The N-15 natural abundance method was used to assess the intermittent occurrence of denitrification in the Kiryu watershed in Shiga Prefecture, Japan. The concentration and isotopic composition (delta(15)N) of NO3--N, as well as some physical and chemical variables that potentially affect denitrification, were measured along the flow path from precipitation to stream water via soil solution and groundwater. High maximum groundwater level promoted a decrease in NO3--N concentration that was associated with an increase in the delta(15)N of NO3--N with soil depth; i.e., denitrification occurred in the soil due to increasing soil moisture. While the maximum level of groundwater fell, no such changes were observed. Dissolved oxygen (DO) and Mn2+ concentrations in soil solutions indicated that strong anaerobic condition did not occur during the study period. These results suggested that denitrification was occurring temporarily in anaerobic microsites such as waterlogged soil aggregates. Upward expansion of groundwater zone thus appeared to play an important role in promoting such microanaerobic sites, resulting in the intermittent occurrence of denitrification. Based on these data, a schematic model for assessing denitrification was proposed based on a NO3--N/Cl- - delta(15)N of NO3--N map for soil-water systems. Our data showed that variations of NO3--N concentration and delta(15)N value are useful indicators for elucidating nitrogen dynamics as affected by water mixing, plant uptake, nitrification, and denitrification in forested ecosystems. Copyright (C) 1997 Elsevier Science Ltd.