Assessment of the regional variation in weathering rates of loess and clay soils in the Netherlands

To calculate critical acid loads or to predict element concentrations in the soil solution, information on weathering rates is essential. Several studies have taken place in the Netherlands to obtain weathering rates for non-calcareous sandy soils. Recently information on weathering rates in less vulnerable loess and clay soils have become available. However, up to now no system is available to estimate weathering rates on a regional scale by relating them to regionally available soil properties. To obtain weathering rates of loess and clay soils on a regional scale for the Netherlands, the applicability of a statistical regression model and the process based PROFILE model have been evaluated. Both models were calibrated on a set of laboratory experiments. To evaluate their predictive power, both methods were validated on a number of sites for which field weathering rates were available. Predictions with the statistical model, for the individual base cations, were generally within a factor 2 of thecalculated historical weathering rates, except for Ca, which was overestimated, by a factor 3 to 4. PROFILE strongly overestimated all weathering rates using both standard parameters and in particular after calibration on the laboratory rates. However, PROFILE predicted weathering rates of the loess soils quite good after calibration on historical weathering rates, indicating that the downscaling procedure used in PROFILE to translate laboratory to field weathering rates is inadequate for the consideredsoils. The statistical model was applied to predict weathering rates, for the Netherlands on a 1 * 1 kmr grid scale. Weathering rates at the present pH values in forested loess and clay soils ranged from 135 to 6000 molc ha-1 a-1 in loess soils and from 100 to 1750 molc ha-1 a-1 in clay soils.