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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: original research article 11 Jun 2019

Submitted as: original research article | 11 Jun 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal SOIL (SOIL).

Variations in soil chemical and physical properties explain basin-wide variations in Amazon forest soil carbon densities

Carlos Alberto Quesada1, Claudia Paz1,2, Erick Oblitas Mendoza1, Oliver Phillips3, Gustavo Saiz4,5, and Jon Lloyd4,6,7 Carlos Alberto Quesada et al.
  • 1Instituto Nacional de Pesquisas da Amazônia, Manaus, Cx. Postal 2223 – CEP 69080-971, Brazil
  • 2Universidade Estadual Paulista, Departamento de Ecologia, CEP 15506-900, Rio Claro, São Paulo, Brazil
  • 3School of Geography, University of Leeds, LS2 9JT, UK
  • 4Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berkshire SL5 7PY, UK
  • 5Department of Environmental Chemistry, Faculty of Sciences, Universidad Católica de la Santísima Concepción, Concepción, Chile
  • 6School of Tropical an Marine Sciences and Centre for Terrestrial Environmental and Sustainability Sciences, James Cook University, Cairns, 4870, Queensland, Australia
  • 7Universidade de São Paulo, Faculdade de Filosofia Ciências e Letras de Ribeirão Preto, Av Bandeirantes, 3900 , CEP 14040-901, Bairro Monte Alegre , Ribeirão Preto, SP, Brazil

Abstract. We investigate the edaphic, mineralogical and climatic controls of soil organic carbon (SOC) concentration utilising data from 147 pristine forest soils sampled in eight different countries across the Amazon Basin. Sampling across 14 different World Reference Base soil groups our data suggest that stabilisation mechanism varies with pedogenetic level. Specifically, although SOC concentrations in Ferralsols and Acrisols were best explained by simple variations in clay content – this presumably being due to their relatively uniform kaolinitic mineralogy – this was not the case for less weathered soils such as Alisols, Cambisols and Plinthosols for which interactions between Al species, soil pH and litter quality seem to be much more important. SOC fractionation studies further showed that, although for more strongly weathered soils the majority of SOC is located within the aggregate fraction, for the less weathered soils most of the SOC is located within the silt and clay fractions. It thus seems that for highly weathered soils SOC storage is mostly influenced by surface area variations arising from clay content, with physical protection inside aggregates rendering an additional level of protection against decomposition. On the other hand, most of SOC in less weathered soils is associated with the precipitation of aluminium-carbon complexes within the fine soil fraction and with this mechanism enhanced by the presence of high levels of aromatic, carboxyl-rich organic matter compounds. Also examined as part of this study were a relatively small number of arenic soils (viz. Arenosols and Podzols) for which there was a small but significant influence of clay and silt content variations on SOM storage and with fractionation studies showing that particulate organic matter may accounting for up to 0.60 of arenic soil SOC. In contrast to what were in all cases strong influences of soil and/or litter quality properties, after accounting for these effects neither wood productivity, above ground biomass nor precipitation/temperature variations were found to exert any significant influence on SOC stocks at all. These results have important implications for our understanding of how Amazon forest soils are likely to respond to ongoing and future climate changes.

Carlos Alberto Quesada et al.
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Carlos Alberto Quesada et al.
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Short summary
Amazon soils hold as much carbon (C) as is contained in the vegetation. In this work we sampled soils across 8 different Amazonian countries trying to understand which soil properties control current Amazonian soil C concentrations. We confirm previous knowledge that highly developed soils hold C through clay content interactions but also show a previously unreported mechanism of soil C stabilization in the younger Amazonian soil types which hold C through aluminiun organic matter interactions.
Amazon soils hold as much carbon (C) as is contained in the vegetation. In this work we sampled...