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

Abstract

AbstractTree species may differ in their influence on biogeochemical cycles, leading to differing rates of soil acidification. Over time, quantitative and qualitative changes develop in the characteristics of the soil exchange complex. Three such characteristics 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, commonly called 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. We examined the importance of these three factors in explaining the differences in soil pH between adjacent stands of conifers [primarily Douglas fir, Pseudotsuga menziesii (Mirbel) Franco] and conifers mixed with N‐fixing red alder (Alnus rubra Bong.). 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 (Cascade Head, OR), 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 underscore the importance of considering qualitative changes in soil organic matter as factors driving changes in soil pH and other parameters.