Inorganic nitrogen supply to the stream ecosystems is regulated by hydrological and biogeochemical processes such as denitrification in riparian zones. To elucidate how the denitrification is controlled by hydrologic properties of this landscape unit, we investigated two forested headwater catchments with different soil physical properties. The Kiryu Experimental Watershed (KEW, 0.6 ha) has sandy soils in riparian zones, while the Fukuroyama-sawa catchment (FEW, 1.1 ha) has clayey soils. Concentrations, d15N and d18O of NO3- in soil water, groundwater and streamwater were monitored. Enrichment in d15N-NO3- was found in the perennial groundwater both in KEW and FEW, indicating active denitrification occurred. However, enrichment of d18O-NO3- of KEW was unclear, while that was clearly found in FEW, indicating the denitrification under a closed system without sufficient dissolved O2 and NO3- supply. These were explainable by hydrologic feature of soils holding groundwater: The mean residence time (MRT) of the groundwater in FEW was three times longer than that in KEW. This is essentially explained by difference in the soil water retention between sandy KEW and clayey FEW. Those suggests the possibility that the difference of soil physics can strongly be reflected on inorganic nitrogen discharge from forested catchments.