Small scale pH buffering in organic horizons of two boreal coniferous forest stands

Factors behind the small-scale variaton of pH were examined in O horizons (humus layers) developed under two stands of Picea abies (L.) Karst. (Site F and K) by combining data on the composition of the cation exchange complex with data from titrations of corresponding H⁺-saturated samples. Cations extractable in 0.5 M CuCl₂ (S=cmol_c kg^–1 [2Ca+2Mg+2Mn+K+Na]), aluminium extractable in 1.0 M KCl (Al_e=cmol_c kg^–1 [3Al]) and in 0.5 M CuCl₂ (Al_org=cmol_c kg^–1 [3Al]), as well as pH measured in 0.01 M CaCl₂ (pH_Ca) were analysed in one-cm-layers of 13 O horizon cores at each site. Composite samples representing each of the one-cm-layers at each site, as well as samples with two different levels of Al saturation at Site K, were H⁺ saturated and titrated with NaOH to chosen end points of pH_Ca=4.0 and 5.5 in a 0.01 M CaCl₂ ionic medium. The Acid Neutralisation Capacity (ANC) was estimated as the amount of base needed to increase pH_Ca of the composite H⁺-saturated samples to the mean pH_Ca of the corresponding natural samples. The ANC was found to be similar in magnitude or higher than the amount of sites binding S+Al_e, which suggests that 1.0 M KCl exchangeable Al ions are nonacidic in acid O horizons. The relative contribution from i) the capacity of acidic functional groups, ii) their acid strength and iii) their degree of neutralisation to differences in pH_Ca between sites, among cm-layers and between samples with different levels of Al saturation were estimated from titration curves adjusted to hold two out of three factors (i, ii and iii) to be constant. The degree of neutralisation explained most of the differences in pH_Ca between the two sites, as well as between samples with different levels of Al saturation at Site K. The pH_Ca decrease by depth at site F was, however, partly explained by an increasing acid strength. The study emphasizes the importance of examining not only changes in the degree of neutralisation, but also changes in the acid strength and the capacity of buffering functional groups before conclusions about causes behind acidification processes can be made. Difficulties of accurately estimating the degree of neutralisation (base saturation) of acidic functional groups from the composition of adsorbed cations, owing to the unknown acidity of adsorbed Al, was also demonstrated.