北関東地方の高齢化したスギ・ヒノキ人工林小流域における炭素と窒素の動態特性  [in Japanese] Characteristics of Carbon and Nitrogen Dynamics in a Small Watershed of Aged Japanese Cedar and Cypress Stands in the Northern Kanto Region, Japan  [in Japanese]

北関東地方の高齢化(94年生)したスギ・ヒノキ人工林小流域における炭素(C)と窒素(N)の動態特性を明らかにするために,無機態N量,純N無機化速度と地表面からのCO_2放出速度などを調査した。無機態N量,純N無機化速度およびCO_2放出速度は脚部スギ林で大きく,尾根部ヒノキ林で小さかった。CO_2放出速度と無機態N量には正の相関性があり,土壌呼吸の盛んな時期に林木に吸収されない無機態Nが存在し,脚部スギ林で多いことがわかった。年間の純N無機化量と水抽出性有機態C消費量には正の相関性があり,C無機化特性の純N無機化量への影響が示唆された。本流域のスギ林はヒノキ林と比べて窒素同化に対する炭酸同化の割合,および無機態N供給に対するC消費割合が小さいと推察された。CとNの循環特性から高齢化した脚部スギ林では,生態系からNが流出しやすいと考えられた。

To clarify the characteristics of carbon (C) and nitrogen (N) dynamics in a small watershed of aged (94 year-old) Japanese cedar (Cryptomeria japonica) and cypress (Chamecyparis obtusa) stands in the northern Kanto region, amounts of net N mineralization and CO_2 efflux in soil were measured. The amounts of inorganic N, the rate of net N mineralization and CO_2 efflux were greater in the lower slope of the cedar stand than in the upper slope of the cypress stand. The amounts of inorganic N had a positive correlation with the rate of CO_2 efflux. These suggested that there was inorganic N retention when the CO_2 efflux rate was high, especially in the lower slope of the cedar stand. The amounts of net N mineralization and the consumption of water-soluble organic carbon (WSOC) had a positive correlation, which suggested the effect of C characteristics on net N mineralization in the soil. In the cedar stand, the amount of inorganic N supply was greater than N uptake, and the ratios of C to N assimilation and soil respiration to inorganic N supply were smaller than in the cypress stand. Therefore, the aged artificial forest on a fertile site, such as the cedar stand on a lower slope position, has a risk of N discharge from the forest ecosystem.