Factors determining differences in soil pH in adjacent conifer and alder-conifer stands

Tree species may differ in their influence on biogeochentical cycles,leading to differing rates of soil acidification. Over time, quantitativeand qualitative changes develop in the characteristics of the soilexchange complex. Three such characteristics regulate soil pH: (i)the quantity of adds present, which can be represented as the totalcation-exchange capacity (CEC)S (ii) the degree of dissociation ofthe adds, commonly called base saturation; and (Hi) the affinity ofthe adds for W, or add strength, which represents the compositepits (negative log of the add ionization constant) of the exchangecomplex. We examined the importance of these three factors in ex-plaining the differences in soil pH between adjacent stands of con-ifers [primarily Douglas fir, Pseadotsagc ineaziesii (Mirbel) Franco'and conifers mixed with N-fixing red alder (Alan mbar Bong.). Ata low-productivity site (Wind River, WA), the pH of 0 to 0.15 m ofsoil from both alder-conifer and conifer stands averaged 4.3 in 0.01M CaCl2. The pH values were the same, however, only becausehigher base saturation in the alder-conifer stand was offset bygreater acid strength. At a more productive site (Cascade Head, OR),soil pH (in 0.01 M CaCl2) averaged 3.7 in the alder-conifer standbut 4.4 in the conifer stand. The difference in pH resulted primarilyfrom greater add strength of soil organic matter under alder, andsecondarily from lower base saturation of the exchange complex.These results underscore the importance of considering qualitativechanges in soil organic matter as factors driving changes in soil pHand other parameters.