Although the amount of acidic deposition has recently decreased in Japan, acid deposition has deteriorated some forest ecosystems during the past few decades. We investigated the yearly variations in 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 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. 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. Given the decreasing trends in magnesium and calcium concentration with no change in bicarbonate concentration, we inferred that previous inputs of acidic deposition enhanced the rate of rock weathering.

To identify the soil carbon stock change from cropland to forest land in Japan, we compared the soil carbon stock of a cropland and that of an adjacent forest land at 23 different sites. With regard to a 0–30 cm depth basis, the soil carbon stock in the cropland was greater than that in the forest land; however, it was less than that in the forest land when an equivalent mass basis was used. In less than an elapsed time of 20 years after a land-use change, the soil carbon stock after afforestation was less than that in the adjacent cropland at the same sites. However, after an elapsed time of 20 years, the soil carbon stock in the afforested site exceeded that in the adjacent cropland at the same sites. The ratio of the soil carbon stock in forest land to that in the cropland was 1.10 on average, which is comparable with the previous mass-corrected paired-sampling studies. The ratio in the conifer-planted forest was significantly greater than that in the hardwood re-generated forest. Some of the previous reviews, including those of the non-mass-corrected data, were possibly biased, and more studies using the paired-sampling method with equivalent mass basis need to provide more general ratios in the future.