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Discussion papers | Copyright
https://doi.org/10.5194/soil-2018-19
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Original research article 29 Jun 2018

Original research article | 29 Jun 2018

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

Aluminium and base cation chemistry in dynamic acidification models – need for a reappraisal?

Jon Petter Gustafsson1,2, Salim Belyazid3, Eric McGivney1, and Stefan Löfgren4 Jon Petter Gustafsson et al.
  • 1Department of Sustainable Devel opment, Environmental Science an d Engineering, KTH Royal Institute of Technology, Teknikringen 10B, 100 44, Stockholm, Sweden
  • 2Department of Soil and Environment, Swedish University of Agric ultural Sciences, P.O. Box 7014, 750 07 Uppsala, Sweden
  • 3Department of Physical Geography, Stockholm University, 106 91 Stockholm, Sweden
  • 4Department of Aquatic Sciences and Assessment, Swedish Universi ty of Agricultural Sciences, P.O. Box 7050, 750 07 Uppsala, Sweden

Abstract. Long-term simulations of the water composition in acid forest soils require that accurate descriptions of aluminium and base cation chemistry are used. Both weathering rates and soil nutrient availability depend on the concentrations of Al3+, of H+, and of base cations (Ca2+, Mg2+, Na+ and K+). Consequently, assessments of the acidification status and base cation availability will depend on the model being used. Here we review in what ways different dynamic soil chemistry models describe the processes governing aluminium and base cation concentrations in the soil water. Furthermore, scenario simulations with the HD-MINTEQ are used to illustrate the difference between model approaches. The results show that all investigated models provide the same type of response to changes in input water chemistry. Still, for base cations we show that the differences in the magnitude of the response may be considerable depending on whether a cation-exchange equation (Gaines-Thomas, Gapon) or an organic complexation model is used. The former approach, which is used in many currently used models (e.g. MAGIC, ForSAFE), causes stronger pH-buffering over a relatively narrow pH range, as compared to state-of-the-art models relying on more advanced descriptions in which organic complexation is important (CHUM, HD-MINTEQ). As for aluminium, a fixed gibbsite constant, as used in MAGIC and ForSAFE, leads to slightly more pH-buffering than in the more advanced models that consider both organic complexation and Al(OH)3(s) precipitation, but in this case the effect is small. We conclude that the descriptions of acid-base chemistry and base cation binding in models such as MAGIC and ForSAFE are only likely to work satisfactorily in a narrow pH range. If the pH varies greatly over time, the use of modern organic complexation models is preferred over cation exchange equations.

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This paper investigates how different dynamic soil chemistry models describe the processes governing aluminium and base cations in acid soil waters. We find that traditional cation-exchange equations, which are still used in many models, diverge from state-of-the-art complexation sub-models such as WHAM, SHM and NICA-Donnan when large fluctuations in pH or ionic strength occur. In conclusion, the complexation models provide a better basis for the modelling of chemical dynamics in acid soils.
This paper investigates how different dynamic soil chemistry models describe the processes...
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