Although atmospheric nitrogen deposition due to throughfall has decreased in Tokyo metropolitan, the forested catchment which composed with Japanese cedar (Cryptomeria japonica), Japanese cypress (Chamaecyparis obtusa), and oak (Quercus serrata) was still nitrogen saturated. We aimed to elucidate effects of decrease in nitrogen deposition on aluminum (Al) dynamics in a forested Andisols. Throughfall and soil percolated water samples were taken under Japanese cedar stand and Japanese cypress stand in Hachioji, Tokyo (Field Museum Tamakyuryo). Major inorganic ions were determined by ion chromatography. Total Al concentrations were measured by atomic absorption spectrometry after concentrating under acidic condition. Aluminum concentrations were significantly correlated with nitrate (NO₃^-) concentration in both Japanese cedar stand and Japanese cypress stand. In case of Japanese cedar stand, Al concentrations tended to decrease with time. Acid load associated with N transformation ([H⁺]load), which was chosen as the sole factor for Al mobilization in Japanese cedar stand, has also decreased. Decrease in nitrogen deposition affected Al dynamics via decrease in [H⁺]load. It was suggested that the calcium (Ca) content in needles may promote recovery from soil acidification in Japanese cedar stand.