Chemical status and their variation of floodplain soils collected from the middlecourse of the Tama River in 2001

Abstract

In the middlecourse of the Tama River, a typical urban river in Japan, invasion of alien plants and decline of floodplain endemic plants are remarkable. Soils were collected from the floodplains of Nagata, Sekido and Koremasa in 2001, and pH, electrical conductiv-ity, oxidation-reduction potential and water soluble ions (NO₃^-, HPO₄^2-P, Cl^-, SO₄^2-, Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺) were determined to reveal the chemical status of floodplain soils in the middlecourse of the Tama River. The pH values in Nagata, where the original environment of the Tama River exists, showed alkalinity while those in Sekido and Koremasa located in more urban area were significantly low. Ca²⁺ concentration was the highest level among analyzed ions, and had strong correlation with electrical conductivity. Positive correlations were significant between NO₃^--K⁺, NO₃^-NH4⁺ and NH₄^+--K⁺, indicating that those ions were responsible for eutrophication in the middlecourse of the Tama River. Furthermore, negative correlation between pH values and NIL concentrations was observed. These results indicated that eutrophication of floodplain soils in the middlecourse of the Tama River was followed by decrease of pH values. Although high level of NO₃^- was detected in soils collected from woodland of false acacia (Robinia pseudoacacia L.) in Nagata when compared to riverbed soils of the same area, decrease of NO₃^- concentrations was found in soils collected after the floods. As HPO₄^2--P and NH₄⁺ concentrations and pH values also showed unique fluctuations, it was expected that chemical status of floodplain soils was greatly affected by floods. In our conclusion, decrease in flooding frequency, eutrophication, and following decrease of pH values could change the original condition of floodplain soils in the middlecourse of the Tama River and facilitate the invation into the floodplain of alien plants.