Long‐Term Changes in Nitrate Runoff in a Temperate Forest Catchment in Japan: Multiple Ecosystem Responses Following a Die‐Back Disturbance by Pine Wilt Disease

Nitrate discharge via stream water is a key indicator of how disturbances influence forest nutrient cycling. In a temperate forest in central Japan, 35 years of hydrological, biogeochemical, and plant uptake monitoring revealed sustained nitrate leaching for over 25 years following red pine dieback due to pine wilt disease in 1989–1990. Stream nitrate concentrations followed three distinct phases: (I) a rapid rise and decline (1990–2000), (II) a gradual decrease (2000–2010), and (III) a slight increase stabilizing above pre‐dieback levels (post‐2010). Phase I was driven by changes in soil nitrate production following pine mortality. In Phase II, prolonged low rainfall accelerated declines in NO 3 − concentrations. During Phase III, natural mortality of stressed cypress trees (2008–2015) coincided with a decline in nitrogen uptake of up to 65.7 kg N ha −1 yr −1 , likely contributing to renewed nitrate leaching. Field experiments demonstrated that plant growth conditions strongly influenced catchment‐scale nitrogen dynamics. Shifts in soil microbial activity were associated with climate variability and changes in vegetation. Notably, the effects of plant mortality on nitrate leaching were delayed, with several years separating disturbance events and hydrological responses. These findings highlight the importance of long‐term monitoring to capture delayed and complex responses of nutrient cycling to ecosystem disturbances, integrating approaches such as dual isotope analyses, biogeochemistry, hydrology, and vegetation uptake.