Long-term Changes in Sulfate and Nitrate Concentrations in Streams in Western Japan Between 1986 and 2023 in Response to Changes in Sulfur and Nitrogen Deposition from the Atmosphere

High levels of atmospheric deposition degrade forest ecosystems and cause ecological problems in streams. The amount of atmospheric deposition in Japan has decreased in recent years, but limited information on how stream water chemistry responds to this change is available. We analyzed historical atmospheric sulfur (S) and nitrogen (N) deposition data for Fukuoka City in western Japan from 1992 to 2021 and compared long-term (1986–2023) changes in stream water chemistry at 11 sampling points to evaluate stream water chemistry responses to atmospheric S and N deposition changes. Atmospheric S and N deposition in Fukuoka City increased from the 1990s to the mid-2000s and then decreased to the same as in the 1990s. The sulfate concentration in stream water was significantly lower in 2023 (105 μmol L⁻¹) than 1986 (131 μmol L⁻¹), reflecting changes in atmospheric S deposition. In contrast, the nitrate (NO₃⁻) concentration in stream water was significantly higher in 2023 (107 μmol L⁻¹) than in 1986 (83 μmol L⁻¹) despite atmospheric N deposition decreasing. The calcium counterion concentration was also higher in 2023 (345 μmol L⁻¹) than in 1986 (297 μmol L⁻¹). Partial least squares regression analysis suggested that the NO₃⁻ concentration was related to the high percentage of area covered by Japanese cedar trees, which promote calcium leaching. We conclude that large areas of Japanese cedar plantations could attenuate the response of stream NO₃⁻ concentrations to the decreased atmospheric N deposition.