Acidic substances, specifically sulfur (S) compounds, derived from atmospheric deposition play a major role in the acidification of forest ecosystems. This study conducted field surveys to clarify a buffering system against seasonal large S inputs in a forested catchment in central Japan that has historically suffered from transboundary air pollution. Results showed that atmospheric S fluxes significantly increased in winter due to north-westerly seasonal winds from the Asian continent; fluxes were 1.1 and 0.3 kmol_c ha⁻¹ in the cold and warm seasons, respectively, due to the large effects of sea salt and transboundary air pollution. Despite the large seasonality within atmospheric deposition, SO₄^2– concentrations in stream water (SW) were found to be relatively stable throughout the year. Similarly, S isotopic ratios (δ³⁴S) in rainwater showed clear seasonal variation, increasing to 12‰ in winter and decreasing to 2‰ in summer, whereas the δ³⁴S value of SW was stable year-round at ~9‰. Flux-weighted mean δ³⁴S values for rainfall (RF), throughfall, stemflow, and SW were similar, i.e. 8.5, 9.5, 9.0, and 9.0‰, respectively. Both the δ³⁴S values and the SO₄^2– concentrations in RF and soil solutions appear to converge at values of SW, suggesting that atmospheric deposition is a primary S origin in SW. The sulfur adsorption-desorption in soil appears to mainly buffer the large sulfur input and prevent sudden acidification, whereas a relatively small biological sulfur cycle was suggested by litterfall. Possible disturbances within this buffering system should be carefully monitored under a changing climate.