Adsorption complex,pH relationships in some acid mediterranean forest soil profiles

Acid soils in some mediterranean forests were investigated for the composition of the adsorption complex and the gradients in soil pH. The effective CEC (235–838 mmol_c kg^?1) and base saturation (93–98 %) are highest in ectorganic horizons. In the mineral horizons the effective CEC (23–52 mmol_c kg^?1) and base saturation (11–40 %) are much lower. The exchange complex of mineral horizons consists for 90 (AEh) to 40 percent (Bw2) of organic matter. The effective CEC of the mineral clay fraction is low (60 mmol_c kg^?1 clay). The clear trends in soil pH within the ectorganic layer of deciduous and sclerophyllous oak forests are attributed to vertical spatial separation of nitrogen mineralization (ammonification and strongly impeded nitrification) and nutrient uptake by roots (mainly NH₄). This leads to a high effective CEC in the fermentation layer and acidification of the uppermost part of the mineral soil. In contrast to the situation in temperate forests this process is impeded in mediterranean coniferous forests, where litter decomposition is extremely slow and both proton production and consumption take place in the organic rich mineral horizon.