Long-term changes in atmospheric nitrogen deposition and stream water nitrate leaching from forested watersheds in...

Japan receives nitrogenous air pollutants via long-range transport from China. However, emissions of nitrogenous air pollutants in China have stabilized or decreased in recent years. This study examined both the long-term trends in atmospheric nitrogen (N) deposition from the 1990s to the 2010s and the response of stream water nitrate (NO₃^-) leaching from forested areas in western Japan. A long-term (1992-2018) temporal analysis of atmospheric N deposition in Fukuoka (western Japan) was conducted. Atmospheric bulk N deposition was collected at forested sites in a suburban forest (S_west) and a rural forest (R_west) in western Japan during 2009-2018. Stream water samples were also collected from four locations at sites S_west and R_west during the same period. Results showed that atmospheric N deposition in Fukuoka started to decrease from the mid-2000s at an annual rate of -2.5% yr^-1. The decrease in atmospheric N deposition was attributable mainly to decreased atmospheric ammonium (NH₄⁺) deposition, which caused greater contribution of NO₃^- deposition to atmospheric N deposition. Concentrations of NO₃^- in the stream water samples from three of the four locations decreased significantly at an annual rate of -3.7 to -0.7% yr^-1. However, stream water NO₃^- concentrations increased in one watershed where understory vegetation has been deteriorating owing to the increased deer population. This might weaken the recovery of N leaching from forested areas.