Andosol genesis: Transition from silandic to aluandic properties and the related changes in organic carbon storage

Andosols are commonly subdivided according to silandic and aluandic features. Both subgroups are considered to be end members of the Andosol genesis. Silandic Andosols are characterized by organic matter (OM) strongly bound to imogolite-type material (ITM), while Aluandic Andosols mainly consist of aluminium-OM complexes (Al-OM complexes). According to current theory, silandic and aluandic properties are direct results of primary weathering, assuming two separate lines of genesis.

Our previous results, however, suggest that silandic horizons can transform into aluandic over time, resulting in additional carbon accumulation. This is likely caused by dissolved organic matter (DOM) entering the subsoil with the percolating soil solution, masking dissolved aluminium as soluble Al-OM complexes. This promotes the dissolution of ITM, releasing aluminium. The latter reacts with DOM, inducing the formation of insoluble, Al-OM complexes, which then precipitate.

To test this hypothesis, we conducted a long-term percolation experiment with material of an Ecuadorian Andosol formed in a homogeneously tephra deposit. This soil exhibits aluandic properties in the topsoil and silandic properties in the subsoil. Ions, pH, and DOC in the feed and eluate solution were monitored over a period of 18 months. The convection-dispersion-equation as implemented in HYDRUS-1D was used to model the percolation experiment as a one-dimensional standard reactive solute transport.

Our results revealed a strong carbon accumulation of 14 g·kg^-1 in the silandic material after 18 months, with the vertical transport of Al-OM complexes only explaining up to 33 % of the carbon accumulation. The HYDRUS-1D model revealed that sorption of DOM dominates at the beginning of the experiment and explains up to 40 % of carbon accumulation (including vertically transported Al-OM complexes). For the silandic material, the results indicate that up to 91% of the carbon accumulation are due to ITM dissolution and subsequent formation of insoluble Al-OM complexes.

In summary, our findings support the hypothesis, that ITM dissolution and the subsequent formation of Al-OM precipitates significantly contribute to the increase in carbon concentration in the silandic material, while the above aluandic material did not change.