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ヒノキ林における土壌中の窒素動態を定量的に解明する目的で, 京都大学大学院農学部附属演習林上賀茂試験地において, 土壌中の無機態窒素現存量, 実験室培養による純窒素無機化速度, イオン交換樹脂を用いた土壌中での無機態窒素移動量, 現地での窒素無機化速度を測定した.土壌の無機態窒素現存量は年間を通じて20～24 ㎏ N ha^ 50 ㎝^で推移し, 71%以上をアンモニア態窒素が占めた.実験室における培養では, 窒素無機化速度は39～74 ㎏ N ha^ 28 days^, 現地培養による窒素無機化速度は0～13.5 ㎏ N ha^ 28 days^で, どちらも春～夏にかけてやや窒素無機化速度は高まった.土壌深度50㎝からの無機態窒素流亡量は0.2～2.1 ㎏ N ha^ 28 days^であった.これらの結果から, 上賀茂ヒノキ林では可給態無機態窒素は年間を通じて供給されていること, 可給態無機態窒素は約75 ㎏ N ha^ 50 ㎝^ year^生成されることがわかった.また, N budget法により地下部には地上部とほぼ同程度の約33 ㎏ N ha^ 50 ㎝^ year^が投下され, 細根の生産量は1.49～2.67 ton ha 50 ㎝^ year^と推定された., Qualitative analysis of soil N dynamics was done at Kamigamo Experimental Forest of Kyoto University. Soil inorganic N pool size, net soil N mineralization rate, N leaching loss and in-situ net soil N mineralization rate were determined. Soil inorganic N pool size ranged from 20 to 24 kg N ha 50cm, ammonium - N dominated over 71%. Net N mineralization rate was higher during spring to summer, it ranged from 39 to 74 kg N ha 28days. In-situ net N mineralization rate was also high during spring to summer and it fluctuated from 0 to 13.5 kg N ha 28days. Leaching loss from 50 cm soil depth varied from 0.2 to 2.0 kg N ha 28days. From these results, N was mineralized throughout the year and available N in this ecosystem was estimated about 75 kg N ha 50 cm year. Using N budget method, it was suggested that 33 kg N ha 50 cm year is used below ground and fine root production is estimated from 1.49 to 2.67 ton ha 50 cm year.