pH buffering in acidic soils developed under Picea abies and Quercus robur -: effects of soil organic matter, adsorbed cations and soil solution ionic strength

Soil solution chemistry, soil acidity and composition of adsorbed cations were determined in two soil profiles developed under a mixed spruce (Picea abies and Picea sitchensis) stand and in one soil profile developed under an oak (Quercus robur) stand. Soils under spruce were classified as Spodosols and soils under oak were classified as Inceptisols. All profiles were developed in the same parent material; a Saahlian sandy till containing less than 2% clay. In the mineral soil, the contribution from mineral surfaces to the total cation-exchange capacity (CECt) was estimated to be less than 3%. Soil solution pH and the percent base saturation of CECt [%BS = 100 (2Ca + 2Mg + Na + K) CECt-1] were substantially lower in the upper 35-40 cm of the two Spodosols, as compared to the Inceptisol. The total amount of soil adsorbed base cations (BC) did not differ among the three profiles on an area basis down to 1 m soil depth. Thus, soil acidification of CECt due to net losses of BC could not explain differences in soil pH and %BS among the soil profiles. A weak acid analogue, taking the pH-effect of metal complexation into consideration, combined with soil solution ionic strength as a covariate, could describe both the pH variation by depth within soil profiles and pH differences between the Inceptisol and the two Spodosol profiles. Our results confirm and extend earlier findings from O and E horizons of Spodosols that the extent to which organic acid groups react with Al minerals to form Al-SOM complexes is a major pH-buffering process in acidic forest soils. We suggest that an increasing Al-saturation of SOM is the major reason for the widely observed pH increase by depth in acidic forest soils with a pH less than approximately 4.5. Our results strongly imply that changes in mass of SOM, the ionic strength in soil solution and the relative composition of soil adsorbed Al and H need to be considered when the causality behind changes in pH and base saturation is investigated.