Cation-exchange reactions involving aluminium ions in podzols disturbed by deep tilling

The stratified structure of a podzolic soil is changed during deep tilling of forest soils. Here, we assessed whether (i) cation-exchange properties and (ii) cation-exchange reactions in podzolic soil were altered when the illuvial (B) horizon was subject to environmental conditions prevailing on the soil surface and the organic (O) horizon was buried within the mineral soil. The samples were taken from undisturbed and disturbed podzolic soil profiles at two forest sites that had been ploughed 17 and 31 years ago and planted with Norway spruce seedlings. An increase in soil organic C and effective cation-exchange capacity (CECe) was observed in the Bs horizons exposed to the soil surface. The accumulation of soil C was mainly due to organic matter produced by the planted Norway spruce. The Al3+-Ca2+ cation-exchange equilibria were similar in both the disturbed and undisturbed O and Bs horizons. This indicates that similar reactions between Al3+ and Ca2+ took place in different horizons of podzols irrespective of the disturbance. A positive correlation of CECe in the Bs horizons with soil C indicated that soil organic matter played a major role in the cation-exchange reactions. The importance of exchangeable cations in determining pH variation in the O horizon was additionally supported by the success in describing the relationship between soil solution pHs and base saturation using the extended Henderson-Hasselbach equation in which exchangeable Ale had nonacidic properties similar to those of base cations. The results of this study suggest that organic matter, accumulated within the soil profile during soil formation and upon ploughing, had an important role in determining the cation-exchange properties and reactions in the studied podzols.