報告書番号

T99016

タイトル（和文）

スギ林小流域における渓流水のアルカリ度の形成メカニズム

タイトル（英文）

Alkalinity formation mechanism for streamwater in a small watershed covered by Japanese cedar forest

概要 （図表や脚注は「報告書全文」に掲載しております）

渓流水のアルカリ度の形成メカニズムを明らかにするため，スギ林小流域で林外雨，林内雨，土壌水，地下水および渓流水の化学組成を調べた．樹冠と土壌層（100cm）でのHCO3-濃度の増加量は，それぞれ6，64μmolc L-1であったのに対し，土壌水（100cm）から地下水になる過程では著しく上昇した．その増加量（384μmolc L-1）はNO3-濃度の低下量（399μmolc L-1）に見合う量であったことから，HCO3-濃度の上昇は脱窒に起因すると推定した．渓流水の化学組成は地下水に類似した．NO3-濃度は土壌層10cmで著しく上昇（78→400μmolc L-1）しており，土壌有機物分解によるNH4+の生成とその硝化が主原因であった．以上の結果から，渓流水のアルカリ度の形成には土壌表層での硝化による大量のNO3-の生成と地下水帯での脱窒が決定的な役割を果たしていることが分かった．

概要 （英文）

In order to clarify the mechanism for formation of streamwater alkalinity, we examined the chemical composition of precipitation, throughfall, soil solution (at depths of 10, 30, 50, 100 cm), groundwater and streamwater for 2 years in a small watershed covered by Japanese cedar forest in Gunma Prefecture, Japan. Annual volume-weighted mean pH values for precipitation and throughfall were 4.60 and 4.92, respectively, showing that precipitation was partially neutralized by the canopy. This caused a slight increase in HCO3- concentration (6 μmolc L-1 in average). As throughfall passed through an uppermost soil layer of 10 cm thick, NO3- concentration increased markedly (from 78 to 400 μmolc L-1), which was mainly caused by mineralization of organic nitrogen followed by nitrification. Then, NO3- concentration increased gradually with depth. Increase in HCO3- concentration within the soil layer of 100 cm thick was only 64 μmolc L-1. In contrast, a considerable increase in HCO3- concentration (384 μmolc L-1) was observed between soil solution at 100 cm depth and groundwater. This value was almost identical to the decrease in NO3- concentration between the two (399 μmolc L-1), implying that the increase in HCO3- concentration was attributed to denitrification in the aquifer. The chemistry of sreamwater was quite similar to that of groundwater. It is concluded from these results that internal NO3- production in the surface soil layer as well as denitrification in the aquifer played a crucial role in the formation of streamwater alkalinity.

報告者

キーワード