Although the amount of acidic deposition has recently decreased in Japan, it has still deteriorated some forest ecosystems during the past several decades. Moreover, recent climate changes can affect stream water chemistry. We investigated the temporal trend and effects of climate conditions on stream water chemistry for more than 20 years in two areas (Yusuhara and Taisho) in the Shmanto River Basin, southern Japan, where the effects of acidic deposition are considered to be modest. Stream water samples were collected monthly from three forest watersheds selected at each site. The annual means of the stream chemistry were predicted by multiple regression analysis. The ammonium, nitrate, and sulfate concentrations in the bulk precipitation have decreased at Yusuhara, and the sodium, magnesium, calcium, chloride, nitrate, and sulfate concentrations in the stream water have decreased in both areas. The nitrate and sulfate concentrations apparently responded to the decreasing input of acidic deposition. The sunlight hours were positively related with the potassium, magnesium, calcium, nitrate, sulfate, and bicarbonate concentrations in stream water. The results suggest that long sunlight hours boost the photosynthetic activities, thus promoting soil respiration and decomposition of soil organic matter. Moreover, a higher carbonic acid concentration in the soil solution promotes cation weathering and carbonic acid dissociation to bicarbonate. Given the decreasing trends in magnesium and calcium concentration with no change in bicarbonate concentration, we inferred that previousinputs of acidic deposition enhanced the rate of rock weathering.

中文翻译：

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气候和酸沉降对日本南部四万十川流域森林流域溪流水化学年际变化的影响

尽管日本的酸性沉降量最近有所减少，但在过去几十年中仍然使一些森林生态系统恶化。此外，最近的气候变化会影响溪流水的化学成分。我们调查了 20 多年来气候条件对日本南部 Shmanto 河流域的两个地区（梼原和大正）溪流水化学的时间趋势和影响，这两个地区的酸性沉积影响被认为是适度的。每月从每个地点选择的三个森林流域收集溪流水样。通过多元回归分析预测了流化学的年度平均值。梼原地区大量降水中的铵、硝酸盐和硫酸盐浓度均有所下降，而这两个地区溪水中的钠、镁、钙、氯化物、硝酸盐和硫酸盐浓度均有所下降。硝酸盐和硫酸盐浓度明显响应酸性沉积输入的减少。日照时数与溪水中钾、镁、钙、硝酸盐、硫酸盐和碳酸氢盐浓度呈正相关。结果表明，较长的日照时间促进光合作用活动，从而促进土壤呼吸和土壤有机质的分解。此外，土壤溶液中较高的碳酸浓度促进阳离子风化和碳酸离解为碳酸氢盐。鉴于镁和钙浓度呈下降趋势，而碳酸氢盐浓度没有变化，我们推断，先前的酸性沉积输入增强了岩石风化的速率。