Factors Determining Differences in Soil pH in Adjacent Conifer and Alder‐Conifer Stands

Abstract

Tree species may influence the following three characteristics that regulate soil pH: (i) the quantity of acids present, which can be represented as the total cation-exchange capacity (CEC); (ii) the degree of dissociation of the acids (base saturation); and (iii) the affinity of the acids for H+, or acid strength, which represents the composite pKa (negative log of the acid ionization constant) of the exchange complex. The importance of these three factors was examined in explaining the differences in soil pH between adjacent stands of conifers (primarily Douglas fir, Pseudotsuga menziesii) and conifers mixed with N-fixing red alder (Alnus rubra). At a low-productivity site (Wind River, WA), the pH of 0 to 0.15 m of soil from both alder-conifer and conifer stands averaged 4.3 in 0.01 M CaCl2. The pH values were the same, however, only because higher base saturation in the alder-conifer stand was offset by greater acid strength. At a more productive site, soil pH (in 0.01 M CaCl2) averaged 3.7 in the alder-conifer stand but 4.4 in the conifer stand. The difference in pH resulted primarily from greater acid strength of soil organic matter under alder, and secondarily from lower base saturation of the exchange complex. These results emphasize the importance of considering qualitative changes in soil organic matter as factors driving changes in soil pH and other parameters.