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

Dep. of Forest and Wood Sciences, Colorado State Univ., Ft. Collins, CO 80523

Phillip Sollins

Dep. of Forest Science, Oregon State Univ., Corvallis, OR 97331

*Corresponding author.

ABSTRACT

Tree 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 pK_a (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.

This project was funded primarily by National Science Foundation Grant BSR-841678, and also by the Integrated Forest Study of the Electric Power Research Institute through Oak Ridge National Laboratory.

Received for publication November 16, 1989.