Nitrogen (N) is a critical element for tree growth in forest ecosystems. As trees absorb inorganic N, the soil N mineralization process is a key process for their development. Although the spatial pattern of soil N mineralization is expected to relate to tree growth, the difference between the upper and lower positions within a small-scale slope is unclear. Therefore, we compared annual and seasonal soil N mineralization rates in Japanese cypress (Chamaecyparis obtusa [Siebold & Zucc.] Endl.), which stands at both the upper and lower positions on a slope. We used the resin-core method to estimate in situ soil N mineralization rates. Additionally, the litter decomposition rate and inorganic N passed through the litter layer, which are primary sources for soil N mineralization, were investigated using the litter bag and resin-core methods. Our findings revealed that the annual soil N mineralization rate at the lower position was 5 times higher than that at the upper position. Moreover, seasonal variations in soil N mineralization rate tended to be higher at the lower position than at the upper position. The temporal change in input ammonium passed through the litter layer was similar to that of the nitrification rate in the soil at the lower position, except for winter. Notably, high nitrification in winter at the lower position may be related to soil frost, which can accelerate the decomposition of organic matter. Despite these differences, the litter decomposition rate was similar between the slope positions. The higher soil N mineralization rate and substrate input may result in higher tree growth at the lower position on a slope.