engleski

Geochemical modelling of suction cup solutions from eroded soils of a lysimeter study with Visual MINTEQ

Data obtained from analysing soil solution extracted by suction cups are often used for quantifying element fluxes and balances of soil ecosystems. However, the extraction procedure itself may significantly alter the soil solution chemistry. Degassing of CO2 increases the pH of the solution which is followed by a cascade of chemical reactions and changes in species composition of chemical components, possibly leading to the misinterpretation of data. Likewise, possible filtering of larger soluble organic acids may be reflected in a modified pH- value and cation complexation. Suction cups extract mainly the mobile fraction of the solution. The chemical analysis of the solution is carried out under atmospheric conditions that can be quite different from the situation in the soil and yields bulk element concentrations. Thus, for using the suction cup data for the description of multi-component reactive solute transport, the data need to be interpreted in terms of actual conditions in the soil solution. We study the effects of (a) DOC on soil solution cation complexation for the decalcified horizons (Ah, Al, Bt-horizons) and, (b) the CO2-partial pressure on solution element speciation, as well on precipitation or dissolution of mineral solids within the calcareous parent material (glacial till, C- horizon). Geochemical speciation is analysed with an equilibrium-based Visual MINTEQ software assuming cation complexation by dissolved organic matter according to NICA- Donnan model. Data are from TERENO SoilCan lysimeter of the ZALF experimental Research Station in Dedelow (Uckermark) located near the city of Prenzlau, Germany. The speciation at a single sampling time, representing solution from a 14-day collection period, shows that calculated solution pH-values are always higher than measured values. Under the assumption of an elevated CO2 partial pressure in the soil air, calcite precipitation is simulated in the subsoil. The complete reconstruction of the original pore water solution chemistry is still limited by unknown effects of the extraction procedure and by porosity of suction cups.

geochemical modelling, suction cup solution, eroded soils, lysimeter

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