Andosol genesis: Transition from silandic to aluandic properties and the related changes for organic carbon storage.
Final Report Abstract
Andosols are commonly subdivided according to silandic and aluandic features. Silandic Andosols are characterized by organic matter (OM) strongly bound to short-range ordered aluminosilicates (SROAS), while aluandic Andosols mainly consist of aluminium–OM complexes (Al–OM complexes). According to current theory, silandic and aluandic properties are direct results of the weathering of different substrates, assuming two separate lines of genesis. However, recent findings suggest that silandic horizons can transform into aluandic over time. This could be caused by dissolved organic matter (DOM) entering the subsoil with the percolating soil solution and promoting the dissolution of SROAS phases by complexing Al. Increasing loading of DOM with Al will ultimately result in the progressing formation of insoluble Al–OM precipitates. To test this hypothesis, we conducted a long-term percolation experiment with material of an Ecuadorian Andosol formed in a homogeneous tephra deposit featuring aluandic properties in the top- and silandic properties in the subsoil. Inorganic solutes, pH, and dissolved organic carbon (DOC) in the feed and eluate solution were monitored over a period of 18 months to track changes during the process. To estimate the amount of retained DOC, we modelled the percolation experiment with the convection–dispersion-equation as implemented in HYDRUS-1D. To determine changes in the solid phases, we analysed the bulk- and different density fractions of the column materials after 0, 6, and 18 months for organic carbon (OC) concentration, ammonium oxalate-extractable Al, Si, and Fe concentration and by X-ray diffraction and X-ray Absorption. Additionally, we sampled the soil solution in the field for 10 days and conducted DOM-sorption and Al–DOM-precipitation experiments. Our results show that Si is preferentially leached from all horizons compared to Al, while Al and OC are retained in the silandic material. We detected a strong OC accumulation of 14 mg∙g-1 in the silandic material after 18 months, with the vertical transport of soluble Al–OM complexes from the overlying topsoil only explaining up to 33 % of the OC accumulation. The modelling revealed that sorption of DOM dominates at the beginning of the experiment and explains up to 40 % of total OC accumulation. In turn, the results indicate that up to 70 % of the OC accumulation are due to SROAS dissolution and subsequent neoformation of insoluble Al–OM complexes. We also showed that previously formed Al–OM complexes increase their OC:Al molar ratio upon contact with fresh DOM solution. Density fractionation showed, that with increasing percolation time the fraction 1.6–2.0 g cm-³, being the dominant fraction in the aluandic material, increased in proportion reaching >90 wt%. Overall, our study provides first and compelling experimental evidence of a pedogenetic transition from silandic to aluandic Andosols.
Publications
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Genese von Andosolen: Übergang von silandischen zu aluandischen Eigenschaften - Konzept und erste Ergebnisse ; receiving a poster award
Martin Kaupenjohann & Klaus Kaiser
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Oral presentation DBG-Tagung Halle 2023, Title: Ein Langzeit-Perkolationsversuch zur Genese von Andosolen: Übergang von silandischen zu aluandischen Eigenschaften und Bedeutung für die Kohlenstoffspeicherung
Martin Kaupenjohann & Klaus Kaiser
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Poster presentation EGU assembly Vienna 2024: Title: Andosol genesis: Transition from silandic to aluandic properties and the related changes for organic carbon storage
Martin Kaupenjohann & Klaus Kaise
