Seasonal Variation of pH(H2O) and pH(CaCl2) in Centimeter-layers of Mor Humus in a Picea abies (L.) Karst. Stand

Ten mor cores (O-horizon) were sampled every month during September 1985 to August 1986 (except December) from a 9 m(2) plot. Each core usually yielded four sliced 1-cm-layers in addition to the S-layer. Each layer was extracted in both distilled water and in 0.01 M CaCl2. The spatial pH variation was found to be greater than the seasonal variation in all layers. With few exceptions, pH(H2O) and pH(CaCl2) showed a similar seasonal trend in the entire mor layer. Seasonal alteration of chemical or biological conditions generally influenced pH of all cm-layers in the same direction. The general seasonal trend of pH(H2O) and pH(CaCl2) was a continuous increase during autumn and winter, with a maximum in April, followed by a more drastic decrease and a pH(H2O) minimum in June. The minimum pH(CaCl2) appeared in August. Cation uptake (May-June) and mineralisation, leaching and alteration of organic acid properties (September-April, July-August) were discussed as the dominating factors behind the seasonal pH variation.