Abstract
This study aims to identify the possible roles of a floodplain inbiogeochemical nitrogen cycles, based on analysis of groundwaterdynamics of inorganic nitrogen species. Groundwater samples collected from boreholes made of poly vinyl chloride located onthe floodplain in the middle reach of the Tama River, Nagata district were analyzed for dissolved organic carbon, dissolved oxygen, inorganic nitrogen compounds, major anions and the stableisotope ratio of nitrate-nitrogen.Dissolved oxygen in groundwater samples collected from marked borehole (G-5) was low (50 ± 16 μM) and nitrate decreasedfrom 63 μM to < 10 μM during our research period. Concentrations of nitrate, nitrite and nitrous oxide decreased one after the other. These decreases in concentration, combinedwith 15N-enrichment of groundwater nitrate suggest denitrification as a significant nitrate sink, yielding an isotopic fractionation of nitrate-nitrogen with an enrichment factor (e= –17.9‰) that is comparable tothose in various soil and groundwater systems. This study provesthat floodplains can perform as nitrogen sinks for groundwaterin river catchments.
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