Assessment of the Regional Variation in Weathering Rates of Loess and Clay Soils in The Netherlands

To calculate critical acid loads or to predict elementconcentrations in the soil solution, information on weatheringrates is essential. Several studies have taken place in theNetherlands to obtain weathering rates for non-calcareous sandysoils. 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 ona regional scale by relating them to regionally available soilproperties.To obtain weathering rates of loess and clay soils on a regionalscale for the Netherlands, the applicability of a statisticalregression model and the process based PROFILE model have beenevaluated. Both models were calibrated on a set of laboratoryexperiments. To evaluate their predictive power, both methodswere validated on a number of sites for which field weatheringrates were available.Predictions with the statistical model, for the individual basecations, were generally within a factor 2 of the calculatedhistorical weathering rates, except for Ca, which wasoverestimated, by a factor 3 to 4. PROFILE stronglyoverestimated all weathering rates using both standard parameters and in particular after calibration on the laboratoryrates. However, PROFILE predicted weathering rates of the loesssoils quite good after calibration on historical weatheringrates, indicating that the downscaling procedure used in PROFILEto translate laboratory to field weathering rates is inadequatefor the considered soils.The statistical model was applied to predict weathering rates,for the Netherlands on a 1 × 1 km grid scale. Weathering rates at the present pH values in forested loess and clay soilsranged from 135 to 6000 mol_c ha^-1 a^-1 in loesssoils and from 100 to 1750 mol_c ha^-1 a^-1 in claysoils.