Modelling chemical and biological reactions during unsaturated flow through sandy soils
B. Ludwig A B , S. Herrmann A and K. Michel AA Department of Environmental Chemistry, University of Kassel, Nordbahnhofstrasse 1a, Witzenhausen 37213, Germany.
B Corresponding author. Email: bludwig@uni-kassel.de
Australian Journal of Soil Research 48(3) 221-230 https://doi.org/10.1071/SR09145
Submitted: 13 August 2009 Accepted: 25 November 2009 Published: 6 May 2010
Abstract
Predicting chemical and biological reactions during unsaturated flow through soils is complex, and the accuracy of model predictions is open to question. Objectives were to test how accurately the transport of nutrients (Ca2+, Mg2+, K+, NH4+, and NO3–) in soils can be predicted when differing amounts of nutrients or acid are added. Undisturbed columns of 2 sandy surface soils from arable sites in Darmstadt, Germany, and Sohar, Oman, were irrigated for 360 and 376 days with 3 mm/day at 10°C. Three treatments were carried out: the columns were irrigated with differing fertilising solutions (Fert-1 or Fert-2 treatments) or with HCl. The model PHREEQC2 was used to calculate 1-dimensional transport, inorganic complexation, dissolution or precipitation of CaCO3, multiple cation exchange, and nitrification. We compared 3 model variants: one with no adjustable parameters, a second with optimised nitrification rates and pCO2, and a third with an additional optimisation of the exchange coefficients. In model variant v2 and v3, the Fert-1 treatment was used for calibration. Model variant v1 was of little use in predicting cation transport in soils. The second was more appropriate for both soils, but an optimisation of the exchange coefficients (model variant v3) was required for a more accurate description (Fert-1) and prediction (Fert-2 and HCl) of ion transport. The included proton buffering reactions resulted in a moderately accurate prediction of pH and release of ions. Nitrification in the Fert-1 and Fert-2 experiments considered in the model variants accounted for a range of 0.03–0.04 µmol N/(day.g soil). Overall, our results indicate that calibration experiments are required for a reliable assessment of ion transport in soils during unsaturated flow. The parameter optimisation program PEST in combination with PHREEQC2 best simulated cation and anion transport in sandy soils under unsaturated conditions.
Acknowledgments
We thank Anja Sawallisch for technical support and Dr Joachim Raupp for permitting sampling on the site of the long-term experiments of the Institute for Biodynamic Research Darmstadt. This study was supported by a grant from the Deutsche Forschungsgemeinschaft in the Research Training Group 1397.
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