To examine the influence of microbial carbon (C) availability on the internal soil nitrogen (N) cycles under moder and mull forest floor types within the same slope sequence, surface mineral soils (0-5 cm depth) taken at upper (moder-type forest floor) and lower (mull-type forest floor) positions on a slope in a Cryptomeria japonica D. Don plantation were incubated for 300 days. During the incubation, changes in net and gross N transformations, the organic C and N pools, and microbial respiration were monitored. Despite relatively small differences in net N mineralization in both soils, very rapid rates of gross N transformations were found in mull soil during the initial 15 days of the experiment. A rapid net nitrification occurred after days 150 and 100 in moder and mull soils, respectively, presumably because of decreased microbial C availability. However, a rapid net nitrification also occurred in the mull soil during the initial 15 days when microbial C availability was high, and gross nitrification was detected in both soils, except at day 0 in the moder soil. Changes in gross N transformations and in organic C and N pools over the experiment suggested that the start of rapid net nitrification might be influenced not only by microbial C availability, but also by the microbial availability of N relative to C.