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

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 (Swest) and a rural forest (Rwest) in western Japan during 2009–2018. Stream water samples were also collected from four locations at sites Swest and Rwest 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.