鉱質土壌へのスギ・ヒノキの落葉混入が土壌中の窒素蓄積に及ぼす影響

鉱質土壌へのスギ・ヒノキの落葉混入が土壌中の窒素蓄積に及ぼす影響を明らかにすることを目的に，関東地方の１３，２１，３４，４８，６６，９３年生の同一斜面に隣接したスギ林およびヒノキ林において，年間リターフォールおよびAo層中のN含有量，鉱質土壌に混入した針葉および鉱質土壌中のN含有量を調査した。ヒノキ林では土壌中に混入したヒノキ葉のC／N比は，Ao層と土壌深０―１０cmの鉱質土壌との中間的な値を示していた。各林齢において，ヒノキ林ではスギ林に比べて鉱質土壌に落葉が移動し，鉱質土壌のNに占める割合が大きいことを明らかにした。また，ヒノキ林では分解されやすいリターが土壌中に混入しやすく，その結果，土壌におけるN含有量に少なからず影響を与えていることが示唆された。一方，スギ林では２１年生以降において，リターフォールによるN含有量は林齢の増加とともに減少するもaのの，有機物分解されにくいリターフォールを林床に供給することで，Ao層量が維持され，土壌中のN含有量も変化しないことが推察された。
Takahiro Ichikawa：Effects of Japanese cedar (Cryptomeria japonica D. Don) and Japanese cypress (Chamaecyparisobtusa Sieb. et Zucc.) needles in the soil on nitrogen accumulation in forest soil. Japanese Journal of Forest Environment 58:17―23, 2016.
The aim of the present study was to investigate the effects of Japanese cedar and Japanese cypress needles in the soil on nitrogen accumulation in forest soil according to stand age in the Kanto region of Japan. We determined nitrogen content of litterfall, forest floor, needle leaves in the soil, and soil in cedar and cypress forests. The study sites were located in forests planted with Japanese cedar and Japanese cypress trees that were adjacent to each other on the same slope with different stand ages (13, 21, 34, 48, 66, and 93 years). In cypress forests, the C/N ratio of needle leaves in the soil was intermediate between that of forest floor and soil nitrogen content at a depth of 0―10 cm. The contribution of nitrogen by needle in the soil nitrogen content at a depth of 0―10 cm in the cypress forests was larger than that in the cedar forests. The easily decomposed cypress litter entered the soil at a depth of 0―10 cm, and increased soil nitrogen content in the cypress forest. On the other hand, the nitrogen content of litterfall decreased more than 21 years old in the cedar forests. However,litterfall of decomposed with difficulty was increasing with stand age in the cedar forest. This suggest that the soil nitrogen content at a depth of 0―10 cm remains unchanged with stand age in the cedar forests.