森林表層土壌における無機態窒素の動態とカチオン濃度の対応 : 土壌水分条件を考慮するライシメータ実験

Abstract

土壌水分条件の違いが, 森林表層土壌における無機態窒素の動態に及ぼす影響を明らかにするために, 森林流域内でライシメータ実験を行った. 実験には, 直径20. 2cm, 長さ30cmのパイプに非撹乱土壌を充填した土壌カラムを用いた. 森林流域内に多様に分布する土壌水分条件を考慮し, 土壌カラム底部の平均圧力水頭を -20. 7kPa, -6. 7kPa, OkPa, 2. 0kPaに制御することで土壌水分条件を4段階に分けた. 採水試料は, 4種のライシメータからの流出水と, 土壌水分条件を制御するために吸引した土壌深度10, 20, 30cmからの土壌溶液とした. 土壌水分条件に関わらず, 無機化および硝化は土壌表層で起きていることが確認された. 無機態窒素の濃度および負荷量は, 土壌水分条件により明らかに異なった. 飽和条件下では無機化は進むが, 硝化は抑制され, さらに脱窒が起きている可能性も示唆された. -20. 7kPa, -6. 7kPaで土壌溶液を吸引した条件下では高い硝化活性を示したが, 吸引圧によってNO_3_-濃度およびNO_3_--N負荷量に違いが生じ, 水分条件の違いにより硝化活性が異なることが明らかとなった. また, 硝化活性が最も大きくなる土壌水分条件は, 特定の範因内にあることが示唆された.

An in situ lysimeter experiment was carried out to examine the relationship between the soil moisture condition and dynamics of inorganic nitrogen in a forest catchment. The lysimeters were prepared by in situ undisturbed soil columns with PVC cylinders (inner diameter 20.2cm, length 30.0cm) . Because of a variety of soil moisture conditions in a forest cathement , four soil moisture conditions were controlled by using an average pressure head at the bottom of each column at -20.7kPa, -6.7kPa, OkPa and 2.0kPa. Samples were gravitationally drained from four lysimeters and soil solutions at 10cm, 20cm and 30cm depths which were sucked to control soil moisture conditions. It was observed that nitrification and mineralization of nitrogen occured in the upper soils in every lysimeters regardless of moisture conditions. The four soil moisture conditions caused significant differences in the concentration and budget of inorganic nitrogen. Under saturated condition, mineralization of nitrogen was observed in the upper 10cm layer of the soil column, but nitrification was limited. It suggested that denitrification occured in the column under saturated condition. Under sucking soil solution at -20.7kPa and -6.7kPa, the concentration and budget of nitric acid were higher than two other columns. Nitrification activity tended to be higher under dry condition. Consequently it was clear that nitrification activity depended on the soil moisture condition. Moreover, it was suggested that at a specific range of soil moisture condition which nitrification activity was maximum.