Nitrogen dynamics were investigated in tandem with groundwater monitoring in a temperate forest catchment in Japan, to determine the effect of the partial dieback of dominant tree stands on nutrient cycling and streamwater chemistry, especially temporal and spatial changes in the NO3- concentrations in soil and ground water. Decreased nitrogen uptake by roots and increased nitrogen supply from the litter fall that occurred with the 1992-94 pine dieback caused by pine-wilt disease brought on a threefold increase in nitrate concentrations in streamwater and subsurface groundwater. It was found that, from 1992 to 1996, seasonal peaks occurred in the stream NO3- concentrations during the rainy season (July to August). This seasonal variation corresponded directly to groundwater levels in the riparian zone near the catchment outlet, suggesting that seasonal changes in groundwater levels were the dominant factor controlling temporal variation in stream NO3- concentrations. The mechanism of the seasonal pattern found in this watershed emphasizes the importance of hydrological seasonality with the high precipitation, groundwater levels, and runoff rates that are seen in summer in Japan, whereas in forested watersheds in Europe and the eastern USA the smaller inorganic nitrogen pools and low transportation forces simultaneously affect NO3- leaching during summer. Copyright (C) 2003 John Wiley Sons, Ltd.