EFFECTS OF AIR-DRYING ON THE MEASUREMENT OF SOIL-PH IN ACIDIC FOREST SOILS OF QUEBEC, CANADA

Nine podzolic soil profiles (n = 67 horizons) were sampled in southern Quebec to evaluate the effects of drying on soil pH measurements in H2O (pH(w)) and in 0.01M CaCl2 (pH(CA)) and to identify the existence of relationships between pH changes on drying and soil properties. The precision of pH measurements (global precision of +/- 0.035 pH unit) varied, with the poorest precision value (0.10) being recorded for pH(w) in field-moist samples of organic horizons (O). For any soil horizon group CO, E, B, C), the pH change attributable to drying was always larger in H2O (up to 0.50 pH unit or 130 mu mol H+ L(-1)) than in CaCl2 (up to 0.25 pH unit or 520 mu mol H+ L(-1)). When expressed in terms of H+ concentration, the trend is inverted because pH(CA) values are often one pH unit more acidic than pH(w) values for the same horizon. Drying generally resulted in soil acidification for all horizon types although both acidification and alkalinization could occur in a given profile. We found positive and significant relationships between pH changes attributable to drying expressed as \Delta H+\ and organic C and exchangeable Al for both electrolytes. The relationships were stronger when soil materials mere grouped on a profile basis (average r value = 0.78 and 0.62 for organic C and exchangeable Al, respectively) than when grouped by horizon type (average r value = 0.39 and 0.49 for organic C and exchangeable AZ, respectively). The DOC concentrations in both electrol;ytes and for any given horizon were always higher in dry than in moist soil samples. Absolute changes in DOC concentrations were correlated to the absolute magnitude of pH changes upon drying (0.67 < r( 0.96; alpha less than or equal to 0.05). However, the relationships between the magnitude of DOC changes and the polarity of pH changes upon drying (Delta H+) varied with the electrolyte used. In H2O, the decrease in soil pH is mostly associated with the acidifying effect of an increase in organic matter solubility after drying. In CaCl2, it is suggested that the potential reduction of AZ availability after soil drying and the flocculation of organic substances by Ca2+ ions could contribute to the increase in pH. Characterizing the suite of organic substances present in individual horizons could also prove very helpful in elucidating the response of soil pH to drying.