sedimentary) sampled along slope gradients. For three soil depths, we measured the potential maximum heterotrophic respiration under stable environmental conditions as well as the radiocarbon content (Δ14C) of the bulk soil and respired CO2. We found that soil microbial communities were able to mineralize C from fossil as well as other poor quality C sources under laboratory conditions representative of tropical topsoils. Furthermore, despite similarities in terms of climate, vegetation, and the size of soil C stocks, soil respiration showed distinct patterns with soil depth and parent material geochemistry. The topographic origin of our samples was not a main determinant of the observed respiration rates and Δ14C. In situ, however, soil hydrological conditions likely influence soil C stability by inhibiting decomposition in valley subsoils. Our study shows that soil fertility conditions are the main determinant of C stability in tropical forest soils. Further, in the presence of organic carbon sources of poor quality or the presence of strong mineral related C stabilization, microorganisms tend to discriminate against these sources in favor of more accessible forms of soil organic matter as energy sources, resulting in a slower rate of C cycling.

Our results demonstrate that even in deeply weathered tropical soils, parent material has a long-lasting effect on soil chemistry that can influence and control microbial activity, the size of subsoil C stocks, and the turnover of C in soil. Soil parent material and its lasting control on soil chemistry need to be taken into account to understand and predict C stabilization and rates of C cycling in tropical forest soils.

Benjamin Bukombe et al.

Status: open (until 11 Apr 2021)

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Benjamin Bukombe et al.

Benjamin Bukombe et al.


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Short summary
This study assesses the controls on soil respiration and the radiocarbon content in soil developed from geochemically varying parent material in African tropical forests. Using incubation experiments, we found that a combination of factors related to soil fertility and the chemistry of the soil solution, organic matter quality, and soil mineralogy drive soil respiration patterns along the investigated geochemical gradients. Radiocarbon signatures predominantly depend on parent material features.