Pedon-scale silicate weathering: comparison of the PROFILE model and the depletion method at 16 forest sites in Sweden

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Weathering of soil minerals is important for the recovery from acidification and for the sustainability of forestry. However, there is still substantial uncertainty about its absolute rate. This study presents a harmonized comparison of field weathering rates estimated with the mechanistic model PROFILE and the depletion method for 16 intensively sampled soil profiles across Sweden representing different site conditions. In general, a correspondence in total weathering rates was found between the two methods except in rare cases where either method yielded deviating results. The weathering rate was higher according to the depletion method than according to PROFILE for Mg, while PROFILE produced higher weathering rates for the other base cations. The Spearman rank correlation (ρ) between the two methods indicated significant correlation for Ca (ρ=0.44, p=0.04) and non-significant correlation for Mg (ρ=0.51, p=0.09), Na (ρ=0.25, p=0.34), K (ρ=0.07, p=0.80), and the sum of the base cations (ρ=0.11, p=0.67). The variation in weathering rates with depth showed opposite gradients in the upper 50cm, which reflects the conceptual differences between the methods. This study shows the potential of using multiple methods to identify a probable weathering rate, if harmonized input data are used. Furthermore, it highlights the importance of making comparisons for individual elements in order to interpret differences between methods. Regardless of the method used, weathering rates were below or at the same level as the losses caused by whole-tree harvesting, particularly in southern Sweden, indicating a risk of negative effects on soils and waters.