Denitrification in Shallow Groundwater in a Forest Adjacent to Farm Land on Joso Upland : Measured Using δ^<15>N Values

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  • δ^<15>N値による平地林浅層地下水中における脱窒の検証 : 茨城県南西部常総台地の例
  • デルタ15Nチ ニヨル ヘイチ リンセンソウ チカスイチュウ ニ オケル ダッ

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Abstract

In order to determine processes of decreasing nitrate concentration in shallow groundwater in a forest on a suburban upland, we investigated the spatial distribution of δ^<15>N values of nitrate and nitrate concentrations in groundwater and soil water on Joso Upland, 60 km northeast of Tokyo. The water samples were collected at four plots along the direction of groundwater flow, which existed in farm land, forest, the boundary between farm land and forest, and grassland near a spring, respectively. Soil water samples were collected at depths of 0.2 m, 1 m and 3 m in the forest. The nitrate concentrations of shallow groundwater in farm land, forest and grassland were from 8 to 11, 6 and 2 mg L^<-1>, respectively. The nitrate concentration of shallow groundwater remarkably decreased throughout the forest. This suggested dilution of shallow groundwater with soil water which had a lower nitrate concentration. The δ^<15>N values of shallow groundwater in the forest (12 ‰) and grassland (more than 14 ‰) were higher than those of the shallow groundwater in the farm land (6 ‰), and the soil water in the forest (about 2 ‰). These results indicated that the nitrate concentration decreased not only by the dilution process with soil water, but also by the denitrification process of shallow groundwater in the forest and grassland.

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