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 SO cm) provided information about the mechanisms by which dissolved AI3+ 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+.