Nitrate discharge via streamwater is an important indicator for how disturbances affect the forest nutrient cycling. A comprehensive review of 35 years of hydrological data in the Matsuzawa catchment, a temperate forest in central Japan, demonstrated that substantial nitrate (NO₃^-) leaching into streams was observed for over 25 years following the red pine dieback attributed to pine wilt disease (PWD) in 1989-1990. The same field experiments that occurred in 2002-2003 and 2022-2024 have implied different N dynamics and NO₃^- leaching patterns in this catchment. Dissolved inorganic nitrogen (DIN) pools of 10-30cm and 30-50cm in 2002-2003 were 17.7-20.9 and 18.6-22.4 kg N/ha, respectively. They were much larger than those in 2022-2024: 2.5-3.1 and 3.5-5.4 kg N/ha. According to the observation data of microbial activities, the annual average net mineralization and nitrification rates of every soil layer decreased in 2022-2024. DIN, mostly NO₃^- leaching through 0-50cm to the groundwater was also measured by the ion exchange resin core method. In 2002-2003, 12.9-26.0 kg N/ha/yr DIN was exported through the 50 cm soil layer, while in 2022-2024, it was 3.3-17.4 kg N/ha/yr. However, nitrate concentrations of streamwater in 2022-2024 did not show significantly declined trends when compared with those in 2002-2003. A plausible explanation is that mature Japanese Cypress has led to a decrease in nitrogen uptake. We calculated the estimation of annual DIN uptake from 0-50 cm in both periods: they were 142.5-180.4 kg N/ha/yr in 2002-2003 and 80.1-97.2 kg N/ha/yr in 2022-2024, respectively. The paired experiments demonstrated that nitrogen dynamics in this catchment varied largely during the past decades since PWD. Both plants and soil microbes performed much more actively in 2002-2003, even though there was less rainfall and less vertical infiltration. NO₃^- leaching via streamwater was a synthesis process that was controlled by both hydrological pathways and biochemical processes. NO₃^- discharge may not respond directly to the alternations of plant growth and microbial activity. Long-term research has been essential in understanding how disturbances influence nutrient cycling in catchments, particularly through diverse monitoring methods that include nitrogen biogeochemistry, hydrology, and plant growth.