Long-term persistence of soil organic matter in Amazonian Dark Earth

Research output: Thesisinternal PhD, WU


Amazonian Dark Earths (ADE), highly fertile pre-Columbian anthropic soils found in the Amazon, have intrigued scientists for decades. Despite tropical conditions, these soils exhibit large contents of carbon (C) and nutrients (particularly Ca and P), contrasting with the poor adjacent soils (ADJ) from the Amazon. High carbon (C) content in ADE has been linked with the occurrence of black carbon (BC) in these soils. BC has been claimed to be highly resistant to decomposition due to its polycondensed aromatic structure. However, recent advancements indicate that intrinsic chemical characteristics do not fully explain long-term persistence of soil organic matter (SOM). Soil minerals are known to play an important role on the mechanisms of SOM stabilization mainly through adsorption of SOM onto the surface of reactive soil minerals (i.e. Fe and Al oxides). However, the relative contribution of mineral protection versus chemical recalcitrance in explaining long-term persistence of SOM in ADE is poorly understood. The overall objective of this PhD thesis was to investigate the interactive roles of BC, Ca and P in explaining long-term persistence of SOM in ADE. Overall, my results (Chapter 3–5) have major implications for a common SOM methodology proposed to isolate a ‘stable’ pool of SOM upon chemical oxidation with NaOCl and to subsequently differentiate and quantify the relative contribution of two mechanisms of SOM stabilization (i.e. mineral association and chemical recalcitrance) upon soil demineralization with HF. Combining data from Chapters 4 and 5, it seems like BC is removed upon chemical oxidation with NaOCl and therefore is part of the operationally defined ‘labile’ rather than the ‘stable’ SOM pool. Removal of BC with NaOCl raises questions about the contrast between low resistance of BC towards chemical oxidation and apparent high resistance of BC against biological oxidation, which certainly deserves more attention in future studies. Furthermore, my results shed light on the importance of high Ca and P inputs (Chapter 2 and 5) besides the occurrence of BC in ADE (Chapter 5) in explaining long-term persistence of SOM in ADE. In my thesis, I propose that it is the interaction between OM inputs (including BC inputs), Ca and P that are responsible for the long-term persistence of large SOM contents in ADE, which brings major implications for the sole use of charred OM (i.e. biochar) as a strategy to reproduce ADE desirable characteristics in other soils.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
  • Kuijper, Thomas, Promotor
  • Hiemstra, Tjisse, Co-promotor
  • Teixeira, W.G., Co-promotor, External person
Award date11 Nov 2019
Place of PublicationWageningen
Print ISBNs9789463951746
Publication statusPublished - 11 Nov 2019

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