Comparison of different isotherm models for dissolved organic carbon (DOC) and nitrogen (DON) sorption to mineral soil

The concentration of dissolved organic matter (DOM) in the soil solution is predominantly determined by sorption processes to mineral surfaces. We compared four models describing DOM sorption isotherms to mineral soil material using the results of batch experiments for dissolved organic carbon (DOC) and nitrogen (DON) sorption to different horizons of a podsol. The Initial Mass and modified Langmuir isotherms could be fit to the data points in all cases. The three curvilinear models (Langmuir, Freundlich and exponential models) were found to result in similar goodness-of-fit and parameter estimates. We found that the most widely used, linear model (the Initial Mass isotherm) could be used for horizons with very small sorption capacity and horizons almost saturated with DOM, but failed to describe sorption in the deeper, DOM-unsaturated horizons characterized by a curvilinear shape of the sorption isotherm. The modified Langmuir isotherm was found to describe DOM sorption to mineral soil best and gave realistic parameter estimates, including a measure for maximum sorption capacity of the soil. Comparing the results of the sorption experiments with DOM concentrations in soil solution in the field indicated that the sorption isotherms can serve as a basis for larger DOM leaching models.