Studies on mineral composition of the discharge water from subtropical broad-leaved evergreen forestland (1) : A case of the Minami-meiji-yama in the northern part Okinawa Island (Department of Environmental Sciences and Technology)

本研究は,沖縄島北部の南明治山における亜熱帯広葉樹林地から流出するpH,塩素・硫酸・カルシウム・マグネシウム・硝酸態チッ素およびアンモニア態チッ素のイオン濃度について調査したものである。その結果を要約すると次のようである。林外雨量と林内雨量の関係は,直線的な相関関係で示された。林内雨量の塩素・硫酸・カルシウムおよびマグネシウムのイオン濃度は,林外雨量のそれと比較して,それぞれ1.74倍,1.64倍,2.35倍および2.25倍も高い値を示した。樹幹流下水のイオン濃度は,林内雨量のイオン濃度より全体的に低い値を示したが,林外雨量のイオン濃度と比較して,特にアンモニア態チッ素とカルシウムイオン濃度の値が高かった。渓流水の場合は,林外雨量の値に比較して塩素・硫酸・カルシウムおよびマグネシウムのイオン濃度が高い値を示した。また,滋賀県の場合は,無機態チッ素(硝酸態チッ素+アンモニア態チッ素)の割合は約6 : 4であるのに対して沖縄の場合は,それが約8 : 2となり,圧倒的に硝酸態チッ素の割合が高かった。An investigation was carried out to find discharge characteristics of mineral nutrients from the subtropical broad-leaved evergreen forestland Minami-meiji-yama in the northern part of Okinawa Island. Determined were pH and such ions as chlorine, sulfate, calcium, magnesium, nitrate, and ammonium in the rain, throughfall and discharge water. There was a linear correlation between the rainfall and the throughfall. However, the ionic concentrations of chlorine, sulfate, calcium, magnesium were 1.74,1.64,2.35,and 2.25 times higher in the throughfall than those in the rainfall, respectively. The ionic concentrations of the stem flow were generally lower than those of the throughfall. The stem flow was about the same with the rainfall in the ionic concentrations except for ammonium and calcium, that were higher in the stem flow. For stream water, ionic concentrations of chlorine, sulfate, calcium and magnesium showed higher values than those for rainfall. It was found that the proportion of nitrate nitrogen and ammonium nitrogen in inorganic nitrogen was about 8 : 2 in the discharge water that was a remarkably higher proportion than the reported as 6 : 4 in Shiga Prefecture