[M-IS09] Biogeochemistry

[MIS09-06] Comparison of stream water chemistry measured in 1999 and 2020 in the Shimanto river watershed

*Yoshiyuki Inagaki¹, Masahiro Inagaki¹, Koji Shichi¹, Shuichiro Yoshinaga¹, Tsuyoshi Yamada¹, Satoru Miura¹, Yoshiki Shinomiya¹, Kazumichi Fujii¹ (1.Forestry and Forest Products Research Institute)

Keywords:nitrate, sulfate, stream water chemistry, calcium

Stream water chemistry in forested ecosystems are affected by atmospheric deposition and climatic condition. Sampling of steam water samples at the same site from multiple locations would provide insights about spatial heterogeneity of changes in stream water chemistry. In this study, stream water was collected from 92 locations at forested watersheds in Shimanto River Basin in 1999 and 2020 during summer and in 2000 and 2021 during winter season. Stream water chemistry after 20 years were highly correlated with that in the initial sampling. The results suggest that stream water chemistry is stable during 20 years. In summer season, mean value of electric conductivity (EC) decreased by 10% after 20 years. The decrease of calcium, magnesium and nitrate concentration (8.7 - 10.3%) was similar to that of EC whereas decrease of chloride and sulfate concentration was greater (22.5 - 24.8%). In winter season, mean value of EC decreased by 4.3% after 20 years. The increase of calcium and magnesium concentration (17.5 - 17.7%) was greater than that of EC. Sulfate concentration did not change appreciably but chloride concentration decreased by 10.8%. Decrease of nitrate concentration (55.9%) was greater than other properties. From these findings, changes of anion concentration for 20 years were different between element. Chloride concentration decreased in both seasons. Nitrate concentration decreased in winter season whereas sulphate concentration decreased in summer season. There are some possible reasons for explaining lower nitrate concentration during winter season. At first, amount of rain before sampling were greater in the initial time. It is suggested that nitrate concentration was higher due to greater nitrate leaching from the surface soils in the initial sampling. Secondly, nitrogen demand of soil microbes and plants would increase during 20 years. Higher biological demand would be caused by lower input of atmospheric nitrogen and higher nitrogen uptake by plants in response to elevated atmospheric carbon dioxide concentration. Because decrease of nitrate concentration was obvious in all though the study areas, large-scale factor would be a primary reason for this pattern.