RATES OF SOIL ACIDIFICATION UNDER DIFFERENT PATTERNS OF NITROGEN MINERALIZATION

To evaluate the relative significance of acid deposition in a forest-soil system, H+-generating and -consuming conditions in the whole system were quantified based on the determination of the H budget. The two sites selected for this study displayed different rates of nitrification, which affected significantly the rates of soil acidification. Since at one site (MIKAMI), 60% of the total acid load was attributed to nitrification, the depletion of the soil acid-neutralizing capacity (ANC(S)) was three and a half times faster than that of the other site (YASU). The decreasing rates of ANC(S) were 5.9 kmol ha-1 y-1 at MIKAMI and 1.7 kmol ha-1 y-1 at YASU, which are classified as sites with ''intermediate'' and ''low'' rates of soil acidification, respectively. Atmospheric input of H+ accounted for only a small part (< 10%) of the total acid load. Comparison of the drainage at two depths (40 and 80 cm) provided information about the mechanisms by which dissolved Al3+ was retained in soil. The YASU soil showed a high sulfate adsorption capacity, which resulted in rise of the solution pH by one unit, and subsequently in the precipitation of dissolved Al3+ as jurbanite. In contrast, at MIKAMI, acid neutralization did not occur in the soil at 40- to 80-cm depths, but the cation exchange reaction decreased the concentration of dissolved Al3+.